PSNR of two unequal matrices

If I am executing a code of run length encoding on an image matrix, giving me an another image matrix of not of that size(due to compression of image data). Is it possible to calculate the PSNR in this situation?

[Guillaume]

You would normally compute the PSNR between your original image and the decompressed image.

Isn't run length encoding non-lossy? In which case, the decompressed image is the same as the original and your PSNR is infinite.

[tina jain]

OK, got it