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This blog post is a result of a collaboration between Visidon, headquartered in Finland and Imagination, based in the UK. Visidon is recognised as an expert in algorithms for camera image enhancement and analysis and Imagination has a series of world-beating neural network accelerators (NNA) with performance up to 100 TOPS per second per core.

The problem tackled in this blog post is denoising images from conventional colour cameras. The solution is in two parts:

• Algorithms that remove the noise without damaging image detail.
• A high-performance convolution engine capable of running a trained neural network that takes a colour image as input and outputs a denoised colour image.

The process of denoising images has a long history. The way modern CMOS imagers work can be thought of as an array of photon counters. Photons arrive at the sensor at an average rate: fewer in dark regions with relatively high fluctuations, but more in brighter regions with relatively lower fluctuations, i.e., better signal-to-noise ratio. The fluctuations are the noise (with Poisson statistics) due to the physics of light and cannot, in general, be avoided. We can, however, remove the noise with further processing. The key thing is to do this without damaging the picture content.

Over the years many solutions have been proposed. These include simply blurring a picture slightly, sophisticated approaches with a bilateral filter, Beltrami filters based on manifold theory, scale-space Kalman filters, etc.

The interesting points about denoising are firstly that noise is most obvious (to us) in flat regions of an image and noise is less visible (to us) near edges. Edges, though, are most likely to be blurred or damaged in some other way by many denoising algorithms. Damaged edges are as bad perceptually as noise!

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