mjpeg to raw rgb24 with video4linux

Question

I'm writing a c++ webcam viewer using video4linux. I need a RGB24 output (interleaved R8B8G8) for displaying. I'm able to get video input for almost all low-resolution webcam, using YUYV, GREY8 or RGB24. But I need to get input also from high-resolution webcams, that use MJPEG for compression when high framerate is needed.

I'm able to get MJPEG stream using V4L2_PIX_FMT_MJPEG as pixel format, but received framebuffer is compressed.

How can I quickly convert it to RGB24?

Can I use libjpeg for this?

Answer 1

The quickest solution I've found is decode_jpeg_raw from mjpegtools which decode jpeg data to planar YUV420. Then the conversion from yuv420 to rgb24 is done by this function:

inline int clip(int value) {
    return (value > 255) ? 255 : (value < 0) ? 0 : value;
}

static void yuv420_to_rgb24(
/* luminance (source) */const uint8_t* const y
/* u chrominance (source) */, const uint8_t* u
/* v chrominance (source) */, const uint8_t* v
/* rgb interleaved (destination) */, uint8_t* const dst
/* jpeg size */, int const size
/* image width */, int const width) {

    const int lineSize = width * 3;

    uint8_t* r1 = dst;
    uint8_t* g1 = r1 + 1;
    uint8_t* b1 = r1 + 2;

    uint8_t* r2 = r1 + lineSize;
    uint8_t* g2 = r2 + 1;
    uint8_t* b2 = r2 + 2;

    const uint8_t* y1 = y;
    const uint8_t* y2 = y + width;

    uint8_t* const end = r1 + size;

    int c1 = 0;
    int c2 = 0;
    int e = 0;
    int d = 0;

    while (r1 != end) {
        uint8_t* const lineEnd = r2;
        /* line by line */
        while (r1 != lineEnd) {
            /* first pixel */
            c1 = *y1 - 16;
            c2 = *y2 - 16;
            d = *u - 128;
            e = *v - 128;

            *r1 = clip(c1 + ((454 * e) >> 8));
            *g1 = clip(c1 - ((88 * e + 183 * d) >> 8));
            *b1 = clip(c1 + ((359 * d) >> 8));

            *r2 = clip(c2 + ((454 * e) >> 8));
            *g2 = clip(c2 - ((88 * e + 183 * d) >> 8));
            *b2 = clip(c2 + ((359 * d) >> 8));

            r1 += 3;
            g1 += 3;
            b1 += 3;

            r2 += 3;
            g2 += 3;
            b2 += 3;

            ++y1;
            ++y2;

            /* second pixel */

            c1 = *y1 - 16;
            c2 = *y2 - 16;
            d = *u - 128;
            e = *v - 128;

            *r1 = clip(c1 + ((454 * e) >> 8));
            *g1 = clip(c1 - ((88 * e + 183 * d) >> 8));
            *b1 = clip(c1 + ((359 * d) >> 8));

            *r2 = clip(c2 + ((454 * e) >> 8));
            *g2 = clip(c2 - ((88 * e + 183 * d) >> 8));
            *b2 = clip(c2 + ((359 * d) >> 8));

            r1 += 3;
            g1 += 3;
            b1 += 3;

            r2 += 3;
            g2 += 3;
            b2 += 3;

            ++y1;
            ++y2;

            ++u;
            ++v;
        }
        r1 += lineSize;
        g1 += lineSize;
        b1 += lineSize;
        r2 += lineSize;
        g2 += lineSize;
        b2 += lineSize;
        y1 += width;
        y2 += width;
    }
}

Answer 2

Yes you can use libjpeg for this, but usually the output of libjpeg is in YUV420 or YUV422. You might instead use that code: http://mxhaard.free.fr/spca50x/Download/gspcav1-20071224.tar.gz (check for decoder source, there's a small jpeg decoder that's working well and deals with color conversion directly so the output is in RGB888)