Setting individual pixels of an RGB frame for ffmpeg encoding - c++

Question

I'm trying to change the test pattern of an ffmpeg streamer, Trouble syncing libavformat/ffmpeg with x264 and RTP , into familiar RGB format. My broader goal is to compute frames of a streamed video on the fly.
So I replaced its AV_PIX_FMT_MONOWHITE with AV_PIX_FMT_RGB24, which is "packed RGB 8:8:8, 24bpp, RGBRGB..." according to http://libav.org/doxygen/master/pixfmt_8h.html .
To stuff its pixel array called data, I've tried many variations on
for (int y=0; y<HEIGHT; ++y) {
for (int x=0; x<WIDTH; ++x) {
uint8_t* rgb = data + ((y*WIDTH + x) *3);
const double i = x/double(WIDTH);
// const double j = y/double(HEIGHT);
rgb[0] = 255*i;
rgb[1] = 0;
rgb[2] = 255*(1-i);
}
}
At HEIGHTxWIDTH= 80x60, this version yields
, when I expect a single blue-to-red horizontal gradient.
640x480 yields the same 4-column pattern, but with far more horizontal stripes.
640x640, 160x160, etc, yield three columns, cyan-ish / magenta-ish / yellow-ish, with the same kind of horizontal stripiness.
Vertical gradients behave even more weirdly.
Appearance was unaffected by an AV_PIX_FMT_RGBA attempt (4 not 3 bytes per pixel, alpha=255). Also unaffected by a port from C to C++.
The argument srcStrides passed to sws_scale() is a length-1 array, containing the single int HEIGHT.
Access each Pixel of AVFrame asks the same question in less detail, so far unanswered.
The streamer emits one warning, which I doubt affects appearance:
[rtp # 0x269c0a0] Encoder did not produce proper pts, making some up.
So. How do you set the RGB value of a pixel in a frame to be sent to sws_scale() (and then to x264_encoder_encode() and av_interleaved_write_frame())?

Answer 1

Use avpicture_fill() as described in Encoding a screenshot into a video using FFMPEG .
Instead of passing data directly to sws_scale(), do this:
AVFrame* pic = avcodec_alloc_frame();
avpicture_fill((AVPicture *)pic, data, AV_PIX_FMT_RGB24, WIDTH, HEIGHT);
and then replace the 2nd and 3rd args of sws_scale() with
pic->data, pic->linesize,
Then the gradients above work properly, at many resolutions.

Answer 2

The argument srcStrides passed to sws_scale() is a length-1 array, containing the single int HEIGHT.
Stride (AKA linesize) is the distance in bytes between two lines. For various reasons having mostly to do with optimization it is often larger than simply width in bytes, so there is padding on the end of each line.
In your case, without any padding, stride should be width * 3.