I used to do all of my PP with the GIMP. I would often make a lot of use of curves but also the levels utility it contains as it has some very useful features.
The first one is auto. It does some interesting things to all channels including individual colour channels. If the result isn’t as it needs to be especially on colour channels it give a good idea of what needs fixing. Sometimes a curve on a colour channel is a better option but in many case auto does show what is wrong and given a good mix of colours can even get it right.
In terms of “normal” levels and contrast the results are mixed on auto however once some one understands what the sliders do it’s far easier to set white and black points and mid tone contrast with them than it is with curves. Some people have problems with curves as tiny changes can make a fair difference so find levels a lot easier.
It also has a slider that sets the output span separately. So if for instance the image needed to be restricted to say 10 to 220 bits it can be done and the contrast can be adjusted with the centre input slider. I’ve found that this can be used to create more space for adjustments quickly and easily. The same can be done with curves but it’s quicker and easier. I do sometimes process jpg’s and it can be very useful for that.
Ok there are other ways of doing the same thing but the interface is far more intuitive. GIMP’s biggest problem is that it can produce spikey histograms due to working in 8 bit but they needn’t be apparent on the display unless pushed too far. I have always wondered if these could be “debanded” in some way but obviously not to a great extent and when working 16bit it’s unlikely to happen anyway.
If I hit auto levels in RT I am yet to get a decent result and don’t find adjusting the settings it gives easy so generally just reset it all. Most people find normal level controls very easy to pick up and a number of shots need very little else.
You may want to take a look at using a tone curve to make adjustments to your photos. Paul Matthijsse has a nice article in his blog which shows how to simply use a RT tone curve that will give you results very similar to Gimp’s levels tool.
As I mentioned I generally do use curves on photo’s and have done so for a long long time.
The point really is that the GIMP levels is a lot easier to use for most people. It’s also present in a lot of pp package although the bit range of the output setting is as far as I am aware unique.
It too can produce excellent images all on it’s own. For instance on the shot on the demo that you linked to setting the left and right main sliders to each end of the histogram and then adjusting the centre slider would have very similar effect to the curve he shows. There is also the option of varying the position of the outer sliders and then adjusting the contrast. What the outer sliders are actually doing is setting the black and white points. The central contrast slider is adding a smooth curve There are a couple of tutorials here that show basic use
Sadly I can’t find the one that make use of auto levels to get colours approx. correct. Having numbers available and not just the sliders is useful too. A more advanced one that GIMP users may find interesting as well is this one that probably throws in some slight use of curves too. The initial use of levels helps with that.
I don’t think it replaces all of the tools in RT or anything like it but have noticed that with people just starting out that they tend to have little trouble using it correctly and often it’s all that is needed to get things more or less right. Can’t say the same about curves. Other things like highlight and shadow recovery are still needed for mapping from raw. I used to use UfRaw a lot. It’s approach is pretty basic - curves only really.
GIMP’s auto may be a bit much in terms of what it does to the colour channels so that might best be an option. It appears to stretch or compress all to the same length which does sometimes work out but may need altering afterwards - that can be umm “great fun”
I suspect being able to spec the output span is aimed at web use and viewers with crap monitors. I’ve used it to say shift the black end up by N bits and maybe the bright end down. The centre input slider can then be used to get the mid tone contrast as wanted which is often the important part. I do like the bits aspect been shown on the output and as most things are displayed on a sRGB screen that’s pretty sensible. If the end is Adobe RGB for printing then 0 to 255 might not be a good idea.
The histogram was bunched up. The left and right input pointer have stretched the output to full white and full black. The middle slider adjusted for the contrast in the lower mid tone range. The output pointers can be used to set where the output will be. In this case full black and full white. It can be helpful in many way. Auto for instance can clean up colour balances or at least indicate which channel is out. It can also be used in much the same way as curves can but is far less delicate to use.
since the thread is marked Raw Therapee and relates to raw processing, let me share some thoughts on HDR processing technicques I was figuring out for HDR RAWs. Though, I use the Darktable instead of the raw Therapee, it won’t make much of a difference due to the following quite generic conclusion I’ve come up with
I was evaluating reference procedure, just for my self, to get the best of my HDRs encompassing contrast and vivid color rendering. Firstly I looked into HDR tone mapping, then after getting poor results I employed the levels and the tone curve that provided contrast yet preserving most colors. Then, after watching a lot of videos on raw processing of landscapes for good colors and contrast rendering (those are two basic factors that give your landscape shot a catchy look), I summarized that the best results were obtained with no levels or tone mapping at all. Pro photographers who showcased their processing used, first of all, just used shadows/highlights and blacks/whites modules, regardless of to balance dark and light image zones. While the tone curve was also applied, it was rather post-shadows/highlights, very secondary module, at the stage where the shot already looked quite punchy and haze-free thus giving it an extra contrast. Also, to my surprise, this method preserved a lot of initially vivid colors as well.
I can post results of my own experiment with that method which proved it works if you would be interested
But… Rawtherapee has something that isn’t present in GIMP (as far as I know) and G’MIC: HSV-equalizer. I would love to have the same functionality in G’MIC or GIMP. (“Equalize HSV”-filter in G’MIC is totally different)
So maybe a little bit crossbreeding between those beautiful programs would be fun.
just to make it clear: I had no problem with shadow details. The scene was relatively dark and I modelled the photo’s exposure to match that impression. And having taken five shots covering ±4EV there are enough details in the shadows. It would have been easy to push it some more in RT. Your version looks much too bright.
Dear Flossie I was responding to this comment you made in the other thread
What do you think? Am I on the right track? I’m a bit unshure, if I preserve all details in the shadows with my RT preprocessing step. The preprocessed images look relatively dull and the post processing is mostly about lifting the shadows via tonemapping.
Levels is a very easy way of getting contract and brightness where ever you want it in your original shots or the final result. It’s a;so even possible to use the output sliders to create more space for further work and a number of other things.
Too bright yes but then if I didn’t do something pretty obvious with it there would be no point in posting.
Just a quick note. The GIMP “Auto” on the levels dialog is the same as running Colors → Auto → White Balance
That is, on each RGB channel independently, discard pixels at each end of the histogram that are used by only 0.05% of the pixels in the image, and then stretch the remaining range as much as possible. This invariably leads to a shift in the hue of the image.
The thing is, curves can do all that levels can do, but much more. If you want to introduce folks to using them simply in a way that might mimic behavior of the Levels dialog, realize that pulling the white and black slider points in that dialog is the same as simply pulling the top and bottom of the curves around (and can be seen using the “Edit these Settings as Curves” option in GIMP).
(I would also strongly recommend to use a Logarithmic histogram in GIMP levels).
Is the same as:
The Output Levels can be controlled through the vertical axis in Curves.
True Pat. I often do the same sort of thing with curves but going on other forums a lot find levels easier to use. Might add especially beginners. It’s also adequate all on it’s own for many shots.
I agree concerning auto but on some shots it is worth running and may need each channel adjusting. I also think it’s easier to adjust individual channels for some of the situations that can turn up due to mixed lighting. That too can be done with curves.
I suspect the reason people find it easier is that small curve changes can make a big difference and also levels wont produce “odd” curves as it can’t.
I’m also pretty sure that you are a very experienced GIMP user. I follow your blog. Some aren’t. I think 2 posts cropped up that agree with my general feelings on the subject. I also think that anyone who stepped back from the fact that they can use curves effectively would also agree that it’s easier to use. Not so capable true but often adequate.
I don’t know how many times I have posted things like this along with an explanation of what it does. Usually to no avail. People don’t seem to have any problems with black points, white points or contrast using levels. Try telling them that this increases mid tone level contrast etc and eyes glaze over. It’s too much in one go. I mean by shifting the black I have kept the contrast. Eventually some then get to grips with curves.
My feelings on the matter are that 86.3% of all statistics are made up on the spot, and the reason 51% of all people think levels are easier is because the 1% who write the 99% say that levels are easier and it must be so.
The middle handle of Levels has more to do with gamma and little to do with contrast, which is itself more or less related to the steepness of the curve. Changing the gamma appears to change contrast because it increases the steepness of the curve at either the low or high ends, but this also means that you cannot improve contrast in the mid-tones that way.
True some people say that using levels is playing with mid tone gamma but some add the word effectively. There’s no reason why a curve couldn’t be applied afterwards and anything that change contrast with tone level is effectively messing with gamma. One of the problems for people who play with curves is appreciating why and where the contrast changes.
In 8-bit[1], each time you play with anything that changes the response curve (be it Levels, Curves, Brightness/Contrast, Hue-Saturation, Color Balance, and a whole bag of scripts and plugins)…) you lose colors (and the more pronounced the effect, the more colors). Curves is the only tool that lets you do the whole effect with one single tool usage, therefore minimizing color loss.
[1] This is also true for larger bit depths, but much less perceptible.
I do sometimes process jpg’s and it can be very useful for that.
I follow your blog. Some aren’t. I think 2 posts cropped up that agree with my general feelings on the subject. I also think that anyone who stepped back from the fact that they can use curves effectively would also agree that it’s easier to use. Not so capable true but often adequate.
I don’t know how many times I have posted things like this along with an explanation of what it does. Usually to no avail. People don’t seem to have any problems with black points, white points or contrast using levels. Try telling them that this increases mid tone level contrast etc and eyes glaze over. It’s too much in one go. I mean by shifting the black I have kept the contrast. Eventually some then get to grips with curves.
I’d guess you are conversant with curves Morgan.