improving contrast with the local laplacian filter

sometimes difficult lighting situations arise which, when taking photographs, result in unappealing pictures. for instance very uniform lighting on a cloudy day may give dull results, while very contrasty illumination (such as back lit) may require to compress the contrast to embrace both highlights and shadows in the limited dynamic range of the output device.

refer to the following two shots as examples:

Have you ever over-exposed your images? Have you ever noticed that your images look flat and dull after you apply negative exposure compensation? Even though the over/underexposed warning says there is no overexposure? Have you ever wondered what is going on? Read on.

the problem

First, why would you want to know which pixels are overexposed, clipped?

Consider this image:

… Why is the sky so white? Why is the image so flat and dull?

modern sensor capture an astonishing dynamic range, namely some sony sensors or canon with magic lantern’s dual iso feature.

this is in a range where the image has to be processed carefully to display it in pleasing ways on a monitor, let alone the limited dynamic range of print media.

example images

use graduated density filter to brighten foreground

using the graudated density iop works well in this case since the horizon here is more or less straight, so we can easily mask it out with a simple gradient in the graduated density module. now what if the objects can’t be masked out so easily?

[update 2016/07/31: there was a section about intermediate export to csv and manually changing that file. this is no longer needed, exporting the style directly from darktable-chart is fine now.]

motivation

for raw photography there exist great presets for nice colour rendition:

• in-camera colour processing such as canon picture styles
• fuji film-emulation-like presets (provia velvia astia classic-chrome)
• pat david’s film emulation luts

unfortunately these are eat-it-or-die canned styles or icc lut profiles. you have to apply them and be happy or tweak them with other tools. but can we extract meaning from these presets? can we have understandable and tweakable styles like these?

Most modules in darktable are working on changing pixels color, lightness, etc. Few modules are moving pixels and when they do they are doing it in a very constraint way like to rotate, fix the lens’ distortions or remove spots.

The liquify module offer more ways to move pixels around by applying some free style distortions to parts of the image. There is three tools to help doing that:

• point
• line
• curve

Today we are happy to announce a big new feature that we will not only ship with the big 2.0 release later this year but also with our next point release, 1.6.4, which is due in about a week: even more darkroom modules!

One of the big strengths of darktable has always been its varied selection of modules to tweak your image. However, that has also been one of the main points of criticism: too much, too many and too complicated to grasp. To make it easier for the user to deal with the flood of tools darktable has had the “more modules” list since many years. It changed its appearance a few times, we added module categories, allowed to select favorite modules, and all of that has proven to be useful. Thus there have always been people that approached us with great new ideas for new modules, especially since we moved to GitHub a while ago with its powerful Pull Request system, yet we couldn’t accept many of them. Some were not that great codewise, some didn’t really fit our product vision – and then there were some that looked nice and certainly benefited some users, but we felt it wasn’t generic enough to justify polluting our module list even more. Of course this was a bad situation, after all these people invested quite some time into providing us with a new feature that we turned down. No one likes to waste their time.

If you overexpose a photo with your digital camera you are in trouble. That’s what most photography related textbooks tell you – and it’s true. So you better pay close attention to your camera’s metering while shooting. However, what to do when the “bad thing” happened and you got this one non-repeatable shot, which is so absolutely brilliant, but unfortunately has some ugly signs of overexposure?

In this blog article I’d like to summarize how darktable can help you to repair overexposed images as much as possible. I’ll cover modules which have been part of darktable for a long time but also touch the brand new module “color reconstruction”.

After being in the camera our pictures deserve some love and to be shared. Every photographer will tell you the joy of having a picture in the hands. At last the pixels have taken form on a piece of paper to give birth to a photography which can be put on the wall!

Though, printing is not easy, there are many technical aspects to take into account. To streamline this process darktable has been added a print module.

Figure 1: the gradient test image – download and play with it in darktable

People using image editors or similar (raster) graphics editors are probably familiar with histograms. You also have them in almost all digital cameras. In darktable you can find it very prominently in the top right corner of darkroom mode, but also as a backdrop of modules like levels, tonecurve and similar.

From a mathematical point of view they are a diagram displaying the amount of pixels in the image that have a specific colour, lightness, value or similar measure. The horizontal axis represents the brightness while the vertical axis corresponds to the amount of pixels that have that brightness.

The next major release of darktable will contain multiple features that have been discussed on this blog and that will make it more powerful than ever. These new features will allow you to process your images in new and creative ways.

However there is one new feature in the upcoming darktable release that is more about Digital Assets Management and simplifying your workflow: Lua scripting.

Lua is a scripting language that is commonly used to add scripting capabilities to programs. It is a programming language that is particularly simple to learn.

In darktable, selective editing was a long awaited feature. Our development version now allow limiting module effects to a region of the image.

Remember the old times, the red light of the darkroom, the smell of the developing bath …

Remember when you were using your hands or a small piece of cardboard to achieve some masking …

Now you can do that in darktable.

example

let take this photo as an example.

One of the upcoming new feature in darktable is the ability to use the same development module several times. By applying the same module multiple times and combining them with blendif it is possible to do some effects that could not be achieved previously without using external tools like the gimp.

Modules that can be instantiated multiple times have a new icon in their header, next to the “reset” button. Clicking that icon will open a pop-up menu that allows you to create a new instance of the module, change the order in which the different instances are applied, or delete an instance of the module. Each instance can have its own parameters, can be activated or deactivates separately and can use presets. Note that the last instance of a module can’t be deleted, it can only be deactivated.

… and how to get rid of it.

[update 02/05/2018 The information how to create camera noise profiles is outdated please read this tutorial instead!]

[update 20/12/2012: ‘how to profile your camera’ includes instructions with the new gen-profile script]

[update 15/12/2012: no more recompile needed, updated the instructions in the benchmark section and how to run make.sh.]

to summarize the current situation in dt: we have a lot of cool tools wrapped around great algorithms with almost all the knobs you need to get perfect results. while you can actually get really great results it’s this sheer number of knobs that makes finding a good parameter set quite a time consuming task. even creating per-iso presets is not straight forward, as most of the current modules depend on a lot more stuff early on in the pipe (whitebalance, exposure, basecurve, etc).

For quite some time people have asked us for a way to geotag their images from within darktable. While that is a nifty feature for sure and really helpful when you take pictures outside of a studio we always had to say something along the lines of “sorry, we don’t have that yet”. Some day however Henrik decided to give it a try and started work in his geo branch. Things started to come together nicely and everything looked really promising, but unfortunately he was a little short in free time so the progress stalled and the code started to bitrot. Since it would be a pity to throw away all the great work Henrik did I kind of adopted the branch and set sails to add the missing bits and pieces to make geotagging a new feature of darktable.

People following the development of darktable might have heard that we added a grouping feature. Everyone who hasn’t heard of that yet: We added a grouping feature.

Now that everybody knows about it I should try to explain what it actually is and how it works/how to use it. For the technical specification you can have a look at the design specs.

The first change in the GUI that can be noticed is a little ‘G’ button. Well, first of all we have to notice that there are two kinds of ‘G’ in the GUI: one in the top toolbar, next to the preferences wheel. The other kind is on images frames which are part of a group, next to the yin-yang-edited symbol.

in an ideal world, an image is piecewise smooth. it has soft gradients, some detail and edges. in particular there’s no noise and the edges are sharp. given these assumptions, you can do a lot of cool things to your pictures, using techniques like frequency space editing, wavelets, or local histograms.

darktable’s equalizer module demonstrates some of this, using the wavelet approach. you can use it to sharpen and denoise, enhance or attenuate certain frequencies in your image, while keeping the edges intact.

darktable is all about providing you the best tools in order to get the most out of your photographies. This blog entry will explain how an existing feature can help you get more detailed exports and it will try to give you a glimpse of what is cooking in an unscheduled but upcoming version of darktable for even better detail preservation.

Make sure to enable High Quality Resampling for exporting your photographies

In darktable, the pixel pipeline is responsible for processing your photography from demosaic up to the point it is passed over to the output subsystem (saving in tiff, jpeg…).

or “If one slider is not enough”

Diligent readers of our small blog series are already aware of the blending feature that darktable offers as part of many modules. Instead of just handing over their result to the subsequent module in pixelpipe, “blending modules” take a moment to reconsider. Based on the blend setting they will take their original output together with their input and do a re-processing. As an example refer to here, where we took blend mode “overlay” with module “lowpass” to do shadow recovery.