PSNR and SSIM on non-rectangular image regions

1. What is the difference between PSNR and SSIM?
2. Why PSNR instead of MSE?
3. How do you find the PSNR of an image?

What is the difference between PSNR and SSIM?

PSNR is used earlier than SSIM, is easy, has been widely used in various digital image measurements, and has been considered tested and valid. SSIM is a newer measurement tool that is designed based on three factors i.e. luminance, contrast, and structure to better suit the workings of the human visual system.

Why PSNR instead of MSE?

The mean-square error (MSE) and the peak signal-to-noise ratio (PSNR) are used to compare image compression quality. The MSE represents the cumulative squared error between the compressed and the original image, whereas PSNR represents a measure of the peak error. The lower the value of MSE, the lower the error.

How do you find the PSNR of an image?

Now let P1 = MAX^2 and P0 = MSE. We then have PSNR = 10 log10(MAX^2/MSE) = 10 log10(MAX/(MSE)^(1/2))^2 = 20 log10(MAX/(MSE)^(1/2)). Therefore, PSNR = 20 log10(MAX/(MSE)^(1/2)).