Convert CMYK image to RGB image

[bogho]

Hi!

I'm want to convert a CMYK image to a RGB image using VisualC++6.0 and CMM.

The code looks like this:

Input :
inputPixels - a structure with the pixels from the image
profile1 - char* the name for the CMYK file profile
profile2 - char* the name for the RGB file profile

Output :
outputPixels- the output pixels


HPROFILE inputProfile, outputProfile;
HTRANSFORM transform;
BOOL result;
int nPixels = columns * rows;

inputProfile = OpenColorProfileFromFile(profile1);
outputProfile = OpenColorProfileFromFile(profile2);

PHPROFILE pProf = (PHPROFILE)malloc(sizeof(HPROFILE)*2);
pProf[0] = inputProfile;
pProf[1] = outputProfile;

PDWORD intent = (PDWORD)malloc(sizeof(DWORD));
intent[0] = INTENT_SATURATION;
transform = CreateMultiProfileTransform(pProf,2, intent,1, BEST_MODE, 2);
if (transform == NULL)
{
ShowError(GetLastError());
}

outputPixels = (PixelStruct*)malloc(sizeof(PixelStruct)*nPixels);

result = TranslateBitmapBits(transform, inputPixels, BM_KYMCQUADS, columns, rows, 0, outputPixels, BM_xBGRQUADS,0,NULL,0);

if (!result)
{
ShowError(GetLastError());
return NULL;
}

return outputPixels;
}

This code show the image, but not quite good.
Those anyone know how to do that.

P.S. Using the profile1 and profile2 files, in Photoshop the result image is correct.

[yiannakop]

My answer won't be code based, cause I don't know how to directly display a CMYK image. But I know that to convert an CMYK image to RGB values you just do the following


CMY2RGB:
--> RED = 1-C
--> GREEN = 1-M
--> BLUE = 1-Y ,

since the RGB ans CMY color spaces are complements. Before doing that though, you have to convert CMYK to CMY by adding the "black" (K) value to the CMY values. The black value is the minimum of the other 3 values:


CMYK2CMY
Cyan = minimum(1,Cyan*(1-Black)+Black)
Magenta = minimum(1,Magenta*(1-Black)+Black)
Yellow = minimum(1,Yellow*(1-Black)+Black)

Hope this helps.

[yiannakop]

I also have found the following link with a demo of such conversion:
http://www.codeproject.com/tools/xcmyk.asp

[softmuthu84]

Give rgb value
r=
g=
b=
double R, G, B;
R = (double) r;
G = (double) g;
B = (double) b;

R = 1.0 - (R / 255.0);
G = 1.0 - (G / 255.0);
B = 1.0 - (B / 255.0);

double C, M, Y, K;
if (R < G)
K = R;
else
K = G;
if (B < K)
K = B;

C = (R - K)/(1.0 - K);
M = (G - K)/(1.0 - K);
Y = (B - K)/(1.0 - K);

C = (C * 100) + 0.5;
M = (M * 100) + 0.5;
Y = (Y * 100) + 0.5;
K = (K * 100) + 0.5;

[gratzielu]

No he wants to convert the image using ICC profiles color conversion. I have the same problem when I convert from CMYK to RGB. My code is similar to yours. The result is not what I expect. If you find the cause to the problem please post it.