How to apply white balance coefficents to RAW image for sRGB output

Question

I want to convert RAW image data (RGGB) to sRGB image. There are many specialized ways to do this but to first understand the basics, I've implemented some easy alogrithms like debayering by resolution-reduction. My current pipeline is:

• Rescale the u16 input data by blacklevel and whitelevel
• Apply white balance coefficents
• Debayer with size reduction, average for G: g=((g0+g1)/2)
• Calculate pseudo-inverse for D65 illuminant XYZ_TO_CAM (from Adobe DNG)
• Convert debayered RGB data to XYZ by CAM_TO_XYZ
• Convert XYZ to D65 sRGB (matrix taken from Bruce Lindbloom)
• Apply gamma correction (simple routine for now, should be replaced by sRGB gamma)
• Rescale from [minval..maxval] to [0..1] and convert f32 to u16
• Save as tiff

The problem is that if I skip the white balance coefficent multiplication (or just replace them by 1.0) the output image already looks acceptable. If I apply the coefficents (taken from AsShot in DNG) the output has a huge color cast. And I'm not sure if I have to multiply by coef or 1/coef.

The first image is the result of the pipeline with wb_coefs set to 1.0.

The second image is the result with the "correct" wb_coefs.

What is wrong in my pipeline?

Additional question:

• I'm not sure about the rescaling process. Do I've to rescale into [0..1] after every step or is it enough to rescale during u16 conversion as final stage?

Full code:

macro_rules! max {
  ($x: expr) => ($x);
  ($x: expr, $($z: expr),+) => {{
      let y = max!($($z),*);
      if $x > y {
          $x
      } else {
          y
      }
  }}
}

macro_rules! min {
  ($x: expr) => ($x);
  ($x: expr, $($z: expr),+) => {{
      let y = min!($($z),*);
      if $x < y {
          $x
      } else {
          y
      }
  }}
}

/// sRGB D65
const XYZD65_TO_SRGB: [[f32; 3]; 4] = [
  [3.2404542, -1.5371385, -0.4985314],
  [-0.9692660, 1.8760108, 0.0415560],
  [0.0556434, -0.2040259, 1.0572252],
  [0.0, 0.0, 0.0],
];

// buf: RAW image data
fn to_srgb(buf: &Vec<u16>, width: usize, height: usize) {
  let w = width / 2;
  let h = height / 2;

  let blacklevel: [u16; 4] = [511, 511, 511, 511];
  let whitelevel: [u16; 4] = [12735, 12735, 12735, 12735];

  let xyz2cam_d65: [[i32; 3]; 4] = [[6722, -635, -963], [-4287, 12460, 2028], [-908, 2162, 5668], [0, 0, 0]];
  let cam2xyz = convert_matrix::<4>(xyz2cam_d65);
  eprintln!("CAM_TO_XYZ: {:?}", cam2xyz);

  // from DNG
  // As Shot Neutral: 0.518481 1 0.545842
  //let wb_coef = [1.0/0.518481, 1.0, 1.0, 1.0/0.545842];
  //let wb_coef = [0.518481, 1.0, 1.0, 0.545842];
  let wb_coef = [1.0, 1.0, 1.0, 1.0];

  // b/w level correction, rescale, debayer
  let mut rgb = vec![0.0_f32; width / 2 * height / 2 * 3];
  for row in 0..h {
    for col in 0..w {
      let r0 = buf[(row * 2 + 0) * width + (col * 2) + 0];
      let g0 = buf[(row * 2 + 0) * width + (col * 2) + 1];
      let g1 = buf[(row * 2 + 1) * width + (col * 2) + 0];
      let b0 = buf[(row * 2 + 1) * width + (col * 2) + 1];
      let r0 = ((r0.saturating_sub(blacklevel[0])) as f32 / (whitelevel[0] - blacklevel[0]) as f32) * wb_coef[0];
      let g0 = ((g0.saturating_sub(blacklevel[1])) as f32 / (whitelevel[1] - blacklevel[1]) as f32) * wb_coef[1];
      let g1 = ((g1.saturating_sub(blacklevel[2])) as f32 / (whitelevel[2] - blacklevel[2]) as f32) * wb_coef[2];
      let b0 = ((b0.saturating_sub(blacklevel[3])) as f32 / (whitelevel[3] - blacklevel[3]) as f32) * wb_coef[3];
      rgb[row * w * 3 + (col * 3) + 0] = r0;
      rgb[row * w * 3 + (col * 3) + 1] = (g0 + g1) / 2.0;
      rgb[row * w * 3 + (col * 3) + 2] = b0;
    }
  }

  // Convert to XYZ by CAM_TO_XYZ from D65 illuminant
  let mut xyz = vec![0.0_f32; w * h * 3];
  for row in 0..h {
    for col in 0..w {
      let r = rgb[row * w * 3 + (col * 3) + 0];
      let g = rgb[row * w * 3 + (col * 3) + 1];
      let b = rgb[row * w * 3 + (col * 3) + 2];
      xyz[row * w * 3 + (col * 3) + 0] = cam2xyz[0][0] * r + cam2xyz[0][1] * g + cam2xyz[0][2] * b;
      xyz[row * w * 3 + (col * 3) + 1] = cam2xyz[1][0] * r + cam2xyz[1][1] * g + cam2xyz[1][2] * b;
      xyz[row * w * 3 + (col * 3) + 2] = cam2xyz[2][0] * r + cam2xyz[2][1] * g + cam2xyz[2][2] * b;
    }
  }

  // Track min/max value for rescaling/clipping
  let mut maxval = 1.0;
  let mut minval = 0.0;

  // Convert to sRGB from XYZ
  let mut srgb = vec![0.0; w * h * 3];
  for row in 0..h {
    for col in 0..w {
      let r = xyz[row * w * 3 + (col * 3) + 0] as f32;
      let g = xyz[row * w * 3 + (col * 3) + 1] as f32;
      let b = xyz[row * w * 3 + (col * 3) + 2] as f32;
      srgb[row * w * 3 + (col * 3) + 0] = XYZD65_TO_SRGB[0][0] * r + XYZD65_TO_SRGB[0][1] * g + XYZD65_TO_SRGB[0][2] * b;
      srgb[row * w * 3 + (col * 3) + 1] = XYZD65_TO_SRGB[1][0] * r + XYZD65_TO_SRGB[1][1] * g + XYZD65_TO_SRGB[1][2] * b;
      srgb[row * w * 3 + (col * 3) + 2] = XYZD65_TO_SRGB[2][0] * r + XYZD65_TO_SRGB[2][1] * g + XYZD65_TO_SRGB[2][2] * b;
      let r = srgb[row * w * 3 + (col * 3) + 0];
      let g = srgb[row * w * 3 + (col * 3) + 1];
      let b = srgb[row * w * 3 + (col * 3) + 2];
      maxval = max!(maxval, r, g, b);
      minval = min!(minval, r, g, b);
    }
  }

  gamma_corr(&mut srgb, w, h, 2.2);

  let mut output = vec![0_u16; w * h * 3];
  for row in 0..h {
    for col in 0..w {
      let r = srgb[row * w * 3 + (col * 3) + 0];
      let g = srgb[row * w * 3 + (col * 3) + 1];
      let b = srgb[row * w * 3 + (col * 3) + 2];
      output[row * w * 3 + (col * 3) + 0] = (clip(r, minval, maxval) * (u16::MAX as f32)) as u16;
      output[row * w * 3 + (col * 3) + 1] = (clip(g, minval, maxval) * (u16::MAX as f32)) as u16;
      output[row * w * 3 + (col * 3) + 2] = (clip(b, minval, maxval) * (u16::MAX as f32)) as u16;
    }
  }

  let img = DynamicImage::ImageRgb16(ImageBuffer::from_raw(w as u32, h as u32, output).unwrap());
  img.save_with_format("/tmp/test.tif", image::ImageFormat::Tiff).unwrap();
}

fn pseudoinverse<const N: usize>(matrix: [[f32; 3]; N]) -> [[f32; 3]; N] {
  let mut result: [[f32; 3]; N] = [Default::default(); N];

  let mut work: [[f32; 6]; 3] = [Default::default(); 3];
  let mut num: f32 = 0.0;
  for i in 0..3 {
    for j in 0..6 {
      work[i][j] = if j == i + 3 { 1.0 } else { 0.0 };
    }
    for j in 0..3 {
      for k in 0..N {
        work[i][j] += matrix[k][i] * matrix[k][j];
      }
    }
  }
  for i in 0..3 {
    num = work[i][i];
    for j in 0..6 {
      work[i][j] /= num;
    }
    for k in 0..3 {
      if k == i {
        continue;
      }
      num = work[k][i];
      for j in 0..6 {
        work[k][j] -= work[i][j] * num;
      }
    }
  }
  for i in 0..N {
    for j in 0..3 {
      result[i][j] = 0.0;
      for k in 0..3 {
        result[i][j] += work[j][k + 3] * matrix[i][k];
      }
    }
  }

  result
}

fn convert_matrix<const N: usize>(adobe_xyz_to_cam: [[i32; 3]; N]) -> [[f32; N]; 3] {
  let mut xyz_to_cam: [[f32; 3]; N] = [[0.0; 3]; N];
  let mut cam_to_xyz: [[f32; N]; 3] = [[0.0; N]; 3];

  for i in 0..N {
    for j in 0..3 {
      xyz_to_cam[i][j] = adobe_xyz_to_cam[i][j] as f32 / 10000.0;
    }
  }
  eprintln!("XYZ_TO_CAM: {:?}", xyz_to_cam);
  let inverse = pseudoinverse::<N>(xyz_to_cam);
  for i in 0..3 {
    for j in 0..N {
      cam_to_xyz[i][j] = inverse[j][i];
    }
  }
  cam_to_xyz
}

fn clip(v: f32, minval: f32, maxval: f32) -> f32 {
  (v + minval.abs()) / (maxval + minval.abs())
}

// https://kosinix.github.io/raster/docs/src/raster/filter.rs.html#339-359
fn gamma_corr(rgb: &mut Vec<f32>, w: usize, h: usize, gamma: f32) {
  for row in 0..h {
    for col in 0..w {
      let r = rgb[row * w * 3 + (col * 3) + 0];
      let g = rgb[row * w * 3 + (col * 3) + 1];
      let b = rgb[row * w * 3 + (col * 3) + 2];
      rgb[row * w * 3 + (col * 3) + 0] = r.powf(1.0 / gamma);
      rgb[row * w * 3 + (col * 3) + 1] = g.powf(1.0 / gamma);
      rgb[row * w * 3 + (col * 3) + 2] = b.powf(1.0 / gamma);
    }
  }
}

The DNG for this example can be found at: https://chaospixel.com/pub/misc/dng/sample.dng (~40 MiB).

Answer 1

The main reason for getting wrong colors is that we have to normalize the rows of rgb2cam matrix to 1, as described in the following guide.

According to DNG spec:

ColorMatrix1 defines a transformation matrix that converts XYZ values to reference camera native color space values, under the first calibration illuminant.

It means that if the calibration illuminant is D65, the ColorMatrix converts XYZ to "camera RGB".
(Convert it as is, without using any white balance scaling coefficients).

• The inverse ColorMatrix, converts from "camera RGB" to XYZ.
  After converting XYZ to sRGB, the result is color balanced sRGB.
  The conclusions is that ColorMatrix includes the while balance coefficients in it (the white balancing coefficients apply D65 illuminant).
• Normalizing the rows of rgb2cam to 1 neutralizes the while balance coefficients, and keeps only the "Color Correction Matrix" (the math is a bit complicated).
• Without normalizing the rows, we are scaling by while balance multipliers two times:

1. Scale coefficients from ColorMatrix that balances the input to D65.
2. Scale coefficients taken from AsShotNatural that balances the input to the illuminant of the scene (illuminant of the scene is close to D65).
   The result of scaling twice is an extreme color cast.

---

Tracking the maximum in order to avoid "magenta cast in the highlights":

Instead of tracking the actual maximum color values in the input image, we suppose to track the "theoretical maximum color value".

• Take whitelevel - blacklevel and scale by the white balance multipliers.
  Track the result...

The guiding rule is that the colors supposed to be the same in both cases:

• Applying the processing to small patches of the image, and places the patches together (where we can't track the global minimum and maximum).
• Applying the processing to the entire image.

I suppose you have to track the maximum of scaled whitelevel - blacklevel, only when white balance multipliers are less than 1.
When all the multipliers are 1 or above, we can clip the result to 1.0, without tracking the maximum.
Note:

• there is probably an advantage of scaling down, and tracking the maximum, but I don't know this subject.
  In my solution we just multiply upper (above 1.0), and clip the result.

---

The solution is based on Processing RAW Images in MATLAB guide.

I am posting both MATLAB implementation and Python implementation (but no Rust implementation).

---

The first step is extracting the raw Bayer image from sample.dng using dcraw command line:

dcraw -4 -D -T sample.dng  

Rename the tiff output to sample__lin_bayer.tif.

---

Conversion process:

• Rescale the uint16 input data by blacklevel and whitelevel (subtract blacklevel from all the pixels and scale by whitelevel - blacklevel).
• Apply white balance scaling coefficients.
  The scaling coefficients equals 1./AsShotNatural.
  Scale the red pixels in the Bayer alignment by the red scaling coefficient, scale the greens by the green scaling, and the blues by the blue scaling.
  Assumption: the minimum scaling is 1.0 and the others are above 1.0 (we my divide by the minimum scaling to make sure).
• Clip the scaled result to [0, 1] (clipping is required due to demosaic implementation limitations).
• Demosaicing (Debayer) using MATLAB demosaic function or cv2.cvtColor in Python.
• Calculate rgb2cam matrix: rgb2cam = ColorMatrix * rgb2xyz.
  rgb2xyz matrix is taken from Bruce Lindbloom site.
• Normalize rows of rgb2cam matrix so the sum of each row equals 1 (divide each row by the sum of the row).
• Compute cam2rgb matrix by inverting rgb2cam: cam2rgb = inv(rgb2cam).
  cam2rgb is the "CCM matrix" (Color Correction Matrix).
• Left multiply matrix cam2rgb by each RGB tuple (apply color correction).
• Apply gamma correction (use sRGB standard gamma).
• Convert to uint8 and save as PNG (PNG format is used for posting in SO website).

---

MATLAB Implementation:

filename = 'sample__lin_bayer.tif'; % Output of: dcraw -4 -D -T sample.dng

% Exif information:
blacklevel = 511; % blacklevel = meta_info.SubIFDs{1}.BlackLevel(1);
whitelevel = 12735; % whitelevel = meta_info.SubIFDs{1}.WhiteLevel;
AsShotNeutral = [0.5185 1 0.5458];
ColorMatrix = [ 0.6722   -0.0635   -0.0963
               -0.4287    1.2460    0.2028
               -0.0908    0.2162    0.5668];
bayer_type = 'rggb';


% Constant matrix for converting sRGB to XYZ(D65):
% http://www.brucelindbloom.com/Eqn_RGB_XYZ_Matrix.html
rgb2xyz = [0.4124564  0.3575761  0.1804375
           0.2126729  0.7151522  0.0721750
           0.0193339  0.1191920  0.9503041];


% Read input image (Bayer mosaic alignment, pixels data type is uint16):
raw = imread(filename);

% "Linearizing":
% There is no LinearizationTable so we only have to subtract the black level.
% Convert from range [blacklevel, whitelevel] to range [0, 1].
lin_bayer = (double(raw) - blacklevel) / (whitelevel - blacklevel);
lin_bayer = max(0, min(lin_bayer, 1));

% The White Balance multipliers are 1./AsShotNeutral
wb_multipliers = 1./AsShotNeutral;
r_scale = wb_multipliers(1); % Assume value is above 1
g_scale = wb_multipliers(2); % Assume value = 1
b_scale = wb_multipliers(3); % Assume value is above 1

% Bayer alignment is RGGB:
% R G
% G B
%
% Apply white balancing to linear Bayer image.
balanced_bayer = lin_bayer;
balanced_bayer(1:2:end, 1:2:end) = balanced_bayer(1:2:end, 1:2:end)*r_scale; % Red   (indices [1, 3, 5,... ; 1, 3, 5,... ])
balanced_bayer(1:2:end, 2:2:end) = balanced_bayer(1:2:end, 2:2:end)*g_scale; % Green (indices [1, 3, 5,... ; 2, 4, 6,... ])
balanced_bayer(2:2:end, 1:2:end) = balanced_bayer(2:2:end, 1:2:end)*g_scale; % Green (indices [2, 4, 6,... ; 1, 3, 5,... ])
balanced_bayer(2:2:end, 2:2:end) = balanced_bayer(2:2:end, 2:2:end)*b_scale; % Blue  (indices [2, 4, 6,... ; 2, 4, 6,... ])

% Clip to range [0, 1] for avoiding "pinkish highlights" (avoiding "magenta cast" in the highlights).
balanced_bayer = min(balanced_bayer, 1);

% Demosaicing
temp = uint16(balanced_bayer*(2^16-1)); % Convert from double to uint16, because MATLAB demosaic() function requires a uint8 or uint16 input.
lin_rgb = double(demosaic(temp, bayer_type))/(2^16-1); % Apply Demosaicing and convert range back type double and range [0, 1].

% Color Space Conversion
xyz2cam = ColorMatrix; % ColorMatrix applies XYZ(D65) to CAM_rgb
rgb2cam = xyz2cam * rgb2xyz;

% Result:
% rgb2cam = [0.2619    0.1835    0.0252
%            0.0921    0.7620    0.2053
%            0.0195    0.1897    0.5379]

% Normalize rows to 1. MATLAB shortcut: rgb2cam = rgb2cam ./ repmat(sum(rgb2cam,2),1,3);

rows_sum = sum(rgb2cam, 2);
% Result:
% rows_sum = [0.4706
%             1.0593
%             0.7470]

% Divide element of every row by the sum of the row:
rgb2cam(1, :) = rgb2cam(1, :) / rows_sum(1); % Divide top row
rgb2cam(2, :) = rgb2cam(2, :) / rows_sum(2); % Divide center row
rgb2cam(3, :) = rgb2cam(3, :) / rows_sum(3); % Divide bottom row
% Result (sum of every row is 1):
% rgb2cam = [0.5566    0.3899    0.0535
%            0.0869    0.7193    0.1938
%            0.0261    0.2539    0.7200]

cam2rgb = inv(rgb2cam); % Invert matrix
% Result:
% cam2rgb = [ 1.9644   -1.1197    0.1553
%            -0.2412    1.6738   -0.4326
%             0.0139   -0.5498    1.5359]

R = lin_rgb(:, :, 1);
G = lin_rgb(:, :, 2);
B = lin_rgb(:, :, 3);

% Left multiply matrix cam2rgb by each RGB tuple (convert from "camera RGB" to "linear sRGB").
sR = cam2rgb(1,1)*R + cam2rgb(1,2)*G + cam2rgb(1,3)*B;
sG = cam2rgb(2,1)*R + cam2rgb(2,2)*G + cam2rgb(2,3)*B;
sB = cam2rgb(3,1)*R + cam2rgb(3,2)*G + cam2rgb(3,3)*B;

lin_srgb = cat(3, sR, sG, sB);
lin_srgb = max(min(lin_srgb, 1), 0); % Clip to range [0, 1]

% Convet from "Linear sRGB" to sRGB (apply gamma)
sRGB = lin2rgb(lin_srgb); % lin2rgb MATLAB functions uses the exact formula [you may approximate it to power of (1/gamma)].

% Show the result, and save to sRGB.png
figure;imshow(sRGB);impixelinfo;title('sRGB'); 
imwrite(im2uint8(sRGB), 'sRGB.png');



% Inverting 3x3 matrix (some help of MATLAB Symbolic Toolbox):
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Assume:
% A = [ a11, a12, a13]
%     [ a21, a22, a23]
%     [ a31, a32, a33]
%
% 1. Compute determinant of A:
% detA = a11*a22*a33 - a11*a23*a32 - a12*a21*a33 + a12*a23*a31 + a13*a21*a32 - a13*a22*a31
%
% 2. Compute the inverse of the matrix A:
% invA = [  (a22*a33 - a23*a32)/detA, -(a12*a33 - a13*a32)/detA,  (a12*a23 - a13*a22)/detA
%          -(a21*a33 - a23*a31)/detA,  (a11*a33 - a13*a31)/detA, -(a11*a23 - a13*a21)/detA
%           (a21*a32 - a22*a31)/detA, -(a11*a32 - a12*a31)/detA,  (a11*a22 - a12*a21)/detA]

---

Python implementation:

import numpy as np
import cv2


def lin2rgb(im):
    """ Convert im from "Linear sRGB" to sRGB - apply Gamma. """
    # sRGB standard applies gamma = 2.4, Break Point = 0.00304 (and computed Slope = 12.92)    
    g = 2.4
    bp = 0.00304
    inv_g = 1/g
    sls = 1 / (g/(bp**(inv_g - 1)) - g*bp + bp)
    fs = g*sls / (bp**(inv_g - 1))
    co = fs*bp**(inv_g) - sls*bp

    srgb = im.copy()
    srgb[im <= bp] = sls * im[im <= bp]
    srgb[im > bp] = np.power(fs*im[im > bp], inv_g) - co
    return srgb


filename = 'sample__lin_bayer.tif'  # Output of: dcraw -4 -D -T sample.dng

# Exif information:
blacklevel = 511
whitelevel = 12735
AsShotNeutral = np.array([0.5185, 1, 0.5458])
ColorMatrix = np.array([[ 0.6722, -0.0635, -0.0963],
                        [-0.4287,  1.2460,  0.2028],
                        [-0.0908,  0.2162,  0.5668]])
# bayer_type = 'rggb'


# Constant matrix for converting sRGB to XYZ(D65):
# http://www.brucelindbloom.com/Eqn_RGB_XYZ_Matrix.html
rgb2xyz = np.array([[0.4124564, 0.3575761, 0.1804375],
                    [0.2126729, 0.7151522, 0.0721750],
                    [0.0193339, 0.1191920, 0.9503041]])


# Read input image (Bayer mosaic alignement, pixeles data type is np.uint16):
raw = cv2.imread(filename, cv2.IMREAD_UNCHANGED)

# "Linearizing":
# There is no LinearizationTable so we only have to subtract the black level.
# Convert from range [blacklevel, whitelevel] to range [0, 1] (convert type to np.float64).
lin_bayer = (raw.astype(np.float64) - blacklevel) / (whitelevel - blacklevel)
lin_bayer = lin_bayer.clip(0, 1)

# The White Balance multipliers are 1./AsShotNeutral
wb_multipliers = 1 / AsShotNeutral
r_scale = wb_multipliers[0]  # Assume value is above 1
g_scale = wb_multipliers[1]  # Assume value = 1
b_scale = wb_multipliers[2]  # Assume value is above 1

# Bayer alignment is RGGB:
# R G
# G B
#
# Apply white balancing to linear Bayer image.
balanced_bayer = lin_bayer.copy()
balanced_bayer[0::2, 0::2] = balanced_bayer[0::2, 0::2]*r_scale  # Red   (indices [0, 2, 4,... ; 0, 2, 4,... ])
balanced_bayer[0::2, 1::2] = balanced_bayer[0::2, 1::2]*g_scale  # Green (indices [0, 2, 4,... ; 1, 3, 5,... ])
balanced_bayer[1::2, 0::2] = balanced_bayer[1::2, 0::2]*g_scale  # Green (indices [1, 3, 5,... ; 0, 2, 4,... ])
balanced_bayer[1::2, 1::2] = balanced_bayer[1::2, 1::2]*b_scale  # Blue  (indices [1, 3, 5,... ; 0, 2, 4,... ])

# Clip to range [0, 1] for avoiding "pinkish highlights" (avoiding "magenta cast" in the highlights).
balanced_bayer = np.minimum(balanced_bayer, 1)

# Demosaicing:
temp = np.round((balanced_bayer*(2**16-1))).astype(np.uint16)  # Convert from double to np.uint16, because OpenCV demosaic() function requires a uint8 or uint16 input.
lin_rgb = cv2.cvtColor(temp, cv2.COLOR_BayerBG2RGB).astype(np.float64)/(2**16-1)  # Apply Demosaicing and convert back to np.float64 in range [0, 1] (is there a bug in OpenCV Bayer naming?).

# Color Space Conversion
xyz2cam = ColorMatrix  # ColorMatrix applies XYZ(D65) to CAM_rgb
rgb2cam = xyz2cam @ rgb2xyz

# Result:
# rgb2cam = [0.2619    0.1835    0.0252
#            0.0921    0.7620    0.2053
#            0.0195    0.1897    0.5379]

# Normalize rows to 1. MATLAB shortcut: rgb2cam = rgb2cam ./ repmat(sum(rgb2cam,2),1,3);

rows_sum = np.sum(rgb2cam, 1)
# Result:
# rows_sum = [0.4706
#             1.0593
#             0.7470]

# Divide element of every row by the sum of the row:
rgb2cam[0, :] = rgb2cam[0, :] / rows_sum[0]  # Divide top row
rgb2cam[1, :] = rgb2cam[1, :] / rows_sum[1]  # Divide center row
rgb2cam[2, :] = rgb2cam[2, :] / rows_sum[2]  # Divide bottom row
# Result (sum of every row is 1):
# rgb2cam = [0.5566    0.3899    0.0535
#            0.0869    0.7193    0.1938
#            0.0261    0.2539    0.7200]

cam2rgb = np.linalg.inv(rgb2cam)  # Invert matrix
# Result:
# cam2rgb = [ 1.9644   -1.1197    0.1553
#            -0.2412    1.6738   -0.4326
#             0.0139   -0.5498    1.5359]

r = lin_rgb[:, :, 0]
g = lin_rgb[:, :, 1]
b = lin_rgb[:, :, 2]

# Left multiply matrix cam2rgb by each RGB tuple (convert from "camera RGB" to "linear sRGB").
sr = cam2rgb[0, 0]*r + cam2rgb[0, 1]*g + cam2rgb[0, 2]*b
sg = cam2rgb[1, 0]*r + cam2rgb[1, 1]*g + cam2rgb[1, 2]*b
sb = cam2rgb[2, 0]*r + cam2rgb[2, 1]*g + cam2rgb[2, 2]*b

lin_srgb = np.dstack([sr, sg, sb])
lin_srgb = lin_srgb.clip(0, 1)  # Clip to range [0, 1]

# Convert from "Linear sRGB" to sRGB (apply gamma)
sRGB = lin2rgb(lin_srgb)  # lin2rgb MATLAB functions uses the exact formula [you may approximate it to power of (1/gamma)].

# Save to sRGB.png
cv2.imwrite('sRGB.png', cv2.cvtColor((sRGB*255).astype(np.uint8), cv2.COLOR_RGB2BGR))

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Results (downscaled):

Result of RawTherapee (all enhancements are disabled):

MATLAB result:

Python result:

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Note:

• The result looks dark due to low exposure (and because we didn't apply any brightness correction).