Multimedia processor-based implementation of an error-diffusion halftoning algorithm exploiting subword parallelism

Multimedia processor-based implementations of digital image processing algorithms have become important since several multimedia processors are now available and can replace special-purpose hardware-based systems because of their flexibility. Multimedia processors increase throughput by processing multiple pixels simultaneously using a subword-parallel arithmetic and logic unit architecture. The error-diffusion halftoning algorithm employs feedback of quantized output signals to faithfully convert a multi-level image to a binary image or to one with fewer levels of quantization. This makes it difficult to achieve speedup by utilizing the multimedia extension. In this study, the error-diffusion halftoning algorithm is implemented for a multimedia processor using three methods: single-pixel, single-line, and multiple-line processing. The single-pixel approach is the closest to conventional implementations, but the multimedia extension is used only in the filter kernel. The single-line approach computes multiple pixels in one scan-line simultaneously, but requires a complex algorithm transformation to remove dependencies between pixels. The multiple-line method exploits parallelism by employing a skewed data structure and processing multiple pixels in different scan-lines. The Pentium MMX instruction set is used for quantitative performance evaluation including run-time overheads and misaligned memory accesses. A speedup of more than ten times is achieved compared to the software (integer C) implementation on a conventional processor for the structurally sequential error-diffusion halftoning algorithm.