Log-hybrid architecture for tonal correction combined with modified un-sharp masking filter algorithm for colour image enhancement

• Developed system system architecture is less computationally complex than Homomorphic Filtering and Multi Scale Retinex
• Developed system yields excellent colour rendition and contrast enhancement with reduced complexity.
• The Log-Hybrid strategy reduces multiplication and avoids complex trigonometric operations.
• The developed architecture is compact and small enough to fit large and small FPGAs.
• The scalable architecture also adds sharpness in addition to colour contrast and rendition.

A novel colour image enhancement architecture combining a Tonal Correction algorithm and a high frequency emphasis circular symmetric filter is implemented in hardware forming a colour image contrast enhancement system of low complexity. The architecture utilizes efficient log-domain calculations, resulting in multiplier-less operations and eliminates costly hardware division. Furthermore, the modularity of the fundamental design enables its ease of incorporation into larger and more complex designs with little modification. The design can be used for both grayscale and RGB colour images by processing each channel individually and the system can serve as a post enhancement module for improved colour rendition. The unit can be configured for the enhancement of over-exposed (too bright) and under-exposed (too dark) images based on adjustable parameters. The architecture easily fits on a basic Spartan III FPGA for low-cost realization evaluation and a Virtex 5 ML505 for real-time use. Results of the implementation and simulation are provided and compared with the software version and show that the system produces images with improved colour contrast and rendition compared to well known image enhancement algorithms such as RGB colour Homomorphic filtering. It also performs better than the Homomorphic filter and the MSRCR for faded colour images.