CODEWITHSUNDEEP
iosobjective-cimage-processingopengl-esgpuimage
rikitikitavi
rikitikitavi

Reputation: 159

GPUImage Histogram Equalization

I would like to use GPUImage's Histogram Equalization filter (link to .h) (link to .m) for a camera app. I'd like to use it in real time and present it as an option to be applied on the live camera feed. I understand this may be an expensive operation and cause some latency.

I'm confused about how this filter works. When selected in GPUImage's example project (Filter Showcase) the filter shows a very dark image that is biased toward red and blue which does not seem to be the way equalization should work.

Also what is the difference between the histogram types kGPUImageHistogramLuminance and kGPUImageHistogramRGB? Filter Showcase uses kGPUImageHistogramLuminance but the default in the init is kGPUImageHistogramRGB. If I switch Filter Showcase to kGPUImageHistogramRGB, I just get a black screen. My goal is an overall contrast optimization.

Does anyone have experience using this filter? Or are there current limitations with this filter that are documented somewhere?

Upvotes: 1

Views: 747

Answers (1)

Piglet
Piglet

Reputation: 28974

Histogram equalization of RGB images is done using the Luminance as equalizing the RGB channels separately would render the colour information useless.

You basically convert RGB to a colour space that separates colour from intensity information. Then equalize the intensity image and finally reconvert it to RGB.

According to the documentation: http://oss.io/p/BradLarson/GPUImage

GPUImageHistogramFilter: This analyzes the incoming image and creates an output histogram with the frequency at which each color value occurs. The output of this filter is a 3-pixel-high, 256-pixel-wide image with the center (vertical) pixels containing pixels that correspond to the frequency at which various color values occurred. Each color value occupies one of the 256 width positions, from 0 on the left to 255 on the right. This histogram can be generated for individual color channels (kGPUImageHistogramRed, kGPUImageHistogramGreen, kGPUImageHistogramBlue), the luminance of the image (kGPUImageHistogramLuminance), or for all three color channels at once (kGPUImageHistogramRGB).

I'm not very familiar with the programming language used so I can't tell if the implementation is correct. But in the end, colours should not change too much. Pixels should just become brighter or darker.

Upvotes: 1

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