CODEWITHSUNDEEP
cimagev4l2
user1505520
user1505520

Reputation: 415

Converting from YUY2 to RGB24 from V4L2 API

I am trying to use the V4L2 API to capture images and put the images into an opencv Mat. The problem is my webcam only captures in YUYV (YUY2) So I need to convert to RGB24 first. Here is the complete V4L2 code that I am using.

I was able to get objects in the picture to be recognizable, but it is all pink and green, and it is stretched horizontally and distorted. I have tried many different conversion formulas and I have had the same basic pink/green distorted image. The formula used for this picture is from http://paulbourke.net/dataformats/yuv/. I am using the shotwell photo viewer on linux to view the .raw image. I couldn't get gimp to open it. I am not that knowledgable with how to save image formats, but I am assuming there has to be some kind of header but the shotwell photo viewer seemed to work. Could this possibly be reason for the incorrect image?

I am not sure if V4l2 is returning a signed or unsigned byte image which is pointed to by p. But if this were the problem woudln't my image would just be off-color? But it seems the geometry is distorted too. I believe I took care of the casting to and from floating point properly.

Could someone help me understand

  1. how to find out the underlying type contained in the *void p variable
  2. the proper formula for converting from YUYV to RGB24 including explanations of which types to use
  3. could saving the image with no format (headers) and viewing with Shotwell be the problem?
  4. is there an easy way to save an RGB24 image properly.
  5. general debugging tips

Thanks

static unsigned char   *bgr_image;

static void process_image(void *p, int size)
{
    frame_number++;
    char filename[15];
    sprintf(filename, "frame-%d.raw", frame_number);
    FILE *fp=fopen(filename,"wb");

    int i;
    float y1, y2, u, v;
    char * bgr_p = bgr_image;
    unsigned char * p_tmp = (unsigned char *) p;

    for (i=0; i < size; i+=4) {

      y1 = p_tmp[i];
      u = p_tmp[i+1];
      y2 = p_tmp[i+2];
      v = p_tmp[i+3];

      bgr_p[0] = (y1 + 1.371*(u - 128.0));
      bgr_p[1] = (y1 - 0.698*(u - 128.0) - 0.336*(v - 128.0));
      bgr_p[2] = (y1 + 1.732*(v - 128.0));
      bgr_p[3] = (y2 + 1.371*(v - 128.0));
      bgr_p[4] = (y2 - 0.698*(v - 128.0) - 0.336*(u - 128.0));
      bgr_p[5] = (y2 + 1.732*(u - 128.0));

      bgr_p+=6;
    }

    fwrite(bgr_image, size, 1, fp);                                       
    fflush(fp);
    fclose(fp);
}

Upvotes: 3

Views: 4799

Answers (2)

uml&#228;ute
uml&#228;ute

Reputation: 31284

do not try to re-invent the wheel. lots of people have written colorspace-converters and chances are high that your implementation (even if it works) is not the "optimal" one (e.g. being slower than necessary).

the canonical way to deal with V4L2 devices of any colourspace is to use the libv4l-library, which will transparently convert the cameras native colorspace to once of BGR24, RGB24 and YUV420 (if you desire that, which i think is true).

as for saving the image, again use what is already there. personally, i would use imagemagick to save a frame in a "proper" format that can be read by any imageviewer (png or tiff, if quality matters)

Upvotes: 1

Ulterior
Ulterior

Reputation: 2902

First, you must understand with what type of YUV422 you are working. 

PIX_FMT_YUYV422,   ///< packed YUV 4:2:2, 16bpp, Y0 Cb Y1 Cr
PIX_FMT_UYVY422,   ///< packed YUV 4:2:2, 16bpp, Cb Y0 Cr Y1

Try replacing y1, u, y2, and v accordingly, but you maybe be not dealing with YUV422 at all, the picture could be a planar, instead of a packed format you are expecting?

I think its better for you to download IrfanViewer, which has a raw yuv file open functionality and try picking the correct values to have a correctly decoded image to find what type of data you are using.

Upvotes: 2

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