kdeui Library API Documentation

KPixmapIO Class Reference

Fast QImage to/from QPixmap conversion. More...

#include <kpixmapio.h>

List of all members.

Public Types

enum  ShmPolicies { ShmDontKeep, ShmKeepAndGrow }

Public Member Functions

QPixmap convertToPixmap (const QImage &image)
QImage convertToImage (const QPixmap &pixmap)
void putImage (QPixmap *dst, int dx, int dy, const QImage *src)
void putImage (QPixmap *dst, const QPoint &offset, const QImage *src)
QImage getImage (const QPixmap *src, int sx, int sy, int sw, int sh)
QImage getImage (const QPixmap *src, const QRect &rect)
void setShmPolicy (int policy)
void preAllocShm (int size)


Detailed Description

Fast QImage to/from QPixmap conversion.

Author:
Geert Jansen <jansen@kde.org>
Version:
Id
kpixmapio.h,v 1.14 2003/08/19 10:00:20 coolo Exp
KPixmapIO implements a fast path for QPixmap to/from QImage conversions. It uses the MIT-SHM shared memory extension for this. If this extension is not available, it will fall back to standard Qt methods.

Typical usage:

You can use KPixmapIO for load/saving pixmaps.

KPixmapIO io; pixmap = io.convertToPixmap(image); image = io.convertToImage(pixmap);

It also has functionality for partially updating/saving pixmaps, see putImage and getImage.

KPixmapIO vs. Qt speed comparison

Speed measurements were taken. These show that usage of KPixmapIO for images up to a certain threshold size, offers no speed advantage over the Qt routines. Below you can see a plot of these measurements.

kpixmapio-perf.png

Performance of KPixmapIO

The threshold size, amongst other causes, is determined by the shared memory allocation policy. If the policy is ShmDontKeep, the shared memory segment is discarded right after usage, and thus needs to be allocated before each transfer. This introduces a a setup penalty not present when the policy is ShmKeepAndGrow. In this case the shared memory segment is kept and resized when necessary, until the KPixmapIO object is destroyed.

The default policy is ShmDontKeep. This policy makes sense when loading pixmaps once. The corresponding threshold is taken at 5.000 pixels as suggested by experiments. Below this threshold, KPixmapIO will not use shared memory and fall back on the Qt routines.

When the policy is ShmKeepAndGrow, the threshold is taken at 2.000 pixels. Using this policy, you might want to use preAllocShm to pre-allocate a certain amount of shared memory, in order to avoid resizes. This allocation policy makes sense in a multimedia type application where you are constantly updating the screen.

Above a couple times the threshold size, KPixmapIO's and Qt's speed become linear in the number of pixels, KPixmapIO being at least 2, and mostly around 4 times faster than Qt, depending on the screen and image depth.

Speed difference seems to be the most at 16 bpp, followed by 32 and 24 bpp respectively. This can be explained by the relatively poor implementation of 16 bit RGB packing in Qt, while at 32 bpp we need to transfer more data, and thus gain more, than at 24 bpp.

Conclusion:

For large pixmaps, there's a definite speed improvement when using KPixmapIO. On the other hand, there's no speed improvement for small pixmaps. When you know you're only transferring small pixmaps, there's no point in using it.

Definition at line 86 of file kpixmapio.h.


Member Enumeration Documentation

enum KPixmapIO::ShmPolicies
 

Shared memory allocation policies.

Definition at line 141 of file kpixmapio.h.


Member Function Documentation

QPixmap KPixmapIO::convertToPixmap const QImage image  ) 
 

Convert an image to a pixmap.

Parameters:
image The image to convert.
Returns:
The pixmap containing the image.

Definition at line 180 of file kpixmapio.cpp.

References QPixmap::convertFromImage(), QImage::depth(), QImage::height(), putImage(), and QImage::width().

Referenced by KRootPixmap::updateBackground().

QImage KPixmapIO::convertToImage const QPixmap pixmap  ) 
 

Convert a pixmap to an image.

Parameters:
pixmap The pixmap to convert.
Returns:
The image.

Definition at line 198 of file kpixmapio.cpp.

References QPixmap::convertToImage(), getImage(), QPixmap::height(), and QPixmap::width().

Referenced by KRootPixmap::updateBackground().

void KPixmapIO::putImage QPixmap dst,
int  dx,
int  dy,
const QImage src
 

Bitblt an image onto a pixmap.

Parameters:
dst The destination pixmap.
dx Destination x offset.
dy Destination y offset.
src The image to load.

Definition at line 217 of file kpixmapio.cpp.

References QPixmap::convertFromImage(), QImage::depth(), QPixmap::height(), QImage::height(), QPixmap::width(), and QImage::width().

Referenced by convertToPixmap(), and putImage().

void KPixmapIO::putImage QPixmap dst,
const QPoint offset,
const QImage src
 

This function is identical to the one above.

It only differs in the arguments it accepts.

Definition at line 210 of file kpixmapio.cpp.

References putImage(), QPoint::x(), and QPoint::y().

QImage KPixmapIO::getImage const QPixmap src,
int  sx,
int  sy,
int  sw,
int  sh
 

Transfer (a part of) a pixmap to an image.

Parameters:
src The source pixmap.
sx Source x offset.
sy Source y offset.
sw Source width.
sh Source height.
Returns:
The image.

Definition at line 251 of file kpixmapio.cpp.

References QPixmap::convertToImage(), QPixmap::height(), and QPixmap::width().

Referenced by convertToImage(), and getImage().

QImage KPixmapIO::getImage const QPixmap src,
const QRect rect
 

This function is identical to the one above.

It only differs in the arguments it accepts.

Definition at line 245 of file kpixmapio.cpp.

References getImage(), QRect::height(), QRect::width(), QRect::x(), and QRect::y().

void KPixmapIO::setShmPolicy int  policy  ) 
 

Set the shared memory allocation policy.

See the introduction for KPixmapIO for a discussion.

Parameters:
policy The alloction policy.

Definition at line 891 of file kpixmapio.cpp.

void KPixmapIO::preAllocShm int  size  ) 
 

Pre-allocate shared memory.

KPixmapIO will be able to transfer images up to this size without resizing.

Parameters:
size The size of the image in pixels.

Definition at line 890 of file kpixmapio.cpp.


The documentation for this class was generated from the following files:
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This file is part of the documentation for kdeui Library Version 3.2.2.
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