CODEWITHSUNDEEP
javascriptwebglmaskclipstencil-buffer
jonobr1
jonobr1

Reputation: 1093

How to use stenciling to create a 2d clipping mask in webgl?

I've been investigating a way to create a clipping mask similar to this one (done in SVG).

Based on my findings I chose to achieve this through stenciling. My implementation however is grossly incorrect. I'm not entirely sure how gl.stencilOp and gl.stencilFunc work because it seems like I need to render the fragment masking my main content twice. Once before I render the main content and once after with varying parameters.

Here's the working test: https://dl.dropboxusercontent.com/u/1595444/experiments/two.js/issues/issue-56/stencil-buffer/test/clip.html

enter image description here

The relevant snippet / portion of this test can be found in ../src/renderers/webgl.js starting at L67:

if (this._mask) {

  gl.enable(gl.STENCIL_TEST);
  gl.stencilFunc(gl.ALWAYS, 1, 1);

  gl.colorMask(false, false, false, true);
  gl.stencilOp(gl.KEEP, gl.KEEP, gl.INCR);

  // Renders the mask through gl.drawArrays L111
  webgl[this._mask._renderer.type].render.call(
    this._mask, gl, program, this);

  gl.colorMask(true, true, true, true);
  gl.stencilFunc(gl.NOTEQUAL, 0, 1);
  gl.stencilOp(gl.KEEP, gl.KEEP, gl.KEEP);

}

// Renders main content through a series of gl.drawArrays calls
_.each(this.children, webgl.group.renderChild, {
  gl: gl,
  program: program
});

if (this._mask) {

  gl.colorMask(false, false, false, false);
  gl.stencilOp(gl.KEEP, gl.KEEP, gl.DECR);

  // Re-render mask so main content doesn't flicker
  webgl[this._mask._renderer.type].render.call(
    this._mask, gl, program, this);

  gl.colorMask(true, true, true, true);
  gl.stencilFunc(gl.NOTEQUAL, 0, 1);
  gl.stencilOp(gl.KEEP, gl.KEEP, gl.KEEP);

  gl.disable(gl.STENCIL_TEST);

}

Guidance to emulate the webgl stenciling to work like the svg example would be much appreciated.

Upvotes: 2

Views: 2817

Answers (1)

Raziza O
Raziza O

Reputation: 1646

What you need to do is:

  • Draw your stencil area (in your case the blue rectangle),
  • Stop drawing into the stencil
  • Draw your scene which you want to consider the stencil
  • Stop stencil

As follow:

if (this._mask) {
    // Clearing the stencil buffer
    gl.clearStencil(0);
    gl.clear(gl.STENCIL_BUFFER_BIT);

    // Replacing the values at the stencil buffer to 1 on every pixel we draw
    gl.stencilFunc(gl.ALWAYS, 1, 1);
    gl.stencilOp(gl.REPLACE, gl.REPLACE, gl.REPLACE);

    // disable color (u can also disable here the depth buffers)
    gl.colorMask(false, false, false, false);

    gl.enable(gl.STENCIL_TEST);

    // Renders the mask through gl.drawArrays L111
    webgl[this._mask._renderer.type].render.call(this._mask, gl, program, this);

    // Telling the stencil now to draw/keep only pixels that equals 1 - which we set earlier
    gl.stencilFunc(gl.EQUAL, 1, 1);
    gl.stencilOp(gl.KEEP, gl.KEEP, gl.KEEP);
    // enabling back the color buffer
    gl.colorMask(true, true, true, true);
}

// Renders main content through a series of gl.drawArrays calls
_.each(this.children, webgl.group.renderChild, {
   gl: gl,
   program: program
});

// Stop considering the stencil
if (this._mask) {
   gl.disable(gl.STENCIL_TEST);
}

Upvotes: 3

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