MATLAB tif file format vrs PNG file format. True lossless compression?

Question

I have been working on a problem where I need to save a matrix as an Image and for it to be a truly lossless example of the matrix in MATLAB.

I have tried writing the image as a png and tiff both in lossless form:

Name              Size                 Bytes  Class     Attributes

  diff            237x354               671184  double              
  imPng           237x354               167796  uint16              
  imPngD          237x354               671184  double              
  imTiff          237x354                83898  uint8               
  imTiffD         237x354               671184  double              
  padded         2042x2170            35449120  double              
  testImage       237x354               671184  double              

testImage is the matrix I would like to save losslessly.

I used the following lines to write the matrix as a tif and png:

EDU>> imwrite(testImage,'imTiff.tif','tiff','compression','none');
EDU>> imwrite(testImage,'imTiff.png','PNG','bitdepth',16);

I then loaded the images back to the work space (imTiff and inPng) and converted them to double.

EDU>> imPngD = im2double(imPng);
EDU>> imTiffD = im2double(imTiff);

but when I subtract either imPngD or imTiffD from testImage there are residual values left.

My questions are:
Is there something wrong in my processing?
If not, does this mean that tif and png are not truly completely lossless?

I can supply images if you think it would help.

I thought these images might help: testImage

Difference between imTiffD and testImage

Difference between imPngD and testImage

Please note: the values of the grey pixels in images 2-3 are zero i.e. the difference between testImage and the reloaded images are zero.

Answer 1

PNG and TIFF are both truly lossless (ignoring the TIFF option to package JPEG, which you are not using). However they are truly lossless given their input, which is either 8 or 16 bits per pixel per color value for PNG, or 8 bits per pixel per color value for TIFF.

If you are looking beyond the precision that can be carried by 8 or 16 bit values, then you will see a difference.