29a.ch experiments by Jonas Wagner

Black and White Conversion using PCA

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I have been hacking on my photo forensics tool lately. I found a few references that suggested that performing PCA on the colors of an image might reveal interesting information hidden to the naked eye. When implementing this feature I noticed that it did a quite good job at doing black & white conversions of photos. Thinking about this it does actually make some sense, the first principal component maximizes the variance of the values. So it should result in a wide tonal range in the resulting photograph. This led me to develop a tool to explore this idea in more detail.

This experimental tool is now available for you to play with:

To give you a quick example let’s start with one of my own photographs: original photo

While the composition with so much empty space is debatable, I find this photo fairly good example of an image where a straight luminosity conversion fails. This is because the really saturated colors in the sky look bright/intense even if the straight up luminosity values do not suggest that.

pca conversion luminosity conversion
Hover it to see the results of a straight luminosity conversion instead.

In this case the PCA conversion does (in my opinion) a better job at reflecting the tonality in the sky. I’d strongly suggest that you experiment with the tool yourself.

If you want a bit more detail on how exactly the conversions work please have a look at the help page.

Do I think this is the best technique for black and white conversions? No. You will always be able to get better results by manually tweaking the conversion to fit your vision. Is it an interesting result? I’d say so.

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