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For subtract_black() call,

For subtract_black() call, the 'black level value' and 'black level data' are the same. Just different terms for the same thing to maintain 'writing style' (by not repeating same words many times).

In my pseudo code, you need to do this way:

LibRaw imgp; // Image processor class;
  for(row=0; row < ..height; row++)
    for(col=0; col < .. width; col++)
        int color_index = imgp.COLOR(row,col);
        int black_level_for_this_color = + imgp.imgdata.color.cblack[color_index];
        int next_pixel_value_adjusted_for_black = VISIBLE_PIXEL(row,col) - black_level_for_this_color;
        if(next_pixel_value_adjusted_for_black < 0)
            next_pixel_value_adjusted_for_black = 0;

VISIBLE_PIXEL is defined in my previous message.

But for astrophotography it is better to average several dark frames (and do not use black/cblack[]) and subtract this value.

-- Alex Tutubalin @LibRaw LLC