write and read the same image but they are different

Question

lol lolol 2019-9-10

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评论： Guillaume 2019-9-11

Hi, I would like to save exact copy of image.

When I use

imwrite(image, 'image.png');

and after that:

image2=imread('image.png')

and try to compare this images they are different- but I dont see any differenceces(imabsdiff also)

psnr(image,image2)
ssim(image,image2)

PSNR = 18.600852547147856

SSIM = 0.091853247350195

both images are double class

What I can do to have the same pixel by pixel images?

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Rik 2019-9-11

Comment posted as answer by lol lolol:

image is a double class, I don't know how imwrite and imread works, but when I try to make image2 a double class, it's still a different image.

I checked one pixel and values are different:

image =

0.298940255146088

image2 =

0.298039215686275

Any suggest how to fix it?

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Answer 1

Rik 2019-9-11

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You should read the documentation of the functions you're using if you want to understand them. Matlab has excellent documentation, it is one of the things that is better than most alternatives.

If you want to have the exact same image, you need to make sure there is no lossy compression in the file format and that you don't have any float rounding. The png format is lossless, so no problem there. If you want to store and retrieve images, they will generally be stored as uint8, instead of double. You haven't attached any example files to reproduce your issue, so I can't be more specific than this.

Also, you shouldn't shadow the internal image function with a variable name. That can in some cases cause issues. Before assigning a variable, you can use the which function to find out if the name is in use by a function you may need.

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Adam 2019-9-11

Why do you need to save it as an image rather than as actual data? If you want that level of accuracy then you should be saving just the data, not an image. Those type of differences are almost impossible to see with the eye though usually, which is the only purpose of an image over just the raw data.

Guillaume 2019-9-11

There's only a few image formats that support storing pixel intensities as floating point values. tiff is one of them but imwrite doesn't support that.

Any double image written by imwrite will therefore be discretised. If using png, you have the option to discretise it to 8 bits (256 levels) or 16 bits (65536 levels) if you convert it to uint16 (with im2uint16).

Other common image formats such as BMP and jpeg are typically 8-bit only.

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