Different images between MATLAB and ImageMagick

Question

Hi, I'm using ImageMagick 7.0.8-64 Q16 for Windows and MATLAB R2019. I'm doing the same operation (resizing with bicubic interpolation method) in both programs.

ImageMagick code:

magick start.png -resize 400x200! -interpolate Catrom -quality 100 start_magick_100.png

MATLAB code

start_matlab = imresize(start, [200 400], "bicubic")
imwrite(start_matlab, "start_matlab.png", "png")

Now, the difference is not visible but reading again both images in MATLAB and ImageJ they are a little bit different in terms of pixels count. I mean that the pixel count is the same but summing the image matrix in MATLAB give me a different result. How can it be possible? Do IM and Matlab could have different functions even for these simple operations?

Answer 1

Different algorithms produces different result and both software handle interpolation differently.

Looking at your code gives a hint that you’re using catmull-rom algorithm for bicubic interpolation in image magic. It gives the smooth result but can’t be compared pixel to pixel with a different algorithm (matlab)

Answer 2

I will answer this quickly before it gets closed. The answer is simply:

Implementation details count.

So even if the math is the same. Processing might not be the same. Math and images themselves do not have a straight forward enough interpretation. Think:

• How do you do the math in integer arithmetic
• How do you round numbers?
• Do you implement it all in one stage or not?
• Do you process the image numbers in linear space?
  • or do you just do gamma correction.
  • or do you implement a cms?
  • or do you take the numbers at face value?

etc.