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KPixmapIO Class Reference

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

#include <kpixmapio.h>

List of all members.

Public Types
enum	ShmPolicies
Public Methods
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)

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Detailed Description

Fast QImage to/from QPixmap conversion.

Author:

Geert Jansen <jansen@kde.org>

Version:

Id:

kpixmapio.h,v 1.8.2.2 2003/06/05 23:42:37 mueller 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.

@sect 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.

@sect 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.

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.

@sect 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.

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Member Enumeration Documentation

enum KPixmapIO::ShmPolicies

Shared memory allocation policies. Definition at line 141 of file kpixmapio.h.

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Member Function Documentation

QImage KPixmapIO::convertToImage (  const QPixmap &    pixmap )

Convert a pixmap to an image. Parameters: pixmap  The pixmap to convert. Returns: The image.

QPixmap KPixmapIO::convertToPixmap (  const QImage &    image )

Convert an image to a pixmap. Parameters: image  The image to convert. Returns: The pixmap containing the image.

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.

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.

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.

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.

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.

void KPixmapIO::setShmPolicy (  int    policy )

Set the shared memory allocation policy. See the introduction for KPixmapIO for a discussion. Parameters: policy  The alloction policy.

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The documentation for this class was generated from the following file:

• kpixmapio.h

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