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This screen shot depicts a problem that I am experiencing with haze removal in Affinity Photo 1.7.1:


Before haze removal the blue tone of the sky was uniform; after dehazing three distinct tones are visible.  The parts of the sky that are visible between parts of the crane and immediately around it are lightest and closest to the original tone.  Left and right of that area the sky has a saturated look.  The blue tone in the right corner of the picture has an unnatural appearance.

Unfortunately, I neglected to make a copy of the original Background pixel layer before dehazing.  My questions about the haze removal in Affinity Photo concern the algorithm, and how I can help it to avoid mishandling the sky.  I have other examples of this, e. g., two exhaust chimneys on a building in the foreground with the sky between them much lighter than the sky to the left and right after dehazing.

  1. What dehazing algorithm is Affinity Photo using?  I suspect it is based on so-called dark channel priors.  Such algorithms are known to struggle with expanses of sky.  Various refinements to the basic algorithm have been proposed to mitigate the sky problem.  I have accepted the default values suggested by the Haze Removal filter.  I realize I can decrease the strength, but the differences in the sky tone remain unless the strength is decreased nearly to zero.
  2. When I develop my RAW photos in the Develop Persona of Affinity Photo I open them with Tone curve set to "Take no action" in the Develop Assistant, then I increase the RAW latitude by pushing the black point a bit to the right of the histogram (ca. 5%), pushing the midtones towards the middle by increasing the brightness, increasing or decreasing the exposure, shadows and highlights as required, and adding a bit of sharpening.  Adding contrast, saturation, clarity, etc. and, importantly, a tone curve, I leave for the Photo Persona.  I have the feeling that such a "flat" image might be creating problems for the dehazing algorithm, and I should perhaps either add a tone curve in the Develop Persona and dehaze the Background layer in the Photo Persona, or do that plus a Merge Visible in the Photo Persona and dehaze the pixel layer produced by Merge Visible.

Comments or suggestions very welcome.

Richard Liu

MacBook Pro 16" 2021 M1 Max & 64 GB memory | macOS Sonoma 14.5 | BenQ SW271 | Affinity Photo 2.5.2

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On 6/28/2019 at 10:03 AM, Richard Liu said:

 I have the feeling that such a "flat" image might be creating problems for the dehazing algorithm

I don't know if it's related to a "flat" image, but I've found that quite small adjustments to a developed image can make a big difference to the dehazing.  I too would like to know how it works (and why it doesn't).

AP, AD & APub user, running Win10

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