[Libav-user] JPEG YUV->RGB

Steffen Ebersbach ebersbac at hs-mittweida.de
Thu May 12 09:12:14 CEST 2011


Am 12.5.2011 09:02, schrieb Dmitriy K:
> Hi all!
>
> I use libav to decode MJPEG frames from IP-camera and to show some 
> pixels from them on a SDL-surface.
> SDL takes only RGB pixels so I have to convert YUV from MJPEG frame to 
> RGB.
> I have AVFrame after avcodec_decode_video() and as I see, there 
> data[0] stands for Y component, data[1] - V component and data[2] - U 
> component.
> Also, U and V components arrays are twice shorter, so that 1 U or V 
> component stands for 2 Y components.
> Then I use formulas found in wikipedia to convert YUV to RGB:
>
Yes it's the YUV 4:2:2 format, that is often use in JPEG.

> R=Y + 1.370705*(U - 128);
> G=Y - 0.698001*(V - 128) - 0.337633*(U - 128);
> B=Y + 1.732446*(V - 128);
>
> But in final picture I got colors mismatch in highly white and highly 
> black regions.
> For example in pixels that should be black, I have blue or red pixels, 
> and in white regions - only green.
>
> So I have two questions:
> 1) Am I right in my vision of YUV components position in AVFrame->data 
> arrays ?
> 2) Is there any way to solve problems with colors mismatch?
>
>

The formulas are right, but this is not recommend. In libav you can use 
the swscale libary to do this conversion. It's an easy way for this task.

Steffen



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