JPEG-compliant perceptual coding for a grayscale image printing pipeline

We describe a procedure by which Joint Photographic Experts Group (JPEG) co mpression may be customized for gray-scale images that are to be compressed before they are scaled, halftoned, and printed. Our technique maintains 10 0% compatibility with the JPEG standard, and is applicable with all scaling and halftoning methods. The JPEG quantization table is designed using freq uency-domain characteristics of the scaling and halftoning operations, as w ell as the frequency sensitivity of the human visual system. In addition, t he Huffman tables are optimized for low-rate coding. Compression artifacts are significantly reduced because they are masked by the halftoning pattern s, and pushed into frequency bands where the eye is less sensitive. We desc ribe how the frequency-domain effects of scaling and halftoning may be meas ured, and how to account for those effects in an iterative design procedure for the JPEG quantization table, We also present experimental results sugg esting that the customized JPEG encoder typically maintains "near visually lossless" image quality at rates below 0.5 b/pixel (with reference to the n umber of pixels in the original image) when it is used with bilinear interp olation and either error diffusion or ordered dithering. Based on these res ults, we believe that in terms of the achieved bit rate, the performance of our encoder is typically at least 20% better than that of a JPEG encoder u sing the suggested baseline tables.