For the colouring, we need some more work (mostly slogging, though). We could stop here, and this is a decent sketch effect. It’s shown here with a white layer behind it. To do this, open the Layers & Channels dialog (if it’s not open already), and add a layer mask to the layer with the inverted edge detect layer ( Edit -> Copy with the highpass layer selected, Add Layer Mask with the inverted edge layer selected, then select the mask and Edit -> Paste) Since we kept the strong edges in the highpass filtered layer, this means that we end up with a rather nice black-on-transparent layer. Invert the Sobel edge detect (you did keep a copy, right?) with Layer -> Colors -> Invert and apply our highpass-filtered copy as a mask. Small trick to get to black-on-transparent. The only problem is it’s white-on-black, when we want black-on-transparent ideally. Just drag the mouse/pen along the bottom of the curves tool. We set the curve type to free (which allows discontinuities), and then for the bottom 3/4 of the curve (or thereabouts) to 0. The easiest way to do this is with the curves tool ( Image -> Colors -> Curves) like this. To get them, we eliminate the edges with small magnitude. We only want the strong edges, otherwise it’ll look crap. Curves window for how to do a highpass filter ¶ Equalised & desaturated Sobel ¶īring out detail with an auto-equalise ( Layer -> Colors -> Auto -> Auto-Equalize) of the sobel edge detect, and convert it to greys using desaturate ( Layer -> Colors -> Desaturate). Straightforward Sobel edge detect ( Filters -> Edge-Detect -> Sobel) of original (don’t forget to save a copy of the original) The Sobel edge detect should be done on the background image (without an alpha channel) rather than a copy of the background (which has an alpha channel). Tutorial on how to make a nice baby & daddy photo into a nice baby & daddy painting.
Text and images Copyright (C) 2002 Dave Neary and may not be used without permission of the author.
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