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:
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.
b) If white points are close enough, a simple scale would
suffice. Somethink like:
XYZout = XYZin * (WPout/WPin)
if white points are not so close, then you would need
something more complex, like an appearance model.
c) Convert back the XYZ to the desired output space,
which can be sRGB or any other.
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.
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
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