// draw above
gl.blendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
// draw below
gl.blendFunc(gl.ONE_MINUS_DST_ALPHA, gl.ONE);
// eraser
gl.blendFunc(gl.ZERO, gl.ONE_MINUS_SRC_ALPHA);
// glow/additive
gl.blendFuncSeparate(gl.ONE, gl.ONE, gl.ZERO, gl.ONE);
// tint/multiplicative (not perfect because it tints the pen layer but not the background)
// the shader should add (1-alpha) to the rgb color channels so that transparent things don't become black
gl.blendFuncSeparate(gl.ONE_MINUS_DST_ALPHA, gl.SRC_COLOR, gl.ONE, gl.ONE_MINUS_SRC_ALPHA);