RGBWorkingSpace and Histogram

[Nocturnal]

Hi,

sorry, bunch of questions in one post.

When I use the histogram transformation in RGB/K mode, am I modifying L or am I simply applying the same curve to R G and B? It seems to be the latter which means you can't curve L with HistogramTransform. If so, why?

When I load a new RGB image do I need to set the RGBWorkingSpace or is the default setting assumed until I modify it?

[mmirot]

I think you effect all RGB channels with the same curve. I am not sure why there is not a seperate L ( like curves).

[mmirot]

I think each image gets the default RGB working space unless you apply a custom one.

Max

[Nocturnal]

Thanks Max. As you've heard me say I've been telling people there's no need to extract L, muck with it and re-apply it. If there's no way to do an L histogram I'll have to change my story

[Carlos Milovic]

Well... in fact there is no way to do a L histogram transform You may write a script to implement that

Just for curiosity... why do you want to perform a Luminance HT? Black/White point setting? MTF?

[Carlos Milovic]

Just checked, and you may do an automatic black/white points setting (autozero) to the luminance with the Rescale process.

[Nocturnal]

As I explained I've been telling people they don't have to extract L out of RGB before processing because most processes allow you to work on just the L component. Since you do this:

- extract L
- HT on L
- reapply L

I imagine doing HT on L should be possible. Even intuitively it doesn't sound strange to want to map brightness levels. Curves does it, why can't HT?

[Carlos Milovic]

Yes, it is possible (theorically), but I don't see the need/point on processing a synthetic luminance with HT instead of working over the RGB channels. I'm thinking on an aggressive HT, like the one we use to delinearize data. In most cases, it just doesn't work. The saturation levels get screwed (too low). You'll still need to work on it. By the other way, working over the RGB works quite well, and as long as you use similar settings, color balance is well maintained.
On the Curves process, by the other hand, contrast changes to the luminance work better than RGB because color balance is ensured, while the RGB may distort the balances at different zones. Also the changes to the saturation are better handled there.