Does local contrast mean local in pixel values or in image coordinates?

Software darktable
1

Hello,

There is something I don’t understand about the local contrast module in Darktable. When we say “local” does that mean local in terms of pixel values or image coordinates?

In the former it would be equivalent to apply a tone curve and make small bumps in the curve, locally for certain pixel values, to gain contrast in certain regions of the histogram. But the processing would be applied the same to all the image.

In the later it would correspond to apply a different tone curve depending on the x y coordinate on the image. The processing is local in the sense of the image coordinates.

Which one is it between the two descriptions?

Thanks in advance for your answers :slight_smile:

2

Image co-ordinates, I think.

“certain regions” in a histogram do not normally correlated with regions in a image.

3

ok! yes when I said regions in a histogram I was talking about portions of the histogram with similar values.

4

You may want to read here, for example: Local Contrast Enhancement

1 Like
5

Terminology again. “portions”. “values”.

Do you mean contiguous sets of levels (X-axis) with similar counts (Y-axis)?

Hopefully you do realize that histograms do not relate to spatial locations in an image.

6

or there is the bilateral filter mode…

7

Yes I mean pixels with similar values. Nothing related to space.

8

thanks!