KPixmapIO Class Reference
Fast QImage to/from QPixmap conversion. More...
#include <kpixmapio.h>
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 1.15 2004/02/09 20:01:19 mlaurent Exp
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.

Performance of KPixmapIO
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 85 of file kpixmapio.h.
Member Enumeration Documentation
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Shared memory allocation policies. Definition at line 140 of file kpixmapio.h. |
Member Function Documentation
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Convert an image to a pixmap.
References putImage(). Referenced by KRootPixmap::updateBackground(). |
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Convert a pixmap to an image.
References getImage(). Referenced by KRootPixmap::updateBackground(). |
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Bitblt an image onto a pixmap.
Referenced by convertToPixmap(), and putImage(). |
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This function is identical to the one above. It only differs in the arguments it accepts. Definition at line 209 of file kpixmapio.cpp. References putImage(). |
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Transfer (a part of) a pixmap to an image.
Referenced by convertToImage(), and getImage(). |
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This function is identical to the one above. It only differs in the arguments it accepts. Definition at line 244 of file kpixmapio.cpp. References getImage(). |
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Set the shared memory allocation policy. See the introduction for KPixmapIO for a discussion.
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Pre-allocate shared memory. KPixmapIO will be able to transfer images up to this size without resizing.
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The documentation for this class was generated from the following files: