A fast and concise parallel implementation of the 8x8 2D forward and inverse DCTs using halide

Abstract

The Discrete Cosine Transform (DCT) is commonly used for image and video coding and very efficient implementations of the forward and inverse transforms are of great importance. The popular libjpeg-turbo library contains handwritten, highly-optimised assembly language DCT implementations utilizing SIMD instruction sets for a variety of architectures. We present an alternative approach, implementing the 8x8 IDCT and FDCT written in the functional image processing language Halide. We show how less than 200 lines of Halide can replace over 20,000 lines of code the libjpeg-turbo library to perform JPEG encoding and decoding. The Halide implementation is compared for ARMv8 NEON and x86-64 SIMD extensions and shows a 5–25 percent performance improvement over the SIMD code in libjpeg-turbo for decoding and a 10–40 percent improvement for encoding. The Halide code is significantly easier to maintain and port to new architectures than the existing code.