Stochastic Floyd-Steinberg dithering on GPU: image quality and processing time improved

Error diffusion dithering is a technique that is used to represent a grey-scale image in a format usable by a printer. At every step, an algorithm converts the grey-scale value of a pixel to a new value within the allowed ones, generating a conversion error. To achieve the effect of continuous-tone illusion, the error is distributed to the neighboring pixels. Among the existent algorithms, the most commonly used is Floyd-Steinberg. However, this algorithm suffers two issues: artifacts and slowness. Regarding artifacts, those are textures that can appear after the image elaboration, making it visually different from the original one. In order to avoid this effect, we will use a stochastic version of Floyd-Steinberg algorithm. To evaluate the results, we will apply the Weighted Signal to Noise Ratio (WSNR), a visual-based model to account for perceptivity of dithered textures. This filter has a low-pass characteristic and, in particular, it uses a Contrast Sensitivity Function to evaluate the similarity between the original image and the final image. Our claim is that the new stochastic algorithm is better suited for both the WSNR measure and the visual analysis. Secondly, we will face slowness: we will describe a parallel version of Floyd-Steinberg algorithm that will exploit GPU (Graphics Processing Unit), drastically reducing the spent time. Specifically, we noticed that the serial version computational time increases quadratically with the input size, while the parallel version one increases linearly. Both the image quality and the computational performance of the parallel algorithm are evaluated on several large-scale images.