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Using Photoshop I have converted a photograph into a sketch/line drawing, I achieved this in photoshop by:

  1. Making image greyscale
  2. Duplicating image then inverting it
  3. Applying a color(colour) dodge
  4. Applying a gaussian blur (level 8)

Here is the effect, as you can see there are undesired effects on the cats face, where the fur is dark, I've discovered that because the area is not all pure black, and different variations of black.

How can I fix the image before applying the effects to prevent such artefacts from accruing?

original:

enter image description here

New:

enter image description here

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I've just tried the same steps on one of my own images and I don't get similar pixelation. Did you upscale the original image? What is the original resolution of the photo? –  Bart Arondson Mar 7 at 11:49
    
@BartArondson No the image is not upscaled, the resolution is 2048 x 1153 full size, I've only shown a section of the image above but it is quite a large, clear image. The cat is black, Could it be something to do with black parts of an image? –  AaronParkes Mar 7 at 12:34
2  
My guess is that image quality and / or contrast is the cause of it. That pixelation probably exists in the image, it's just that it's so subtle, that you can't see it normally and this technique reveals it. -- I would throw in another wild guess and say that the cat's cheek is black(dark) and there is hardly any visible hair(detail) in that area in your original image. -- Edit: You just almost confirmed my suspicion with your comment, almost. I think your best bet might be to either fake the hair in that cheek or brush out the pixelated stuff with white soft round brush. –  Joonas Mar 7 at 12:39
    
It might help to try and select just the cheek area ( with some feathering ) and use the same technique, just use bigger gaussian blur amount to try end blend in that pixelation. Or try to blend some of that plain greyscale image into the cheek. –  Joonas Mar 7 at 12:49
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Joonas is right. The original image has some compression artefacts in the dark area. If you open the image in Photoshop and increase the exposure to +8 you'll see the same patterns. I assume this is a simple snap, so you don't have a RAW file of the photo to re-export it with less compression. It's probably better to reword the question to something like How to reduce compression artefacts in dark areas. The process you're applying does not introduce this pixelation (hence not relevant to your problem), it just makes it visible. –  Bart Arondson Mar 7 at 14:52

1 Answer 1

You can try to use the Reduce Noise filter in Photoshop on the original image. So the steps for you would be then:

  1. Convert image to B&W (Layer 1)
  2. Duplicate the image, then invert it (this is Layer 2)
  3. Set the layer blending mode of Layer 2 to Colour Dodge.
  4. Apply a Gaussian Blur to Layer 2.
  5. Select Layer 1, go to Filter -> Noise -> Reduce Noise. You'll get the following window:
    enter image description here
  6. Now tweak the parameters of this filter such that the noise is minimised. Don't forget to check the Remove JPEG Artifact option.
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I will give it a go tonight. –  AaronParkes Mar 7 at 15:29
    
Did not seem to do much other then make the square distortion appear round circles. Will keep trying. –  AaronParkes Mar 10 at 10:21

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