Grey Card Color Balancing Like Lightroom

[nits.bikash]

I am trying to implement Grey Card color balancing of Adobe Lightroom using ImageMagic. I got some information from viewtopic.php?f=1&t=15564&sid=f885b9f7f ... f&start=30 to balance color using grey card:-

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convert MAIN.JPG ^
( +clone ( REFERENCE.JPG -gravity Center -crop "128x128+0+0" -scale "1x1" -negate ) +dither -interpolate Integer -clut ) ^
-compose Overlay -composite ^
FIXED.JPG

I tried this one but the result is different from lightroom. Could anyone tell me why this is not accurate and how will get the accurate image?

Here are the images:-

Original one:-


Balanced using Lightroom:-


Balanced using ImageMagick:-

[fmw42]

Try using -color-matrix. I have cropped the image in a region inside the gray card and get its average and divide that into 0.5 to get a ratio of true gray to average gray for each channel. See http://www.imagemagick.org/script/comma ... lor-matrix

In unix syntax, this would be either of the following:

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infile="8Mvyi.jpg"
gray=0.5
rratio=`convert "$infile" -crop 90x40+106+208 +repage -format "%[fx:$gray/mean.r]" info:`
gratio=`convert "$infile" -crop 90x40+106+208 +repage -format "%[fx:$gray/mean.g]" info:`
bratio=`convert "$infile" -crop 90x40+106+208 +repage -format "%[fx:$gray/mean.b]" info:`
echo "$rratio $gratio $bratio"
convert "$infile" -color-matrix \
"$rratio 0 0 \
0 $gratio 0 \
0 0 $bratio" result.jpg

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infile="8Mvyi.jpg"
gray=0.5
declare `convert "$infile" -crop 90x40+106+208 +repage -format "rratio=%[fx:$gray/mean.r]\ngratio=%[fx:$gray/mean.g]\nbratio=%[fx:$gray/mean.b]\n" info:`
echo "$rratio $gratio $bratio"
convert "$infile" -color-matrix \
"$rratio 0 0 \
0 $gratio 0 \
0 0 $bratio" result.jpg

However, if I measure the gray card in the result from Lightroom, I find a gray value, not 0.5 (middle gray), but

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convert YQykH.jpg -crop 90x40+106+208 +repage -format "%[fx:mean]" info:

0.67397

So I use that value, then

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infile="8Mvyi.jpg"
gray=0.67397
declare `convert "$infile" -crop 90x40+106+208 +repage -format "rratio=%[fx:$gray/mean.r]\ngratio=%[fx:$gray/mean.g]\nbratio=%[fx:$gray/mean.b]\n" info:`
echo "$rratio $gratio $bratio"
convert "$infile" -color-matrix \
"$rratio 0 0 \
0 $gratio 0 \
0 0 $bratio" result2.jpg

Adjust the gray=value as desired.

Note that Wikipedia specifies the sRGB color for a gray card as 46.6% or 0.466. See https://en.wikipedia.org/wiki/Middle_gray


Alternately, Lightroom is likely processing this in linear RGB. So this looks like what they are doing:

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infile="8Mvyi.jpg"
gray=0.5
declare `convert "$infile" -colorspace RGB -crop 90x40+106+208 +repage -format "rratio=%[fx:$gray/mean.r]\ngratio=%[fx:$gray/mean.g]\nbratio=%[fx:$gray/mean.b]\n" info:`
echo "$rratio $gratio $bratio"
convert "$infile" -colorspace RGB -color-matrix \
"$rratio 0 0 \
0 $gratio 0 \
0 0 $bratio" -colorspace sRGB result3.jpg