class KPixmapIO

Public Types

• enum ShmPolicies { ShmDontKeep, ShmKeepAndGrow }
• enum ByteOrders { bo32_ARGB, bo32_BGRA, bo24_RGB, bo24_BGR, bo16_RGB_565, bo16_BGR_565, bo16_RGB_555, bo16_BGR_555, bo8 }

Public Methods

•  KPixmapIO ()
•  ~KPixmapIO ()
• 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)
• void  initXImage (int w, int h)
• void  doneXImage ()
• void  createXImage (int w, int h)
• void  destroyXImage ()
• void  createShmSegment (int size)
• void  destroyShmSegment ()
• void  convertToXImage (const QImage &)
• QImage  convertFromXImage ()

Public Members

• bool m_bShm

Detailed Description

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.

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.

Convert an image to a pixmap.

Parameters:

image

The image to convert.

Returns: The pixmap containing the image.

Convert a pixmap to an image.

Parameters:

pixmap

The pixmap to convert.

Returns: The image.

Bitblt an image onto a pixmap.

Parameters:

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

This function is identical to the one above. It only differs in the arguments it accepts.

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.

This function is identical to the one above. It only differs in the arguments it accepts.

Shared memory allocation policies.

Set the shared memory allocation policy. See the introduction for KPixmapIO for a discussion.

Parameters:

policy

The alloction policy.

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.

• Version: $Id: KPixmapIO.html 149726 2002-04-16 07:39:43Z dfaure $
• Author: Geert Jansen
• Generated: dfaure on faure on Tue Apr 16 08:49:25 2002, using kdoc 2.0a53.