In this paper, we present the high-resolution rate-distortion function
for stationary sources with finite differential entropy and under a
simplified version of the SSIM index distance measure used in im-
age coding. We first show that this simplified SSIM index is locally
quadratic. We then show that recent results of Linder and Zamir
on locally quadratic rate-distortion theory are applicable, and finally
derive the high-resolution rate-distortion function.