Reading and Writing OpenEXR Image Files with the IlmImf Library

This document shows how to write C++ code that reads and writes OpenEXR image files. The text assumes that the reader is familiar with OpenEXR terms like "channel", "attribute", or "data window". For an explanation of those terms, see details.html. The OpenEXR source distribution contains a subdirectory, IlmImfExamples, with most of the code examples below. A Makefile is also provided, so that the examples can easily be compiled and run.

Contents of this document:

• 0 Scan-line-based and Tiled OpenEXR files

• 1 Using the RGBA-only Interface for Scan-line-based Files
  • 1.1 Writing an RGBA Image File
  • 1.2 Writing a Cropped Image
  • 1.3 Storing Custom Attributes
  • 1.4 Reading an RGBA Image File
  • 1.5 Reading an RGBA Image File in Chunks
  • 1.6 Reading Custom Attributes

• 2 Using the General Interface for Scan-line-based Files
  • 2.1 Writing an Image File
  • 2.2 Writing a Cropped Image
  • 2.3 Reading an Image File
  • 2.4 Interleaving Image Channels in the Frame Buffer
  • 2.5 Which Channels are in a File?

• 3 Tiles, Levels and Level Modes

• 4 Using the RGBA-only Interface for Tiled Files
  • 4.1 Writing a Tiled RGBA Image File with One Resolution Level
  • 4.2 Writing a Tiled RGBA Image File with Mipmap Levels
  • 4.3 Writing a Tiled RGBA Image File with Ripmap Levels
  • 4.3 Reading a Tiled RGBA Image File

• 5 Using the General Interface for Tiled Files
  • 5.1 Writing a Tiled Image File
  • 5.2 Reading a Tiled Image File

In an OpenEXR file, pixel data can be stored either as scan lines or as tiles. Files that store pixels as tiles can also store multiresolution images. For each of the two storage formats (scan line or tile-based), the IlmImf library supports two reading and writing interfaces: The first, fully general, interface allows access to arbitrary channels, and supports many different in-memory pixel data layouts. The second interface is easier to use, but limits access to 16-bit (HALF) RGBA (red, green, blue, alpha) channels, and provides fewer options for laying out pixels in memory.

The interfaces for reading and writing OpenEXR files are implemented in the following eight C++ classes:

	tiles	scan lines	scan lines and tiles
arbitrary channels	TiledInputFile		InputFile
	TiledOutputFile	OutputFile
RGBA only	TiledRgbaInputFile		RgbaInputFile
	TiledRgbaOutputFile	RgbaOutputFile

The classes for reading scan-line-based images (InputFile and RgbaInputFile) can also be used to read tiled image files. This way, programs that do not need support for tiled or multiresolution images can always use the rather straightforward scan-line interfaces, without worrying about complications related to tiling and multiple resolutions. When a multiresolution file is read via a scan-line interface, only the highest-resolution version of the image is accessible.

1 Using the RGBA-only Interface for Scan-line-based Files

1.1 Writing an RGBA Image File

Writing a simple RGBA image file is fairly straightforward:

    void
    writeRgba1 (const char fileName[],
		const Rgba *pixels,
		int width,
		int height)
    {
	RgbaOutputFile file (fileName, width, height, WRITE_RGBA);	// 1
	file.setFrameBuffer (pixels, 1, width);				// 2
	file.writePixels (height);					// 3
    }

Line 2 specifies how the pixel data are laid out in memory. In our example, the pixels pointer is assumed to point to the beginning of an array of width*height pixels. The pixels are represented as Rgba structs, which are defined like this:

    struct Rgba
    {
	half r;    // red
	half g;    // green
	half b;    // blue
	half a;    // alpha (opacity)
    };

    base + x * xStride + y * yStride.

    pixels + 1 * x + width * y.

The call to writePixels(), in line 3, copies the image's pixels from memory to the file. The argument to writePixels(), height, specifies how many scan lines worth of data are copied.

Finally, returning from function writeRgba1() destroys the local RgbaOutputFile object, thereby closing the file.

Why do we have to tell the writePixels() function how many scan lines we want to write? Shouldn't the RgbaOutputFile object be able to derive the number of scan lines from the data window? The IlmImf library doesn't require writing all scan lines with a single writePixels() call. Many programs want to write scan lines individually, or in small blocks. For example, rendering computer-generated images can take a significant amount of time, and many rendering programs want to store each scan line in the image file as soon as all of the pixels for that scan line are available. This way, users can look at a partial image before rendering is finished. The IlmImf library allows writing the scan lines in top-to-bottom or bottom-to-top direction. The direction is defined by the file header's line order attribute (INCREASING_Y or DECREASING_Y). By default, scan lines are written top to bottom (INCREASING_Y).

You may have noticed that in the example above, there are no explicit checks to verify that writing the file actually succeeded. If the IlmImf library detects an error, it throws a C++ exception instead of returning a C-style error code. With exceptions, error handling tends to be easier to get right than with error return values. For instance, a program that calls our writeRgba1() function can handle all possible error conditions with a single try/catch block:

    try
    {
	writeRgba1 (fileName, pixels, width, height);
    }
    catch (const std::exception &exc)
    {
	std::cerr << exc.what() << std::endl;
    }

1.2 Writing a Cropped Image

Now we are going to store a cropped image in a file. For this example, we assume that we have a frame buffer that is large enough to hold an image with width by height pixels, but only part of the frame buffer contains valid data. In the file's header, the size of the whole image is indicated by the display window, (0, 0) - (width-1, height-1), and the data window specifies the region for which valid pixel data exist. Only the pixels in the data window are stored in the file.

    void
    writeRgba2 (const char fileName[],
		const Rgba *pixels,
		int width,
		int height,
		const Box2i &dataWindow)
    {
	Box2i displayWindow (V2i (0, 0), V2i (width - 1, height - 1));
	RgbaOutputFile file (fileName, displayWindow, dataWindow, WRITE_RGBA);
	file.setFrameBuffer (pixels, 1, width);
	file.writePixels (dataWindow.max.y - dataWindow.min.y + 1);
    }

Even though we are storing only part of the image in the file, the frame buffer is still large enough to hold the whole image. In order to save memory, a smaller frame buffer could have been allocated, just big enough to hold the contents of the data window. Assuming that the pixels were still stored in contiguous scan lines, with the pixels pointer pointing to the pixel at the upper left corner of the data window, at coordinates (dataWindow.min.x, dataWindow.min.y), the arguments to the setFrameBuffer() call would have to be to be changed as follows:

    int dwWidth = dataWindow.max.x - dataWindow.min.x + 1;

    file.setFrameBuffer
	(pixels - dataWindow.min.x - dataWindow.min.y * dwWidth, 1, dwWidth);

    base + x * xStride + y * yStride

      pixels - dataWindow.min.x - dataWindow.min.y * dwWidth
	+ dataWindow.min.x * 1
	+ dataWindow.min.y * dwWidth

    = pixels -
	- dataWindow.min.x
	- dataWindow.min.y * (dataWindow.max.x - dataWindow.min.x + 1)
	+ dataWindow.min.x
	+ dataWindow.min.y * (dataWindow.max.x - dataWindow.min.x + 1)

    = pixels,

1.3 Storing Custom Attributes

Now we want to store an image in a file, and we want to add two extra data in the image file header: A string, called "comments", and a 4x4 matrix, called "cameraTransform".

    void
    writeRgba3 (const char fileName[],
		const Rgba *pixels,
		int width,
		int height,
		const char comments[],
		const M44f &cameraTransform)
    {
	Header header (width, height);
	header.insert ("comments", StringAttribute (comments));
	header.insert ("cameraTransform", M44fAttribute (cameraTransform));

	RgbaOutputFile file (fileName, header, WRITE_RGBA);
	file.setFrameBuffer (pixels, 1, width);
	file.writePixels (height);
    }

In order to make it easier to exchange data between programs written by different people, the IlmImf library defines a set of standard attributes for commonly used data, such as colorimetric information, time and place where an image was recorded, or the owner of an image file's content. For the current list of standard attributes, see the header file ImfStandardAttributes.h. The list is expected to grow over time, as OpenEXR users identify new types of data they would like to represent in a standard format. If you need to store some piece of information in an OpenEXR file header, it is probably a good idea to check if a suitable standard attribute exists, before you define a new attribute.

1.4 Reading an RGBA Image File

Reading an RGBA image is almost as easy as writing one:

    void
    readRgba1 (const char fileName[],
	       Array2D<Rgba> &pixels,
	       int &width,
	       int &height)
    {
	RgbaInputFile file (fileName);
	Box2i dw = file.dataWindow();

	width  = dw.max.x - dw.min.x + 1;
	height = dw.max.y - dw.min.y + 1;
	pixels.resizeErase (height, width);

	file.setFrameBuffer (&pixels[0][0] - dw.min.x - dw.min.y * width, 1, width);
	file.readPixels (dw.min.y, dw.max.y);
    }

After asking the RgbaInputFile object for the file's data window, we allocate a buffer for the pixels. For convenience, we use the IlmImf library's Array2D class template (the call to resizeErase() does the actual allocation). The number of scan lines in the buffer is equal to the height of the data window, and the number of pixels per scan line is equal to the width of the data window. The pixels are represented as Rgba structs.

Note that we ignore the display window; in a program that wanted to place the pixels in the data window correctly in an overall image, the display window would have to be taken into account.

Just as for writing a file, calling setFrameBuffer() tells the RgbaInputFile object how to access individual pixels in the buffer (see also section 1.2, Writing a Cropped Image).

Calling readPixels() copies the pixel data from the file into the buffer. If one or more of the R, G, B, and A channels are missing in the file, the corresponding field in the pixels is filled with an appropriate default value. The default value for R, G and B is 0.0, or black; the default value for A is 1.0, or opaque.

Finally, returning from function readRgba1() destroys the local RgbaInputFile object, thereby closing the file.

Unlike the RgbaOutputFile's writePixels() method, readPixels() has two arguments. Calling readPixels(y1,y2) copies the pixels for all scan lines with y coordinates from y1 to y2 into the frame buffer. This allows access to the the scan lines in any order. The image can be read all at once, one scan line at a time, or in small blocks of a few scan lines. It is also possible to skip parts of the image.

Note that even though random access is possible, reading the scan lines in the same order as they were written, is more efficient. Random access to the file requires seek operations, which tend to be slow. Calling the RgbaInputFile's lineOrder() method returns the order in which the scan lines in the file were written (INCREASING_Y or DECREASING_Y). If successive calls to readPixels() access the scan lines in the right order, the IlmImf library reads the file as fast as possible, without seek operations.

1.5 Reading an RGBA Image File in Chunks

The following shows how to read an RGBA image in blocks of a few scan lines. This is useful for programs that want to process high-resolution images without allocating allocating enough memory to hold the complete image. Those programs typically read a few scan lines worth of pixels into a memory buffer, process the pixels, and store them in another file. The buffer is then re-used for the next set of scan lines. Image operations like color-correction or compositing ("A over B") are very easy to do incrementally this way. With clever buffering of a few extra scan lines, incremental versions of operations that require access to neighboring pixels, like blurring or sharpening, are also possible.

    void
    readRgba2 (const char fileName[])
    {
	RgbaInputFile file (fileName);
	Box2i dw = file.dataWindow();

	int width  = dw.max.x - dw.min.x + 1;
	int height = dw.max.y - dw.min.y + 1;
	Array2D<Rgba> pixels (10, width);

	while (dw.min.y <= dw.max.y)
	{
	    file.setFrameBuffer (&pixels[0][0] - dw.min.x - dw.min.y * width,
				 1, width);

	    file.readPixels (dw.min.y, min (dw.min.y + 9, dw.max.y));
	    // processPixels (pixels)
	    
	    dw.min.y += 10;
	}
    }

1.6 Reading Custom Attributes

In section 1.3, we showed how to store custom attributes in the image file header. Here we show how to test whether a given file's header contains particular attributes, and how to read those attributes' values.

    void
    readHeader (const char fileName[])
    {
	RgbaInputFile file (fileName);

	const StringAttribute *comments =
	    file.header().findTypedAttribute <StringAttribute> ("comments");

	const M44fAttribute *cameraTransform = 
	    file.header().findTypedAttribute <M44fAttribute> ("cameraTransform");

	if (comments)
	    cout << "comments\n   " << comments->value() << endl;

	if (cameraTransform)
	    cout << "cameraTransform\n" << cameraTransform->value() << flush;
    }

In this example, we handle the possibility that the attributes we want may not exist by explicitly checking for 0 pointers. Sometimes it is more convenient to rely on exceptions instead. Function typedAttribute(), a variation of findTypedAttribute(), also searches the header for an attribute with a given name and type, but if the attribute in question does not exist, typedAttribute() throws an exception rather than returning 0.

Note that the pointers returned by findTypedAttribute() point to data that are part of the RgbaInputFile object. The pointers become invalid as soon as the RgbaInputFile object is destroyed. Therefore, the following will not work:

    void
    readComments (const char fileName[], StringAttribute *&comments)
    {
    	// error: comments pointer is invalid after this function returns
	RgbaInputFile file (fileName);
	comments = file.header().findTypedAttribute <StringAttribute> ("comments");
    }

    void
    readComments (const char fileName[], string &comments)
    {

	RgbaInputFile file (fileName);
	comments = file.header().typedAttribute<StringAttribute>("comments").value();
    }

2 Using the General Interface for Scan-line-based Files

2.1 Writing an Image File

This example demonstrates how to write an OpenEXR image file with two channels: One channel, of type HALF, is called G, and the other, of type FLOAT, is called Z. The size of the image is width by height pixels. The data for the two channels are supplied in two separate buffers, gPixels and zPixels. Within each buffer, the pixels of each scan line are contiguous in memory.

    void
    writeGZ1 (const char fileName[],
	      const half *gPixels,
	      const float *zPixels,
	      int width,
	      int height)
    {
	Header header (width, height);					  // 1
	header.channels().insert ("G", Channel (HALF));			  // 2
	header.channels().insert ("Z", Channel (FLOAT));		  // 3
    
	OutputFile file (fileName, header);				  // 4

	FrameBuffer frameBuffer;					  // 5

	frameBuffer.insert ("G",				// name   // 6
			    Slice (HALF,			// type	  // 7
				   (char *) gPixels,		// base	  // 8
				   sizeof (*gPixels) * 1,	// xStride// 9
				   sizeof (*gPixels) * width));	// yStride// 10

	frameBuffer.insert ("Z",				// name   // 11
			    Slice (FLOAT,			// type   // 12
				   (char *) zPixels,		// base   // 13
				   sizeof (*zPixels) * 1,	// xStride// 14
				   sizeof (*zPixels) * width));	// yStride// 15

	file.setFrameBuffer (frameBuffer);				  // 16
	file.writePixels (height);					  // 17
    }

Lines 2 and 3 specify the names and types of the image channels that will be stored in the file.

Constructing an OutputFile object, in line 4, opens the file with the specified name, and stores the header in the file.

Lines 5 through 16 tell the OutputFile object how the pixel data for the image channels are laid out in memory. After constructing a FrameBuffer object, a Slice is added for each of the image file's channels. A Slice describes the memory layout of one channel. The constructor for the Slice object takes four arguments, type, base, xStride, and yStride. type specifies the pixel data type (HALF, FLOAT, or UINT); the other three arguments define the memory address of pixel (x,y) as

    base + x * xStride + y * yStride.

With the values specified in our example, the IlmImf library computes the address of the G channel of pixel (x,y) like this:

    (half*)((char*)gPixels + x * sizeof(half) * 1 + y * sizeof(half) * width)
  = (half*)((char*)gPixels + x * 2 + y * 2 * width),

    (float*)((char*)zPixels + x * sizeof(float) * 1 + y * sizeof(float) * width)
  = (float*)((char*)zPixels + x * 4 + y * 4 * width).

If the image file contains a channel for which the FrameBuffer object has no corresponding Slice, then the pixels for that channel in the file are filled with zeroes. If the FrameBuffer object contains a Slice for which the file has no channel, then the Slice is ignored.

Returning from function writeGZ1() destroys the local OutputFile object, and closes the file.

2.2 Writing a Cropped Image

Writing a cropped image using the general interface is analogous to writing a cropped image using the RGBA-only interface, as shown in section 1.2: In the file's header, the data window is set explicitly, instead of being generated automatically from the image's width and height. The number of scan lines that are stored in the file is equal to the height of the data window, instead of the height of the entire image. As in section 1.2, the example code below assumes that the memory buffers for the pixels are large enough to hold width by height pixels, but only the region that corresponds to the data window will be stored in the file. For smaller memory buffers with room only for the pixels in the data window, the base, xStride and yStride arguments for the FrameBuffer object's slices would have to be adjusted accordingly (again, see section 1.2).

    void
    writeGZ2 (const char fileName[],
	      const half *gPixels,
	      const float *zPixels,
	      int width,
	      int height,
	      const Box2i &dataWindow)
    {
	Header header (width, height);
	header.dataWindow() = dataWindow;
	header.channels().insert ("G", Channel (HALF));
	header.channels().insert ("Z", Channel (FLOAT));

	OutputFile file (fileName, header);

	FrameBuffer frameBuffer;

	frameBuffer.insert ("G",				// name
			    Slice (HALF,			// type
				   (char *) gPixels,		// base
				   sizeof (*gPixels) * 1,	// xStride
				   sizeof (*gPixels) * width));	// yStride

	frameBuffer.insert ("Z",				// name
			    Slice (FLOAT,			// type
				   (char *) zPixels,		// base
				   sizeof (*zPixels) * 1,	// xStride
				   sizeof (*zPixels) * width));	// yStride

	file.setFrameBuffer (frameBuffer);
	file.writePixels (dataWindow.max.y - dataWindow.min.y + 1);
    }

2.3 Reading an Image File

In this example, we read an OpenEXR image file, using the IlmImf library's general interface. We assume that the file contains two channels, R, and G, of type HALF, and one channel, Z, of type FLOAT. If one of those channels is not present in the image file, the corresponding memory buffer for the pixels will be filled with an appropriate default value.

    void
    readGZ1 (const char fileName[],
	     Array2D<half> &rPixels,
	     Array2D<half> &gPixels,
	     Array2D<float> &zPixels,
	     int &width, int &height)
    {
	InputFile file (fileName);

	Box2i dw = file.header().dataWindow();
	width  = dw.max.x - dw.min.x + 1;
	height = dw.max.y - dw.min.y + 1;

	rPixels.resizeErase (height, width);
	gPixels.resizeErase (height, width);
	zPixels.resizeErase (height, width);

	FrameBuffer frameBuffer;

	frameBuffer.insert ("R",				  // name
			    Slice (HALF,			  // type
				   (char *) (&rPixels[0][0] -	  // base
					     dw.min.x -
					     dw.min.y * width),
				   sizeof (rPixels[0][0]) * 1,	  // xStride
				   sizeof (rPixels[0][0]) * width,// yStride
				   1, 1,			  // x/y sampling
				   0.0));			  // fillValue

	frameBuffer.insert ("G",				  // name
			    Slice (HALF,			  // type
				   (char *) (&gPixels[0][0] -	  // base
					     dw.min.x -
					     dw.min.y * width),
				   sizeof (gPixels[0][0]) * 1,	  // xStride
				   sizeof (gPixels[0][0]) * width,// yStride
				   1, 1,			  // x/y sampling
				   0.0));			  // fillValue

	frameBuffer.insert ("Z",				  // name
			    Slice (FLOAT,			  // type
				   (char *) (&zPixels[0][0] -	  // base
					     dw.min.x -
					     dw.min.y * width),
				   sizeof (zPixels[0][0]) * 1,	  // xStride
				   sizeof (zPixels[0][0]) * width,// yStride
				   1, 1,			  // x/y sampling
				   FLT_MAX));			  // fillValue

	file.setFrameBuffer (frameBuffer);
	file.readPixels (dw.min.y, dw.max.y);
    }

Using the Array2D class template, we allocate memory buffers for the image's R, G and Z channels. The buffers are big enough to hold all pixels in the file's data window.

Next, we create a FrameBuffer object, which describes our buffers to the IlmImf library. For each image channel, we add a slice to the FrameBuffer.

As usual, the slice's type, xStride, and yStride describe the corresponding buffer's layout. For the R channel, pixel (dw.min.x, dw.min.y) is at address &rPixels[0][0]. By setting the type, xStride and yStride of the corresponding Slice object as shown above, evaluating

    base + x * xStride + y * yStride

      (char*)(&rPixels[0][0] - dw.min.x - dw.min.y * width)
       + dw.min.x * sizeof (rPixels[0][0]) * 1
       + dw.min.y * sizeof (rPixels[0][0]) * width

    = (char*)&rPixels[0][0]
       - dw.min.x * sizeof (rPixels[0][0])
       - dw.min.y * sizeof (rPixels[0][0]) * width
       + dw.min.x * sizeof (rPixels[0][0])
       + dw.min.y * sizeof (rPixels[0][0]) * width

    = &rPixels[0][0]

Each Slice has a fillValue. If the image file does not contain an image channel for the Slice, then the corresponding memory buffer will be filled with the fillValue.

The Slice's remaining two parameters, xSampling and ySampling are used for images where some of the channels are subsampled, for instance YUV video data. Unless an image contains subsampled channels, xSampling and ySampling should always be set to 1. For details, see header files ImfFrameBuffer.h and ImfChannelList.h.

After describing our memory buffers' layout, we call readPixels() to copy the pixel data from the file into the buffers. Just as with the RGBA-only interface, readPixels() allows random-access to the scan lines in the file (see section 1.4, Reading an RGBA Image File).

2.4 Interleaving Image Channels in the Frame Buffer

Here is a variation of the previous example. We are reading an image file, but instead of storing each image channel in a separate memory buffer, we interleave the channels in a single buffer. The buffer is an array of structs, which are defined like this:

    typedef struct GZ
    {
	half  g;
	float z;
    };

    void
    readGZ2 (const char fileName[],
	     Array2D<GZ> &pixels,
	     int &width, int &height)
    {
	InputFile file (fileName);

	Box2i dw = file.header().dataWindow();
	width  = dw.max.x - dw.min.x + 1;
	height = dw.max.y - dw.min.y + 1;
	int dx = dw.min.x;
	int dy = dw.min.y;

	pixels.resizeErase (height, width);

	FrameBuffer frameBuffer;

	frameBuffer.insert ("G",
			    Slice (HALF,
				   (char *) &pixels[-dy][-dx].g,
				    sizeof (pixels[0][0]) * 1,
				    sizeof (pixels[0][0]) * width));

	frameBuffer.insert ("Z",
			    Slice (FLOAT,
				   (char *) &pixels[-dy][-dx].z,
				    sizeof (pixels[0][0]) * 1,
				    sizeof (pixels[0][0]) * width));

	file.setFrameBuffer (frameBuffer);
	file.readPixels (dw.min.y, dw.max.y);
    }

2.5 Which Channels are in a File?

In functions readGZ1() and readGZ2(), above, we simply assumed that the files we were trying to read contained a certain set of channels. We relied on the IlmImf library to do "something reasonable" in case our assumption was not true. Sometimes we want to know exactly what channels are in an image file before reading any pixels, so that we can do what we think is appropriate.

The file's header contains the file's channel list. Using STL-style iterators, we can iterate over the channels:

    const ChannelList &channels = file.header().channels();

    for (ChannelList::ConstIterator i = channels.begin(); i != channels.end(); ++i)
    {
	const Channel &channel = i->second;
	// ...
    }

    const ChannelList &channels = file.header().channels();
    const Channel &channel = channelList["G"];
    const Channel *channelPtr = channelList.findChannel("G");

3 Tiles, Levels and Level Modes

A single tiled OpenEXR file can hold multiple versions of an image, each with a different resolution. Each version is called a "level". A tiled file's "level mode" defines how many levels are stored in the file. There are three different level modes:

name	description
ONE_LEVEL	The file contains only a single, full-resolution level. A ONE_LEVEL image file is equivalent to a scan-line-based file; the only difference is that the pixels are accessed by tile instead of by scan line.
MIPMAP_LEVELS	The file contains multiple levels. The first level holds the image at full resolution. Each successive level is half the resolution of the previous level in x and y direction. The last level contains only a single pixel. MIPMAP_LEVELS files are used for texture-mapping and similar applications.
RIPMAP_LEVELS	Like MIPMAP_LEVELS, but with more levels. The levels include all combinations of reducing the resolution of the image by powers of two independently in x and y direction. Used for texture mapping, like MIPMAP_LEVELS; the additional levels in a RIPMAP_LEVELS file can help to accelerate anisotropic filtering during texture lookups.

In MIPMAP_LEVELS and RIPMAP_LEVELS mode, the size (width or height) of each level is computed by halving the size of the level with the next higher resolution. If the size of the higher-resolution level is odd, then the size of the lower-resolution level must be rounded up or down in order to avoid arriving at a non-integer width or height. The rounding direction is determined by the file's "level size rounding mode".

Within each level, the pixels of the image are stored in a two-dimensional array of tiles. The tiles in an OpenEXR file can be any rectangular shape, but all tiles in a file have the same size. This means that lower-resolution levels contain fewer, rather than smaller, tiles.

An OpenEXR file's level mode and rounding mode, and the size of the tiles are stored in an attribute in the file header. The value of this attribute is a TileDescription object:

    enum LevelMode
    {
	ONE_LEVEL,
	MIPMAP_LEVELS,
	RIPMAP_LEVELS
    };

    enum LevelRoundingMode
    {
	ROUND_DOWN,
	ROUND_UP
    };

    class TileDescription
    {
      public:

	unsigned int	  xSize;	// size of a tile in the x dimension
	unsigned int	  ySize;	// size of a tile in the y dimension
	LevelMode	  mode;
	LevelRoundingMode roundingMode;

	... 				// (methods omitted)
    };

4 Using the RGBA-only Interface for Tiled Files

4.1 Writing a Tiled RGBA Image File with One Resolution Level

Writing a tiled RGBA image with a single level is easy:

    void
    writeTiledRgbaONE1 (const char fileName[],
			const Rgba *pixels,
			int width, int height,
			int tileWidth, int tileHeight)
    {
	TiledRgbaOutputFile out (fileName,
				 width, height,		// image size
				 tileWidth, tileHeight,	// tile size
				 ONE_LEVEL,		// level mode
				 ROUND_DOWN,		// rounding mode
				 WRITE_RGBA);		// channels in file // 1

	out.setFrameBuffer (&pixels[0][0], 1, width);                       // 2

	for (int tileY = 0; tileY < out.numYTiles (); ++tileY)              // 3
	    for (int tileX = 0; tileX < out.numXTiles (); ++tileX)          // 4
		out.writeTile (tileX, tileY);                               // 5
    }

Opening the file, and defining the pixel data layout in memory are done in almost the same way as for scan-line-based files:

Construction of the TiledRgbaOutputFile object, in line 1, creates an OpenEXR header, sets the header's attributes, opens the file with the specified name, and stores the header in the file. The header's display window and data window are both set to (0, 0) - (width-1, height-1). The size of each tile in the file will be tileWidth by tileHeight pixels. The channel list contains four channels, R, G, B, and A, of type HALF.

Line 2 specifies how the pixel data are laid out in memory. The arithmetic involved in calculating the memory address of a specific pixel is the same as for the scan-line-based interface (see section 1.1). We assume that the pixels pointer points to an array of width*height pixels, which contains the entire image.

Lines 3 and 4 loop over all tiles within the image. The TiledRgbaOutputFile's numXTiles() method returns the number of tiles in the x direction, and similarly, the numYTiles() method returns the number of tiles in the y dimension. During these loops, line 5 writes out each tile in the image.

This simple method works well when enough memory is available to allocate a frame buffer for the entire image. When allocating a frame buffer for the whole image is not desirable, for example, because the image is very large, a smaller frame buffer can be used. Even a frame buffer that can hold only a single tile is sufficient, as demonstrated in the following example:

    void
    writeTiledRgbaONE2 (const char fileName[],
			int width, int height,
			int tileWidth, int tileHeight)
    {
	TiledRgbaOutputFile out (fileName,
				 width, height,		// image size
				 tileWidth, tileHeight,	// tile size
				 ONE_LEVEL,		// level mode
				 ROUND_DOWN,		// rounding mode
				 WRITE_RGBA);		// channels in file // 1

	Array2D<Rgba> pixels (tileHeight, tileWidth);			    // 2

	for (int tileY = 0; tileY < out.numYTiles (); ++tileY)		    // 3
	{
	    for (int tileX = 0; tileX < out.numXTiles (); ++tileX)	    // 4
	    {
		Box2i range = out.dataWindowForTile (tileX, tileY);         // 5

		generatePixels (pixels, width, height, range);		    // 6

		out.setFrameBuffer (&pixels[-range.min.y][-range.min.x],
				    1,		// xStride
				    tileWidth);	// yStride		    // 7

		out.writeTile (tileX, tileY);				    // 8
	    }
	}
    }

In line 2 we allocate a pixels array with tileWidth*tileHeight elements, which is just enough for one tile. Line 5 computes the data window range for each tile, that is, the set of pixel coordinates covered by the tile. The generatePixels() function, in line 6, fills the pixels array with one tile's worth of image data. The same pixels array is reused for all tiles. We have to call setFrameBuffer(), in line 7, before writing each tile so that the pixels in the array are accessed properly in the writeTile() call in line 8. Again, the address arithmetic to access the pixels, is the same as for scan-line-based files. The values for the base, xStride, and yStride arguments to the setFrameBuffer() call must be chosen so that evaluating the expression

    base + x * xStride + y * yStride

4.2 Writing a Tiled RGBA Image File with Mipmap Levels

In order to store a multiresolution image in a file, we can allocate a frame buffer large enough for the highest-resolution level, (0,0), and reuse it for all levels:

    void
    writeTiledRgbaMIP1 (const char fileName[],
			int width, int height,
			int tileWidth, int tileHeight)
    {
	TiledRgbaOutputFile out (fileName,
				 width, height,
				 tileWidth, tileHeight,
				 MIPMAP_LEVELS,	
				 ROUND_DOWN,
				 WRITE_RGBA);					// 1

	Array2D<Rgba> pixels (height, width);					// 2
	out.setFrameBuffer (&pixels[0][0], 1, width);				// 3

	for (int level = 0; level < out.numLevels (); ++level)			// 4
	{
	    generatePixels (pixels, width, height, level);			// 5

	    for (int tileY = 0; tileY < out.numYTiles (level); ++tileY)		// 6
		for (int tileX = 0; tileX < out.numXTiles (level); ++tileX)	// 7
		    out.writeTile (tileX, tileY, level);			// 8
	}
    }

The main difference here is the use of MIPMAP_LEVELS in line 1 for the TiledRgbaOutputFile constructor. This signifies that the file will contain multiple levels, each level being a factor of 2 smaller in both dimensions than the previous level. Mipmap images contain n levels, with level numbers

    (0,0), (1,1), ... (n-1,n-1),

    n = floor (log (max (width, height)) / log (2)) + 1

    n = ceil (log (max (width, height)) / log (2)) + 1

Line 2 allocates a pixels array with width by height pixels, big enough to hold the highest-resolution level.

In addition to looping over all tiles (lines 6 and 7), we must loop over all levels in the image (line 4). numLevels() returns the number of levels, n, in our mipmapped image. Since the tile sizes remain the same in all levels, the number of tiles in both dimensions varies between levels. numXTiles() and numYTiles() take a level number as an optional argument, and return the number of tiles in the x or y direction for the corresponding level. Line 5 fills the pixels array with appropriate data for each level.

As with ONE_LEVEL images, we can choose to only allocate a frame buffer for a single tile and reuse it for all tiles in the image:

    void
    writeTiledRgbaMIP2 (const char fileName[],
			int width, int height,
			int tileWidth, int tileHeight)
    {
	TiledRgbaOutputFile out (fileName,
				 width, height,
				 tileWidth, tileHeight,
				 MIPMAP_LEVELS,
				 ROUND_DOWN,
				 WRITE_RGBA);

	Array2D<Rgba> pixels (tileHeight, tileWidth);

	for (int level = 0; level < out.numLevels (); ++level)
	{
	    for (int tileY = 0; tileY < out.numYTiles (level); ++tileY)
	    {
		for (int tileX = 0; tileX < out.numXTiles (level); ++tileX)
		{
		    Box2i range = out.dataWindowForTile (tileX, tileY, level);

		    generatePixels (pixels, width, height, range, level);

		    out.setFrameBuffer (&pixels[-range.min.y][-range.min.x],
					1,		// xStride
					tileWidth);	// yStride

		    out.writeTile (tileX, tileY, level);
		}
	    }
	}
    }

The structure of this code is the same as for writing a ONE_LEVEL image using a tile-sized frame buffer, but we have to loop over more tiles. Also, dataWindowForTile() takes an additional level argument to determine the pixel range for the tile at the specified level.

4.3 Writing a Tiled RGBA Image File with Ripmap Levels

The ripmap level mode allows for storing all combinations of reducing the resolution of the image by powers of two independently in both dimensions. Ripmap files contains nx*ny levels, with level numbers:

    (0, 0),   (1, 0),   ... (nx-1, 0),
    (0, 1),   (1, 1),   ... (nx-1, 1),
     ...
    (0,ny-1), (1,ny-1), ... (nx-1,ny-1)

    nx = floor (log (width) / log (2)) + 1
    ny = floor (log (height) / log (2)) + 1

    nx = ceil (log (width) / log (2)) + 1
    ny = ceil (log (height) / log (2)) + 1

With a frame buffer that is large enough to hold level (0,0), we can write a ripmap file like this:

    void
    writeTiledRgbaRIP1 (const char fileName[],
			int width, int height,
			int tileWidth, int tileHeight)
    {
	TiledRgbaOutputFile out (fileName,
				 width, height,
				 tileWidth, tileHeight,
				 RIPMAP_LEVELS,
				 ROUND_DOWN,
				 WRITE_RGBA);

	Array2D<Rgba> pixels (height, width);
	out.setFrameBuffer (&pixels[0][0], 1, width);

	for (int yLevel = 0; yLevel < out.numYLevels (); ++yLevel)
	{
	    for (int xLevel = 0; xLevel < out.numXLevels (); ++xLevel)
	    {
		generatePixels (pixels, width, height, xLevel, yLevel);

		for (int tileY = 0; tileY < out.numYTiles (yLevel); ++tileY)
		    for (int tileX = 0; tileX < out.numXTiles (xLevel); ++tileX)
			out.writeTile (tileX, tileY, xLevel, yLevel);
	    }
	}
    }

As for ONE_LEVEL and MIPMAP_LEVELS files, the frame buffer doesn't have to be large enough to hold a whole level. Any frame buffer big enough to hold at least a single tile will work.

4.4 Reading a Tiled RGBA Image File

Reading a tiled RGBA image file is done similarly to writing one:

    void
    readTiledRgba1 (const char fileName[],
		    Array2D<Rgba> &pixels,
		    int &width,
		    int &height)
    {
	TiledRgbaInputFile in (fileName);
	Box2i dw = in.dataWindow();

	width  = dw.max.x - dw.min.x + 1;
	height = dw.max.y - dw.min.y + 1;
	int dx = dw.min.x;
	int dy = dw.min.y;

	pixels.resizeErase (height, width);

	in.setFrameBuffer (&pixels[-dy][-dx], 1, width);

	for (int tileY = 0; tileY < in.numYTiles(); ++tileY)
	    for (int tileX = 0; tileX < in.numXTiles(); ++tileX)
		in.readTile (tileX, tileY);
    }

This example only reads the highest-resolution level of the image. It can be extended to read all levels, for multiresolution images, by also iterating over all levels within the image, analogous to the examples in sections section 4.2 and 4.3.

5 Using the General Interface for Tiled Files

5.1 Writing a Tiled Image File

This example is a variation of the one in section 2.1. We are writing a ONE_LEVEL image file with two channels, G, and Z, of type HALF, and FLOAT respectively, but here the file is tiled instead of scan-line-based:

    void
    writeTiled1 (const char fileName[],
		 Array2D<GZ> &pixels,
		 int width, int height,
		 int tileWidth, int tileHeight)
    {
	Header header (width, height);						// 1
	header.channels().insert ("G", Channel (HALF));				// 2
	header.channels().insert ("Z", Channel (FLOAT));			// 3

	header.setTileDescription
	    (TileDescription (tileWidth, tileHeight, ONE_LEVEL));		// 4
	
	TiledOutputFile out (fileName, header);					// 5

	FrameBuffer frameBuffer;						// 6

	frameBuffer.insert ("G",				     // name	// 7
			    Slice (HALF,			     // type	// 8
				   (char *) &pixels[0][0].g,	     // base	// 9
				    sizeof (pixels[0][0]) * 1,	     // xStride	// 10
				    sizeof (pixels[0][0]) * width)); // yStride	// 11

	frameBuffer.insert ("Z",				     // name	// 12
			    Slice (FLOAT,			     // type	// 13
				   (char *) &pixels[0][0].z,	     // base	// 14
				    sizeof (pixels[0][0]) * 1,	     // xStride	// 15
				    sizeof (pixels[0][0]) * width)); // yStride	// 16

	out.setFrameBuffer (frameBuffer);					// 17

	for (int tileY = 0; tileY < out.numYTiles (); ++tileY)			// 18
	    for (int tileX = 0; tileX < out.numXTiles (); ++tileX)		// 19
		out.writeTile (tileX, tileY);					// 20
    }

5.2 Reading a Tiled Image File

Reading a tiled file with the general interface is virtually identical to reading a scan-line-based file, as shown in section 2.4; only the last three lines are different: Instead of reading all scan lines at once with a single function call, here we must iterate over all tiles we want to read.

    void
    readTiled1 (const char fileName[],
		Array2D<GZ> &pixels,
		int &width, int &height)
    {
	TiledInputFile in (fileName);

	Box2i dw = in.header().dataWindow();
	width  = dw.max.x - dw.min.x + 1;
	height = dw.max.y - dw.min.y + 1;
	int dx = dw.min.x;
	int dy = dw.min.y;

	pixels.resizeErase (height, width);

	FrameBuffer frameBuffer;

	frameBuffer.insert ("G",
			    Slice (HALF,
				   (char *) &pixels[-dy][-dx].g,
				    sizeof (pixels[0][0]) * 1,
				    sizeof (pixels[0][0]) * width));

	frameBuffer.insert ("Z",
			    Slice (FLOAT,
				   (char *) &pixels[-dy][-dx].z,
				    sizeof (pixels[0][0]) * 1,
				    sizeof (pixels[0][0]) * width));

	in.setFrameBuffer (frameBuffer);

	for (int tileY = 0; tileY < in.numYTiles(); ++tileY)
	    for (int tileX = 0; tileX < in.numXTiles(); ++tileX)
		in.readTile (tileX, tileY);
    }

    in.readTile (tileX, tileY, levelX, levelY);