How Qt writes to the frame buffer

[Shiva]

Hi All,

I have been using Qt with Linux for last one month. I have also managed to develop quite a few applications. Now the thing I don't know is how Qt writes to the frame buffer. What is the refreshing rate at which the Qt writes to the FB. This information is required to do a comparison study between Qt and other UI framewroks. So please let me know about any details or documents available for this.

Regards,
Siva

[ardvark]

It only draws to the frame buffer as needed. If your code does qlabel->setText("updated"); and the text was not 'updated' previous and the widget and all parents have called show(), then it will draw to the frame buffer. After that, the frame buffer memory refreshes the screen. You can also 'batch' updates. This might make sense to avoid flicker if you are drawing a complicated scene.

When you call 'qlabel->setText('newer text'), then the code goes and updates the frame buffer. You should be careful in comparing graphic frame works. Some are object oriented others just do blitting operations. They operate at different levels. Most 'object oriented' (have buttons, menus, lists, etc) also have the primitives to draw lines, display text, etc. However, they also have a framework to react and update the screen according to mouse and key events. WindML, Zinc, Peg, NanoX, etc all have different features. Having graphics acceleration to draw lines, text, fill areas, etc will increase the frame rates possible with some of these systems.

[DavidC]

I also has similar question. Test Qt4.5 on ARM Linux. I used painter to draw a sin wave with 600 points, but found refresh rate has only about 5~7Hz, cannot be higher. Then I try QWT curvdemo2 example (>300 data points) on ARM linux for comparison and found refresh rate also very slow, is it normal? How to increase refresh rate with same data points? Does QT provide class to directly write to framebuffer?

[ardvark]

Does QT provide class to directly write to framebuffer?

Follow these instructions,
http://doc.qt.nokia.com/4.7/qt-perfo...-backing-store

You should compare Qt to direct writes to the frame buffer. Your linux frame buffer driver might not be high performance. Some fb drivers may transfer data across and SPI bus, etc. You can open '/dev/fb0', mmap the file and run a standard memory benchmarking algorithm against that memory.

Other possibilities are to use 'mplayer -benchmark' with an mjpeg video of you sine wave. I have an ARM 926 running at about 200MHz and video rates are approx. 30fps with the 'demo/embedded/raycasting' example in the Qt directory. You should also try this demo and see if it gets better frame rates. If so, then your Qt programming could be improved by looking at the source for the raycasting demo.

[bhamadicharef]

raycasting at 30fps ... at what resolution ? On my Gumstix Overo (OMAP3530) at 500mHz
I get 4fps at 1024x768, 10fps at 640x480 and 30fps+ only at much smaller resolution.
What I dont understand is that even without moving into the maze using the key ... the
program eats lots of CPU ... if there are no actions from the user ... should not eat so
much CPU ... is the refreshment need so much ! (note that my system has no SGX
acceleration yet)

Regards

Brahim

[Sriraj]

Hi All,

I have been using Qt with Linux for last one month. I have also managed to develop quite a few applications. Now the thing I don't know is how Qt writes to the frame buffer. What is the refreshing rate at which the Qt writes to the FB. This information is required to do a comparison study between Qt and other UI framewroks. So please let me know about any details or documents available for this.

Regards,
Siva

Hi All,

I have also been been using Qt with Linux for last 6 months, i want to capture all the output(RGB) data and need to send server, if i got to know how qt writes data into frame buffer then its easy to capture those data, so please let me konw the display mechanisum of qt.

Thanks & Regards,
Sriraj

[wysota]

Which Qt version, what platform?