cam_mul array to Temperature and Tint and back

Submitted by spex on

Hi guys,

is there some way how calculate Temperature and Tint value from camera white balance array and back?

I have
cam_mul = {1413, 1024, 2110, 1024};

and I know that it is for Temperature 3200 Kelvin.

For white balance I am using cameraWbCoeff = {1413/512, 1024/512, 2110/512, 1024/512}; and RAW after all changes is OK. But I need change required temperature like as in ACR or UFraw.

I am using Canon EOS-1DX

Thank you for advice

Best Regards Ladislav

Forums: 

Thank for reply.

Submitted by spex on

Thank for reply.

I looked to DNG SDK but I am not able move cam_mul[] to DNG SDK required parameters dng_xy_coord. Calculated temperature is different than on camera.

What is right way how convert cam_mul to xy or XYZ?

Thank you
Ladislav

Calculated temperature *will*

Submitted by lexa on

Calculated temperature *will* differ from camera because camera and LibRaw uses different color profile (because color profile is used for xy-calculation)

The only way to check results is to compare to Adobe software (Lightroom or Camera Raw) with same color profiles (so, for example, DNG file with built-in camera matrices: ACR/Lr uses embedded color datain DNG case and LibRaw may be forced to do the same)

-- Alex Tutubalin @LibRaw LLC

White balance

Submitted by spex on

Hi Alex,

Can I have some questions to you:

1] When I want convert RAW image to XYZ color space I multiply rgb_cam * cam_xyz * RAW pixel?

2] Does LibRaw contain parameters of camera temperature? Or is there some way how reconstruct this value from camera?

3] Is possible reconstruct from Libraw parameters RAW pixel values where camera measure white balance coeficients?

Thank you Ladislav

1) rgb_cam is Camera to sRGB.

Submitted by lexa on

1) rgb_cam is Camera to sRGB. You may multiply this matrix to srgb2xyz matrix to get camera-to-xyz matrix.
2) LibRaw do not do anything with camera CCT/tint
3) Could you please re-formulate your question, I cannot understand what you want to obtain.

-- Alex Tutubalin @LibRaw LLC

White balance values

Submitted by spex on

Thanks for reply.

reformulate 3] Camera must calculate white balance coefficients from some RGB raw values. It is possible get from LibRaw these RGB values?

Thanks

The rule is simple: rgb

Submitted by lexa on

The rule is simple: rgb values, multiplied by WB coeffs will result into equal values (e.g. gray). So, assume Green to be, for example, 1024, than Red/Blue will 1024-divieded-by-WB coeff.

-- Alex Tutubalin @LibRaw LLC

White point reference

Submitted by spex on

Hi Alex,
1] yes it is clear for me, but must be some reference point for white reconstruct point where camera measure rgb values. It is possible get this reference point from LibRaw? I have function which get CCT from XYZ and I need some reference point from camera to get this white point reference.

and one other question.
2] What is different between pre_mul and cam_mul arrays?

For me working pre_mul as camera temperature setting. I am using matrix

for(int row=0; row < 3; row++)
{
for(int col=0; col < 3; col++)
{
cam2RGB[row][col] = rgb_cam[row][col] * pre_mul[col];
}
}

than multiply all RGB values for each pixel

int correction_R = cam2RGB[0][0] * pixel_R[col] + cam2RGB[0][1] * pixel_G[col] + cam2RGB[0][2] * pixel_B[col];
int correction_G = cam2RGB[1][0] * pixel_R[col] + cam2RGB[1][1] * pixel_G[col] + cam2RGB[1][2] * pixel_B[col];
int correction_B = cam2RGB[2][0] * pixel_R[col] + cam2RGB[2][1] * pixel_G[col] + cam2RGB[2][2] * pixel_B[col];

cam_mul working too but image is without color cast and image is not same as in other image editors.

Thanks Ladislav

cam_mul is camera (as shot)

Submitted by lexa on

1) For some cameras you may get some additional WB data from EXIF/Makernotes fields. LibRaw only parses camera 'as shot' coefficients for most cameras.

2)
cam_mul is camera (as shot) white balance.
pre_mul is daylight white balance (calculated from Adobe camera matrix)

-- Alex Tutubalin @LibRaw LLC

Am I right to think that rgb

Submitted by Sylvain on

Am I right to think that rgb_cam is filled with the values of pre_mul?

Thanks,
Sylvain.

rgb_cam is [3][4] matrix,

Submitted by lexa on

rgb_cam is [3][4] matrix, while pre_mul are [4] WB multipliers.

-- Alex Tutubalin @LibRaw LLC