[Libav-user] Help with unpacking PIX_FMT_YUV4* data types

[Ricky Huang]

On Mar 10, 2014, at 4:28 PM, J Decker <d3ck0r at gmail.com> wrote:

> I would think you would just use 'sws_getContext' and 'sws_scale'
> worked good for me... 

Thank you for the pointer.  May I have a little more info regarding how you used sws_getContext()?  I found its API definition as follows:

struct SwsContext * 	sws_getContext (int srcW, int srcH, enum PixelFormat srcFormat, int dstW, int dstH, enum PixelFormat dstFormat, int flags, SwsFilter *srcFilter, SwsFilter *dstFilter, const double *param)

and it allocates and returns a struct SwsContext*.

In my case I am not modifying anything in my filter, why is this function good for me? (I am assuming this performs some kind of transformation because there's a source and destination).  Also in the returned SwsContext struct, I see there are 4 members:

int 	lumXInc
int 	chrXInc
int 	lumYInc
int 	chrYInc

perhaps this is where I can get the luminance?  In that case, does it mean I have to perform some kind of transformation before even able to get the Context?


Thank you for in advance for clarification.



> On Mon, Mar 10, 2014 at 4:18 PM, Ricky Huang <rhuang.work at gmail.com> wrote:
> Hello all,
> 
> I am writing code that extracts the luminance component of an input video using my own custom filter in libavfilter - specifically I am extracting it from "PIX_FMT_YUV420P" a video and I am wondering how to go about doing so.  According to the pixfmt.h header: 
>> 
>> PIX_FMT_YUV420P,   ///< planar YUV 4:2:0, 12bpp, (1 Cr & Cb sample per 2x2 Y samples)
> 
> I am not sure how to interpret that.  Assuming I am doing this in my draw_slice() function. It has the definition of:
>> 
>> static void draw_slice(AVFilterLink *inlink, int y0, int h, int slice_dir)
> 
> I know I can get to the input data plane by:
>> 
>> AVFilterBufferRef *cur_pic = link->cur_buf;
>> uint8_t *data = cur_pic->data[0];
> 
> But there are multiple "planes" in the data.  Does data[0], data[1], data[2] correspond to each of the Y, U, V channels?
> 
> Also, once I am able to point my pointer at the correct coordinate, how should I interpret the extracted result (float, int, etc)?
> 
> 
> Thank you in advance.
> 
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