How do I tell if an image is gamma encoded when imported to numpy?

Question

I am a bit confused about when an image is gamma encoded/decoded and when I need to raise it to a gamma function.

Given an image 'boat.jpg' where the colour representation is labeled 'sRGB'. My assumption is that the pixel values are encoded in the file by raising the arrays to ^(1/2.2) during the save process.

When I import the image into numpy using scikit-image or opencv I end up with a 3-dim array of uint8 values. Do these values need to be raised to ^2.2 in order to generate a histogram of the values, or when I apply the imread function, does that map the image into linear space in the array?

from skimage import data,io
boat = io.imread('boat.jpg')

score 1 · Answer 1 · answered Oct 03 '19 at 03:40

1

if you get your image anywhere on the internet, it has gamma 2.2.
unless the image has an image profile encoded, then you get the gamma from that profile.
imread() reads the pixel values 'as-is', no conversion.
there's no point converting image to gamma 1.0 for any kind of the processing, unless you specifically know that you have to. basically, nobody does that.

answered Oct 03 '19 at 03:40

lenik

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Thanks. I "specifically know that I have to". Is the image profile encoded as metadata? – Jesse McMullen-Crummey Oct 04 '19 at 12:36
@JesseMcMullen-Crummey most of the images on the net do not contain metadata, but if you are lucky, yet, usually gamma value is somewhere there. – lenik Oct 04 '19 at 14:11

score 1 · Answer 2 · answered Feb 23 '23 at 08:15

OpenCV (as of v 4) usually does the gamma conversion for you, depending on the image format. It appears to do it automatically with PNG, but it's pretty easy to test. Just generate a 256x256 8-bit color image with a linear color ramps along x and y, then check to see what the pixel values at given image coords. If the sRGB mapping/unmapping is done correctly at every point, x=i should have pixel value i and so on. If you imwrite to PNG in OpenCV, it will convert to sRGB, tag that in the image format, and GIMP or whatever will happily decode it back back to linear.

Most image files are stored as sRGB, and there's a tendency for most image manipulation APIs to handle it correctly, since well, if they didn't, they'd work wrong most of the time. In the odd instance where you read an sRGB file as linear or vice versa, it will make a significant difference though, especially if you're doing any kind of image processing. Mixing up sRGB and linear causes very significant problems, and you will absolutely notice it if it gets messed up; fortunately, the software world usually handles it automagically in the file read/write stage so casual app developers don't usually have to worry about it.

score 0 · Answer 3 · answered Oct 03 '19 at 02:05

As you probably know, skimage uses a handful of different plugins when reading in images (seen here). The values you get should not have to be adjusted...that happens under the hood. I would also recommend you don't use the jpeg file format because you lose data with the compression.

How do I tell if an image is gamma encoded when imported to numpy?

3 Answers3