Structural Similarity Index Measure (SSIM)

The Structural Similarity Index Measure (SSIM) is a non-linear full-reference metric that compares the luminance, contrast and structure of the original and degraded image. SSIM was first introduced in 2004 as a new way to assess image quality. Instead of measuring the absolute errors between the reference and the degraded pixels (like in PSNR), SSIM measures the structural elements of the pixels.

Example: Imagine an original, high-quality photograph of a city skyline. A software developer compresses this image into a JPEG file, which introduces artifacts and blurriness. When a quality assurance team uses SSIM to test the new image, it doesn't just look for how many pixels are different from the original. Instead, it measures how well the skyline's structure — the sharp lines of the buildings and the patterns of the windows — are preserved. The SSIM score would be high if the compressed image still has clear, defined buildings (even if the colors are slightly off), but it would drop significantly if the buildings become blurry and the structural details are lost, reflecting a poor user experience.