Greyscale histogram for adjusting contrast

ROOT
1

Hi,

I’d like to implement a tool that would be useful to improve the visibility of some images I am getting from a detector. The tool is standard in all image manipulation programmes, but I don’t know if it’s easy or even advisable to have it in ROOT. It usually looks like this:

lpgallery.mb.ca/gimp/images/g90.gif

As far as I understand the way the histograms (TH2D) are displayed now in my code, the smallest value in the histogram is mapped to a black colour, and the largest value is mapped to white.
I would like to have a tool that displays the histogram shown in the link, with two sliders that can change this default behaviour.

I think I could do it in the following way:

  • draw a TH1D with the distribution of the values in the image (easy enough)
  • add two “sliders” (maybe with TLine?) whose position can be changed with a click
  • get the position of the sliders and feed it to the parameters of TColor::CreateColorGradientTable() to change the greyscale
  • set the new palette in the new image

Would this work? Is there any more efficient way of doing that? Should I consider saving the data as TASImage instead of TH2D? I’m not doing that at the moment because I’m really using them as histograms (calculating profiles and projections, fitting functions etc.)

2

Yes that way would be ok.

3

I implemented this, actually with a TSlider, which is even nicer.

I can then successfully create a colour table, but apparently changing the style is not enough. Do I also have to re-draw the histogram?

4

Yes … or update the canvas. Note that only one palette is available at a time.

5

Updating the canvas is not working :frowning:

6

Try the sequence (with “YourCanvas” or the “gPad”):
YourCanvas->Modified(); YourCanvas->Update();

7

Yes, that made it! Thanks