Thanks you so much for the reply Marti wrote: > > > Hi, > > Alex, I don't fully undestand all the math you are using here. > I think it would be better to separate things in order to make > it clear. Assuming you want to change the image white point > the steps would be: Yeah .. I don't really understand 100% either, but yes I'm basiclly alter the white point. (Colortemp?) So the white reference of different white point can be calculated by using colortemp to XY and XY to XYZ (assume Y=1). Is that ok? > > a) Convert image to XYZ space. This can be done in several > ways. If the image is in sRGB then you could use the > gamma/matrix method, but not all images are in sRGB space. > Right coverting image to xyz for me is also a difficult choice there are so many difficult matrix I can use but I pick the Bradford...( I forget why already :) I remember something something related to 2 and 10 degree observation point So let say I get a digitial capture jpg output what should I do to convert that RGB to XYZ? > > b) If white points are close enough, a simple scale would > suffice. Somethink like: > > XYZout = XYZin * (WPout/WPin) > I think this is what I'm trying to do in step 4
> if white points are not so close, then you would need > something more complex, like an appearance model. is there any link I can understand more about that? > > c) Convert back the XYZ to the desired output space, > which can be sRGB or any other. right here I get to use the Reverse Bradford ... since I come in XYZ with Bradford, with my logical guess I just assume what I use to come, after transform I go back out RGB the same way :) > > Bradford matrix is supposed to handle chromatic adaptation, > which is how our brain gets rid of white point. This is used > on perceptual & rel.colorimetric intents. I think you should > use abs.colorimetric instead. hum ...any example of abs colorimetric? I can try it on my calculation and see it provide better result :) I see F-Spot using lcms doing the exact same thing but the result is a little different ( not big just a little ) and wonder how you did it. Thanks > > Regards > Marti Maria > The littleCMS project > http://www.littlecms.com > > > > ----- Original Message ----- From: "Alex Lau" <[EMAIL PROTECTED]> > To: <lcms-user@lists.sourceforge.net> > Sent: Tuesday, January 17, 2006 9:26 AM > Subject: [Lcms-user] Color Temp > > >> Dear all >> I'm new to the list. >> >> I have wrote some color temp transform function. >> And realized lcms able to do the same, I'm just trying clear my >> misunderstanding and make sure my implementation is correct. >> ( even though it look very close to photoshop raw plugin already ) >> >> What I do is this >> >> 1) first I get colortemp from and to and make a xy ( assume no lower >> then 2000k ) >> if( colortemp >= 2000 && colortemp <= 7000 ){ >> xyy->x =-4.607 * pow(10, 9)/ pow(colortemp, 3) + >> 2.967800 * pow(10,6) / pow(colortemp, 2) + >> 99.11 / colortemp + 0.244063; >> } else { >> xyy->x = -2.0064 * pow(10,9)/ pow(colortemp, 3) + >> 1.9018 * pow(10,6) / pow(colortemp, 2) + >> 247.48 / colortemp + 0.237040; >> } >> xyy->y = -3 * pow( xyy->x, 2 ) + 2.87 * xyy->x - 0.275; >> 2) Then XY to XYZ by assuming y = 1 >> new_xyz->X = orig_xyy->x / orig_xyy->y; >> new_xyz->Y = 1; >> new_xyz->Z = (1-orig_xyy->x - orig_xyy->y)/ orig_xyy->y; >> 3) Then using BradFord matrix to get the white reference rgb >> R = new_xyz->X * BradFord[0][0] + >> new_xyz->Y * BradFord[1][0] + >> new_xyz->Z * BradFord[2][0]; >> G = new_xyz->X * BradFord[0][1] + >> new_xyz->Y * BradFord[1][1] + >> new_xyz->Z * BradFord[2][1]; >> B = new_xyz->X * BradFord[0][2] + >> new_xyz->Y * BradFord[1][2] + >> new_xyz->Z * BradFord[2][2]; >> 4) New we get Source Color RGB white reference and Dest Color RGB White >> Reference for making the transformation matrix >> >> matrix[0][0] = BradFord[0][0]*destRGB->R/srcRGB->R; >> matrix[0][1] = BradFord[0][1]*destRGB->G/srcRGB->G; >> matrix[0][2] = BradFord[0][2]*destRGB->B/srcRGB->B; >> >> matrix[1][0] = BradFord[1][0]*destRGB->R/srcRGB->R; >> matrix[1][1] = BradFord[1][1]*destRGB->G/srcRGB->G; >> matrix[1][2] = BradFord[1][2]*destRGB->B/srcRGB->B; >> >> matrix[2][0] = BradFord[2][0]*destRGB->R/srcRGB->R; >> matrix[2][1] = BradFord[2][1]*destRGB->G/srcRGB->G; >> matrix[2][2] = BradFord[2][2]*destRGB->B/srcRGB->B; >> >> 5) transformation matrix = matrix * Reverse_BradFord >> >> Now we can start working on RGB >> >> 6) RGB->sRGB (by gamma apply) >> if( RGB <= 0.04045 ) >> RGB / 12.92 >> else >> pow ( ( (0.055 + RGB)/ 1.055), 2.4 ) ; >> >> 7) sRGB->XYZ (by multiple D65 transform matrix) >> X = R * D65[0][0] + G * D65[1][0] + B * D65[2][0]; >> Y = R * D65[0][1] + G * D65[1][1] + B * D65[2][1]; >> Z = R * D65[0][2] + G * D65[1][2] + B * D65[2][2]; >> Here is the question.. I see you are using D50 transform matrix >> which is for printing color space? I'm not sure how to choose, any idea? >> >> 8) XYZ * transformation matrix >> >> 9) XYZ back to sRGB >> >> 10) sRGB back to RGB >> >> So basically is >> >> RGB->sRGB->XYZ->Transform->XYZ->sRGB->RGB >> >> I'm not sure what I'm talking about ... could someone tell me I'm right >> or wrong :) >> >> Thanks >> >> Alex >> >> >> >> >> >> ------------------------------------------------------- >> This SF.net email is sponsored by: Splunk Inc. Do you grep through log >> files >> for problems? Stop! Download the new AJAX search engine that makes >> searching your log files as easy as surfing the web. 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