# On Finding Gray Pixels

## TL;DR

This paper introduces a simple, learning-free grayness index based on the Dichromatic Reflection Model for efficient and accurate illumination estimation in color images, outperforming many existing methods.

## Contribution

The paper presents a novel grayness index (GI) that is easy to implement, fast, and effective for estimating illumination in images, including multiple light sources.

## Key findings

- GI outperforms state-of-the-art statistical methods.
- GI surpasses many recent deep learning approaches.
- Processing a 1080p image takes approximately 0.4 seconds.

## Abstract

We propose a novel grayness index for finding gray pixels and demonstrate its effectiveness and efficiency in illumination estimation. The grayness index, GI in short, is derived using the Dichromatic Reflection Model and is learning-free. GI allows to estimate one or multiple illumination sources in color-biased images. On standard single-illumination and multiple-illumination estimation benchmarks, GI outperforms state-of-the-art statistical methods and many recent deep methods. GI is simple and fast, written in a few dozen lines of code, processing a 1080p image in ~0.4 seconds with a non-optimized Matlab code.