Legacy ImageMagick Discussions Archive

from app/operations/gimplevelsconfig.c


gimp_levels_config_adjust_by_colors (GimpLevelsConfig *config,
GimpHistogramChannel channel,
const GimpRGB *black,
const GimpRGB *gray,
const GimpRGB *white)
{
[...]
if (gray)
{
gdouble input;
gdouble range;
gdouble inten;
gdouble out_light ...

ok maybe I wasn't clear enough.

in gimp the user clicks on "pick grey point" from "levels" dialog and picks a color.
gimp magically transforms that color in some gamma values for each channel and applies them, resulting a new image.

now what I want is to simulate this behavior with imagemagick ...

i'm getting close.

so for value (r,g,b) picked by user I'll calculate the gamma for each channel:



static double sRGB_to_linear(double x) {
if (x < 0.04045) return x/12.92;
return Math.pow((x+0.055)/1.055, 2.4);
}

double R_linear = sRGB_to_linear(r/255.0);
double G_linear = sRGB_to_linear ...

thank you all of you for replies!

@fmw42: nice tip the one with hald image. but I'm not sure I understand first part. so I have an image and a picked rgb value.
how this rgb values fits in the convert example. AFAIK gamma value is between 0.1 ant 10. do I miss something ?

hi,

I trying to simulate "pick grey point" from gimp "levels" dialog with convert.
for "black point" and "white point" I use:

$ convert -level-colors 'rgb(0,0,0),rgb($RED,$GREEN,$BLUE)' in.jpg out.jpg
and
$ convert -level-colors 'rgb($RED,$GREEN,$BLUE),rgb(255,255,255)' in.jpg out.jpg

any idea ...