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Structural dissimilarity (DSSIM) is an image distance metric, first described in "Structural Similarity-Based Object Tracking in Video Sequences" by Loza et al., that corresponds better to the human perception than MAE or RMSE. It is derived from structural similarity (SSIM), first described in "Image quality assessment: from error visibility to structural similarity" by Wang et al.
For "ftc - floating precision texture compression" by Philipp Klaus Krause a C++ implementation of DSSIM has been created based on Rabah Mehdi's C++ implementation of SSIM.
Download C++ source