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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