CODEWITHSUNDEEP
imagemagickgimp
SEU
SEU

Reputation: 1390

Gimp AutoInputLevels in ImageMagick

I am trying to recreate Gimp's AutoInputLevels (Colors>Levels>AutoInputLevels) in Imagemagick (need to batch process 1000 files). This is for an infrared image. I tried contrast_stretch, normalize and auto level, but they didn't help. Any suggestions?

Thanks.

Edit: I won't be able to provide the representative images. However, when I say they didn't help, I am using other operations in GIMP. Doing the same (auto level and hard_light in ImageMagick) are not providing equivalent results.

Upvotes: 1

Views: 261

Answers (2)

fmw42
fmw42

Reputation: 53154

Adding to @Mark Setchell's answer, I can tweak it a bit and get close using:

Input:

enter image description here

convert redhat.jpg -channel rgb -contrast-stretch 0.6%x0.6% im.png

ImageMagick Result:

enter image description here

GIMP AutoInputLevels Result:

enter image description here

And get a numerical comparison:

compare -metric rmse gimp.png im.png null:

363.484 (0.00554641)

which is about 0.5% difference.

Upvotes: 2

Mark Setchell
Mark Setchell

Reputation: 207660

As you didn't provide a representative image, or expected result, I synthesised an image with three different distributions of red, green and blue pixels, using code like this:

#!/usr/bin/env python3

from PIL import Image
import numpy as np

w, h = 640, 480

# Synthesize red channel, mu=64, sigma=3
r = np.random.normal(64, 3, size=h*w)

# Synthesize green channel, mu=128, sigma=10
g = np.random.normal(128, 10, size=h*w)

# Synthesize blue channel, mu=192, sigma=6
b = np.random.normal(192, 6, size=h*w)

# Merge channels to RGB and round
RGB = np.dstack((r,g,b)).reshape((h,w,3)).astype(np.uint8)

# Save
Image.fromarray(RGB).save('result.png')

enter image description here

Then I applied the GIMP AutoInputLevels contrast stretch that you are asking about, and saved the result. The resulting histogram is:

enter image description here

which seems to show that the black-point and white-point levels have been set independently for each channel. So, in ImageMagick I guess you would want to separate the channels, apply some contrast-stretch to each channel independently and then recombine the channels along these lines:

magick input.png -separate -contrast-stretch 0.35%x0.7%  -combine result.png

enter image description here

If you provide representative input and output images, you may get a better answer.

Upvotes: 0

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