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Brush stroke blending

Brush stroke blending is somewhat of a black art. Which is a shame, since Photoshop's been doing it for more than 15 years now.

Lemme try and explain.

Suppose you've got a pressure-sensitive drawing tablet where the pressure controls the opacity of the brush. If you do a low-pressure stroke, you'd like to have a flat surface of color with roughly the same alpha everywhere (say, alpha 0.5 for half pressure). If you use the usual source-over blend, each of the brush stroke segments would add to the alpha since it's src.a + (1-src.a) * dst.a. As the brush stroke intersects with itself, the alpha builds up all the way to 1. Which is not what you want if you're trying to paint an alpha 0.5 surface.

For a brush stroke with fixed alpha, this would be no problem, you could just vary the blend opacity of the stroke layer. But if you want to have varying alpha inside the stroke layer, you need a different blend. What I was using for hard-edged brushes (and what my Drawmore Touch prototype is using) is MAX blend for the alpha: max(src.a, dst.a). If an alpha 0.5 brush is stamped over a previous alpha 0.5 brush stroke pixel, the result is going to be alpha 0.5. This prevents the stroke layer from accumulating opacity above the brush opacity and makes it possible to do smooth surfaces using pressure-controlled opacity.

But it's broken for soft brushes. With soft brushes you'd like to paint a smooth surface with a soft edge. If you do alpha max, the brush intersections have cross-shaped blending artifacts and it's very difficult to do a smooth surface (you can try with the Drawmore thing: drag left on the brush 100% control to make it 0% hardness, then try to paint a smooth surface. No can do.) GIMP & Krita probably suffer from this too, Photoshop does something more magical.

What I've got in ShaderPaint is my latest attempt at solving this. It does brush stamping and alpha max mixed with source-over clamped to current stamp alpha.

void strokeBlend(vec4 src, float srcA, vec4 dst, out vec4 color)
    // Source-over blend for non-premultiplied alpha.
    color.a = src.a + (1.0-src.a)*dst.a;
    color.rgb = src.rgb*src.a + dst.rgb*dst.a*(1.0-src.a);
    color.rgb /= color.a;

    // Saturate color alpha to brush stamp max alpha.
    // For hard brushes this should be roughly the same as max(dst.a, src.a).
    // For soft brushes, the stroke accumulates alpha up to the brush stamp alpha,
    // which results in a flat stroke area with a smooth edge.
    if (color.a > srcA) {
        color.a = max(dst.a, srcA);

The brush stamping shader is pretty simple. You pass it the last stamp position and the current mouse position (& use them to calculate the last stamp position for the next line segment). The shader then steps along the last stamp -> mouse position -vector with the brush spacing offset. For each of the brush stamp points, accumulate brush color with the in-stroke blending function.

strokeDirection = normalize(strokeDirection);
float stampSeparation = brushRadius * 0.5;
float stampCount = floor(strokeLength / stampSeparation + 0.5);

for (float i = 1.0; i < 200.0; i++) { // Max 200 stamps per segment.
    if (i > stampCount) { // Break once we're done with the stamps.

    // Distance from circular brush.
    float d = length(fragCoord - (lastPoint + strokeDirection*i*stampSeparation)) / brushRadius;

    if (d < 1.0) { // The pixel is inside the brush stamp.
        vec4 src = currentColor;
        // Create a soft border for the brush.
        src.a *= smoothstep(1.0, hardness * max(0.1, 1.0 - (2.0 / (brushRadius))), d);
        strokeBlend(src, opacity, fragColor, fragColor); // Blend the brush stamp into the stroke layer.

Dunno if there's a nicer solution, given the messiness of the blending function.

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Built art installations, web sites, graphics libraries, web browsers, mobile apps, desktop apps, media player themes, many nutty prototypes, much bad code, much bad art.

Have freelanced for Verizon, Google, Mozilla, Warner Bros, Sony Pictures, Yahoo!, Microsoft, Valve Software, TDK Electronics.

Ex-Chrome Developer Relations.