CODEWITHSUNDEEP
rgbmultimediayuvlibav
user2063909
user2063909

Reputation: 21

Encode rgb to yuv420p using libav

I'm trying to convert an vector of RGB image data (derived from a .png image) to YUV420p format using libav.

In the libav sample code the following is used to create a dummy image:

/* prepare a dummy image */
static void fill_yuv_image(AVFrame *pict, int frame_index, int width, int height)
{
    int x, y, i;

    i = frame_index;

    /* Y */
    for(y=0;y<height;y++) {
        for(x=0;x<width;x++) {
            pict->data[0][y * pict->linesize[0] + x] = x + y + i * 3;
        }
    }

    /* Cb and Cr */
    for(y=0;y<height/2;y++) {
        for(x=0;x<width/2;x++) {
            pict->data[1][y * pict->linesize[1] + x] = 128 + y + i * 2;
            pict->data[2][y * pict->linesize[2] + x] = 64 + x + i * 5;
        }
    }
}

I'm not clear about a few things here:

Firstly, do I need to rearrange the RGB data in the input vector so that it's suitable for encoding as YUV420p?

Secondly, I understand that there's a Y value for every pixel and that the Cb and Cr values are used for four (2x2) pixels. What I don't understand is how the RGB data gets "reduced" to the Cb and Cr values - is there an example of how to do this anywhere?

Upvotes: 2

Views: 1312

Answers (2)

Андрей Тернити
Андрей Тернити

Reputation: 358

Convert vector of RGB image data (derived from a .png image) to YUV420p format using libav example.

Upvotes: -1

Multimedia Mike
Multimedia Mike

Reputation: 13256

I'm not entirely sure what you're trying to achieve exactly, so I'll just directly answer your questions as best I can (feel free to follow up with clarifying comments):

1) You will be transforming the RGB data to YUV which will involve some rearrangement. The packed RGB data is fine where it is. You don't really need to adjust it. Actually, it would probably be better to leave it packed the way it is for cache locality reasons.

2) As you already understand, YUV 4:2:0 encodes a Y sample for each pixel but each 2x2 block shares a Cb and a Cr value. However, there is also YUV 4:4:4 data. This is where each pixel gets its own Y, Cb, and Cr sample. A simple strategy for converting RGB -> YUV 4:2:0 is to convert RGB -> YUV 4:4:4 and then average (arithmetic mean) each block of 2x2 Cb samples. There are other algorithms (like filters that involve more of the surrounding samples), but this should work if you're just experimenting with how this stuff works.

Another strategy for experimentation (and speed) is to only compute the Y plane and hold the Cb and Cr planes constant at 128. That will result in a grayscale image.

For real work, you would probably want to leverage the built-in conversion facilities that libav has to offer.

Upvotes: 0

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