How to make a smudge brush tool

WebGL Fundamentals

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WebGLFundame · 2025-02-26 · via WebGL Fundamentals

Question:

I need idea how i can make brush who can color smudge.

Example in picture: right side painting with basic brush with two different colors in left also painting but additional use smudge tool, the result should be something like left side

enter image description here

I need advice how i can try to do it

Answer:

Here is one attempt

On mousedown grab a copy of the area under the mouse into a separate canvas
On mousemove draw that copy one pixel at a time from the previous mouse position to the current mouse position at 50% alpha, grabbing a new copy after each move.

In pseudo code

on mouse down
   grab copy of canvas at mouse position
   prevMousePos = currentMousePos

on mouse move
  for (pos = prevMousePos to currentMousePos step 1 pixel) 
    draw copy at pos with 50% alpha
    grab new copy of canvas at pos
  prevMousePos = currentMousePos

The brush is feathered by drawing a rgba(0,0,0,0) to rgba(0,0,0,1) radial gradient over it using globalCompositeOperation = 'destination-out'.

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