A quality heatmap is the picture that answers where: it colors a single frame by local quality, so a blocky sky reads as a hot patch while a clean face stays cool, showing which region failed while the rest held rather than reporting one pooled average. It is possible because some metrics are computed locally before being pooled into a frame score - SSIM is the clearest case, calculated over small overlapping windows whose intermediate values form a quality map you can render over the frame. Reading it is a skill: a dark blob in a detailed region usually means blur or compression noise, while a low-scoring band along the top and bottom edges is often not a quality problem but a letterbox or aspect-ratio mismatch in the setup. The firm rule is that color must never be the only thing carrying meaning - pair the scale with numbers and label the worst region - because a color-only map is unreadable to a colorblind reader or on a grayscale print.
