How to calculate the video overlay size? (xf86-video-geode & xf86-video-ati)

[Maarten Maathuis]

On Thu, Aug 12, 2010 at 12:15 PM, Cui, Hunk <Hunk.Cui at amd.com> wrote:
> Hi, Alex,
>
>> > In xf86-video-geode:
>> >        lx_video.c -> LXCopyPlanar function, some codes make me confuse,
>> >
>> > (
>> > http://cgit.freedesktop.org/xorg/driver/xf86-video-geode/tree/src/lx_vid
>> > eo.c#n224 )
>> >    YSrcPitch = (width + 3) & ~3;
>> >    YDstPitch = (width + 31) & ~31;
>> >
>> >    UVSrcPitch = ((width >> 1) + 3) & ~3;
>> >    UVDstPitch = ((width >> 1) + 15) & ~15;
>> >
>> >    USrcOffset = YSrcPitch * height;
>> >    VSrcOffset = USrcOffset + (UVSrcPitch * (height >> 1));
>> >
>> >    UDstOffset = YDstPitch * height;
>> >    VDstOffset = UDstOffset + (UVDstPitch * (height >> 1));
>> >
>> >    size = YDstPitch * height;
>> >    size += UVDstPitch * height;
>> >
>> > What is the formula for reference?
>>
>> See http://fourcc.org/
>>
>> > How to define and calculate the YDstPitch and UVDstPitch?
>>
>> Planar formats store the YUV data in separate planes rather than
>> packed as pixel tuples.  Depending on the format the UV portion has
>> half the resolution as the Y component.
>
>        Could you give me some more example to explain this? Confuse about "UVDstPitch = ((width >> 1) + 15) & ~15;"
>        Why the UV portion need to the half of the resolution as the Y portion?

That's because (some) YUV formats have 1 U and 1 V pixel per 4 Y
pixels, it's a way to save space. The grayscale is the most important
part, and the colors are lower resolution (this gives an effective 12
bits per pixel).

>
> Thanks,
> Hunk Cui
>
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