imread seems to alter the image saturation?

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Steven Pan 2024-2-27

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https://ww2.mathworks.cn/matlabcentral/answers/2087786-imread-seems-to-alter-the-image-saturation

评论： Steven Pan 2024-2-28

img_original.jpg

Why does the image read by imread() seem to have a reduced saturation compared to the original image? This also happens when the image is read through an imageDatastore object, I guess the underlying method is still imread(). The codes I used are included below:

img = imread("img_original.jpg");

figure, imshow(img)

which produce something like this:

while the original image file, open in FastStone looks like this:

You can also open the img_original.jpg to see it yourself.

While inspecting the histogram, I see that the blue channel has been altered the most when comparing to the histogram from FastStone. The original image seems to have a lot of blue pixels saturated at 0, while the Matlab's blue histogram shows a slight shift to the right. This is also evident from the red channel.

Factors I have investigated include:

bit-depth
JPG format
imread optional arguments
Monitor display setting: the colour histogram is evident that colour has changed
FastStone: I also tried the default Windows image viewer and it looks the same as in FastStone

Can someone please explain what may cause this colour discrepancy when I just simply read the image? Thank you!

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Steven Pan 2024-2-28

I also tested with OpenCV. The result is the same, the colour discepancy is there too.

Steven Pan 2024-2-28

I believe I found the reason. Turns out the FastStone image viewer may have added extra saturation to enhance the image and makes it more appealing (like many other image viewing software do). However, if you try something like IrfanView you should see the same colour viewed in Matlab. I will attach a comparison here. Therefore, the slightly dull colour from Matlab is the "true" colour of this image.