[maemo-developers] [maemo-developers] SDL vertical blank (vbl) sync.

From: Visti Andresen talpa at galnet.dk
Date: Sun May 28 18:40:38 EEST 2006
On Sun, 28 May 2006 14:47:29 +0200
hcz at hczim.de (Heike C. Zimmerer) wrote:

> Visti Andresen <talpa at galnet.dk> writes:
> > Hi I was wondering if anyone knows how to sync. the SDL_Flip to the
> > vertical blanking of the display.
> You're sure there's anything like vertical blanking for the 770?  My
> understanding is that VBL is closely tied to the transmission of video
> data through an analog line.

Sort of, vertical blanking occurs when the electron beam retraces back to the top of the CRT screen.
And yes it has a dedicated analog line (on a vga/monitor cable, it is shared with the luminance/colour data on a composite signal).

> I doubt you will find anything like that
> in digital-only LCD circuitry.  There will, however, be some kind of
> display refresh which uses some timing of its own.  If you look that
> direction, maybe there's some interval which can be used for similar
> tasks.
As you mention internal TFT/LCD displays use different and usually digital (until the LCD/TFT controller chip) signalling.
But the controller chip still receives a "top of frame" or "vertical blank" signal (at least in those data sheets I have read for LCD displays).

Anyway I would like to sync the SDL_Flip's to the "top of frame" or "vertical blank" signal.

The reason for this wish is, that you really can't do anything animated if you refresh the display at a random point in time.
Okay you can, but there will be artifacts, these are caused by the top part of the display containing one frame and the bottom one containing the next frame.

For those not familiar with the problem I have attached two tiny programs (compiled for arm)

flashingTest. Displays some "random" colours, I wrote it to "benchmark" the graphics performance of the 770 and to detect the native screen format (its 16bit R5G6B5 btw.). It do however show the artifacts.

vblSync. Was written especially to show the artifacts. The high DPI of the 770 display makes them less noticeable but they are still there.

Oh and for those of you who still have the IceAge2 trailer that came with the 770, the Nokia movie player also shows these artifacts quite clearly in the beginning where there are a lot of panning going on.

I can just hope that there are a "hacker" out there who knows how get this kind of "scanline" information out of the 770. Otherwise I'll just have to live with sub Amiga 500 graphics capabilities :)

> - Heike
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