Apply exp() to the Retinex output before normalization, restoring a
linear tone distribution. This improves contrast overall: light gray
content is preserved, and dark content stays dark even when the image
contains deep blacks. The previous normalization compressed all tones
toward the same gray level. Trade-off: shadows and unwanted background
variations are also more visible in some images.
Other:
- stretch toward white using histogram mode
- fall back to simple normalization on documents with large dark areas
- add bilateral denoising
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