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Dark, cloudy lake in the Tatras moutains | by Gael Varoquaux
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Dark, cloudy lake in the Tatras moutains

On a rainy evening, the "Popradské Pleso" lake in the Tatras mountains, Slovakia.


This shot was taken with a mobile phone, as my camera broke down at the very beginning of this trip. I locked the ISO setting of the phone to 100 ISO to try to get as much as possible out of the camera phone, and then resorted to post-processing to try to get a quality image.


For post-processing, I used darktable. I used non-local means denoising, and adjusted the tone curves to have a strong highlight in the center of the image. I use both the equalizer filter (in clarity mode) and the local contrast filter, with masks selecting the center of the image, to increase the contrast gradually toward the center. My goal was to make more visible the different layers of mountains.


Then, I worked a lot on the lower part of the image, the lake, to enhance the highlights in the reflection. For this, I started with a strong modification of the tone curves. However, this created a lot of noise as the sensor of the phone is not too good in the shadows. To fight this noise, applied a set of filters only on the lower part of the image: I used a stronger non-local means that I combined with a wavelet-based denoising (using the "profiled" denoising filter of darktables) as these have different types of artifacts. To cancel out the artifacts, I added to these a low-pass filter that smooths out the image. On top of this, I added artificial grain, to restore the grain of the image destroyed while smoothing.


Finally, I worked a bit on the clouds. I wanted to crop the top of the image, in order to focus more on the horizon line, but I was interested in the shapes of the cloud. So I used a liquify filter to warp these clouds down. Last, in a minor and classic trick, I created a vignetting-like effect by making the top and left clouds darker with a tone-curve filter.


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Taken on August 17, 2017