Recently after scanning in some of my sketches I've been using the liquify tool in Photoshop to modify and test concepts for characters. While this technique may not be art 101 or considered as 'cheating' for traditional artists it's a great way to get quick character ideas from 1 basic sketch. I see it as a similar thing to using a basemesh or a Squirkle in Zbrush for some quick character concepts (sculpting and moving the original mesh).

I don't think Liquify needs much of a tutorial, the icons are self explanatory and you will find it under the filters menu (near the top). Alternatively if you want a shortcut for it --> Shift+Ctrl+x by default.

For this example I scanned in a sketch I did a while ago; After doing some basic colouring for this an extremely rough blurred skin highlight and lowlight and a blue shadow layer, gaussian blurred and then flattened. I bulked the character out a bit and raised his nose, mouth and shrank his ear a little to help with the proportions. Following on from this I tried a few more things, a bit more extreme but just to show the range of characters you can create. These were both done extremely quickly so things like the collar are still distorted but are just examples; While I wouldn't recommend this for final work it's great to just get a few ideas going or work on expressions or proportions of characters. I'd personally recommend just sticking with the move brush for bulking out characters as the bloat or pucker tools can quickly get out of hand - use the reconstruct tool rather than the global reconstruct option to bring major distortions back. Worth giving a go anyway!
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