extract YCbCr values from a jpeg-image

[Wilbert]

Hi there!

I'm using MagickCore API to read an image and extract pixels. I would like to know to know if it is possible to extract the YCbCr values from a jpeg image for example (without any conversion to RGB in between).

I'm using the following code to extract pixels:

Code: Select all

void main(int argc, char *argv[])
	
{
    ExceptionInfo *exception;
    Image *image, *images;
    ImageInfo *image_info;
    int z0,z1,z2,z3;

	MagickCoreGenesis(argv[1], MagickTrue);
	image_info = CloneImageInfo((ImageInfo *) NULL);
	exception = AcquireExceptionInfo();
	
	(void) strcpy(image_info->filename, "red.jpg");
	image_info->colorspace = YCbCrColorspace;
	images = ReadImage(image_info, exception);
	
	images = CoalesceImages(images, exception);
	image = GetImageFromList(images, 0);
	
	register const PixelPacket *p;
	
	for (int y=0; y < 480; y++) {
		p = AcquireImagePixels(image, 0, 480-1-y, 640, 1, exception);
		for (int x=0; x < 480; x++) {
			z0=p->blue;
			z1=p->green;
			z2=p->red;
			z3=255-p->opacity;
char BUF[256];
sprintf(BUF, "blue %d, green %d, red %d\n", z0, z1, z2);
OutputDebugString(BUF);
			p++;
		}
	}

    DestroyImage(image);
    DestroyImageInfo(image_info);
    DestroyExceptionInfo(exception);
    MagickCoreTerminus();
}

But as I understand it, PixelPacket supports only RGBA and CMYK. Is there some other way to get the YCbCr values (if the image is stored as YCbCr)?

[magick]

Before you read your JPEG image, set the image_info->colorspace member to YCbCrColorspace.

[Wilbert]

Before you read your JPEG image, set the image_info->colorspace member to YCbCrColorspace.

I can't get it to work. I replaced my code above with something compilable and i added the 'image_info->colorspace = YCbCrColorspace' line before reading the image.

If i extract the blue, green and red values with PixelPacket i get B=G=0, R=254. But I expected the YCbCr values, thus something like G=76, B=85, R=254.

[magick]

See if it works from the command line. We're using ImageMagick 6.5.6-1:

• convert logo: logo.jpg
  convert -colorspace ycbcr logo.jpg logo.miff
  identify -verbose logo.miff

We get a colorspace of YCbCr:

• -> identify -verbose logo.miff
  Image: logo.miff
  Format: MIFF (Magick Image File Format)
  Class: DirectClass
  Geometry: 640x480+0+0
  Resolution: 72x72
  Print size: 8.88889x6.66667
  Units: PixelsPerInch
  Type: TrueColor
  Base type: TrueColor
  Endianess: Undefined
  Colorspace: YCbCr
  ...

as expected.

[Wilbert]

Thanks for the fast response!

I get the following

Code: Select all

-> identify -verbose red.miff
Image: red.miff
Format: MIFF (Magick Image File Format)
Class: DirectClass
Geometry: 640x480+0+0
Resolution: 72x72
Print size: 8.88889x6.66667
Units: PixelsPerInch
Type: Palette
Base type: TrueColor
Endianess: Undefined
Colorspace: YCbCr
Channel depth:
  red: 7-bit
  green: 2-bit
  blue: 1-bit
...

The Type is different. Is that a problem? (I'm using 6.5.5.)

edit:

after '(void) strcpy(image_info->filename, "red.jpg")':

image_info->colorspace returns 0

after 'image_info->colorspace = YCbCrColorspace;' :

image_info->colorspace returns 7

edit2: the value of image->colorspace is 1 and doesn't change when adding the 'image_info->colorspace = YCbCrColorspace' line.

[magick]

We applied a patch to ImageMagick 6.5.6-2 Beta to fix the problem you reported. The patch will be available by sometime tomorrow. Thanks.

[Wilbert]

It seems to work fine in the latest build! I'm wondering about one thing though. How can i see in which format the picture is stored?

If i use this code:

Code: Select all

   MagickCoreGenesis(argv[1], MagickTrue);
   image_info = CloneImageInfo((ImageInfo *) NULL);
   exception = AcquireExceptionInfo();
   
   (void) strcpy(image_info->filename, "red.jpg");
   images = ReadImage(image_info, exception);
   
   images = CoalesceImages(images, exception);
   image = GetImageFromList(images, 0);
   sprintf(BUF, "color space %d\n", image->colorspace);

Then the colorspace is 1 (RGB), although i know that for jpeg it's always YCbCr or Y. But for other formats i have no idea in which color format it is stored.

[tobeFrank]

Hi there!

I successfully created a ycbcr colospace .miff file using this:

• convert -colorspace ycbcr logo.jpg logo.miff
  identify -verbose logo.miff

but now, how can I convert it to a ycbcr colospace .jpg?

Hope the question is not too stupid but I am totally new with image editing...

Thanks a lot for the help!!!