Is there a simple and accurate way to stretch the histogram of a rendered image using the compositor.
Increase the contrast with the Bright/Contrast node.
@Benu
I’ve been separating the RGB channels and modifying the white and black levels for each channel, Recombining them and adding another RGB node as a master control. I was thinking more along the lines of how to use the histogram in the render window to get accurate results.
By “stretch” you mean change its graph display or changing the actual distribution of luma/chroma?
Btw, in the compositor you need to render first and then inspect the distribution of luma/chroma.
the basic idea
http://www.biorust.com/tutorials/detail/198/en/
the complex view
http://blendervse.wordpress.com/2009/09/10/video-import-into-blender/
the future
http://wiki.blender.org/index.php/Org:Institute/Open_projects/Durian/Features_wishlist
hope this helps
O.K. So Blender 2.5 has a new output node called levels. I’ve just stumbled upon it.
So I guess the answer is: separate the RGB channels and modify the white and black levels for each channel, Recombine them and add another RGB node as a master control then connect up the new levels output node to see the resulting histograms.
Alternately add the master RGB first and then split, combine and add the new levels output node to view the results.
It hasn’t been explicitly stated here (though I think we both know this already), so in case anyone else is reading this, you can press the letter P when over your render window and get a histogram, waveform monitor, and vectorscope (and some other useful stuff) with your rendered image. I don’t think it works in 2.49, though… 2.5 only?
First off, I should correct myself because I’ve just realized that I don’t need to render out first to use the scopes (with P as benu explained). In the default compositing layout one can use the properties tab of the uv/image editor window to access the scopes in real time. See attached img.
Now is this truly awesome or what?
Secondly, in addition to the curves node, one could also use the color balance node to push specific colors to highs or lows.
I would highly recommend the following two sources on the topic:
http://www.packtpub.com/article/introduction-to-blender-2-5-color-grading (also check the sequel at the bottom of the article)
http://www.luceri.it/2010/12/color-correction-preview-and-inspiration-part-5/ (not just the latest part but the whole series)
@blendercomp
Excellent tip, had to try this right away. It’s better than I expected so much easier to navigate with the nodes and the image at the same time.
Also thanks for the links I’ve already read the packtpub one. The Luceri one looks pretty interesting.
@benu
My first approach was to use the render window and the render window curves tool. But the render window curve is flaky and the scopes don’t update.
Second approach was to add a scene to the sequencer, it works but its slow and you need to constantly refresh the scene in order to update the scopes.