Lindsay Kay

BioDigital Tech Talk

July 14, 2017

Silhouettes

Emphasis

for

in

WebGL

Silhouettes: Stencil Technique

  1. Draw non-emphasized objects
  2. Enable stencil testing
  3. Draw emphasized objects
  4. Configure to not render on top of pixels already in stencil buffer
  5. Draw emphasized objects again, with flat color, expanding mesh along vertex normals

Step (3)

Step (5)

Combined result

Silhouettes: Stencil Technique (2)

We expand along the normals because simply scaling an object will create a uneven silhouette:

Silhouettes: a small enhancement

Expand each outline vertex by amount proportional to its depth

Silhouettes: The JavaScript

// Render object using standard shader

gl.clearStencil(0);
gl.clear(GL_STENCIL_BUFFER_BIT);
gl.enable(GL_STENCIL_TEST);
gl.stencilFunc(GL_ALWAYS, 1, -1);
gl.stencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);

drawObject();

// Render glowing, expanded version of object

gl.stencilFunc(GL_NOTEQUAL, 1, -1);
gl.stencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);

drawObject();

Silhouettes: The GLSL

attribute   vec4 position;
uniform     mat4 modelMat;
uniform     mat4 viewMat;
uniform     mat4 projMat;
attribute   vec3 normal;
uniform     float outlineWidth;

void main(void) {
    vec4 projPos = projMat * viewMat * modelMat * vec4(position.xyz, 1.0);
    vec3 offset = (normal * (outlineWidth * (projPos.z/1.0)));
    vec4 worldVertex = modelMat * vec4(modelVertex.xyz + offset, 1.0);
    mat4 viewMatrix = viewMat;
    gl_Position = projMat * (viewMatrix * worldVertex);
}
uniform vec3  outlineColor;
void main(void) {
    gl_FragColor = vec4(outlineColor, 1.0);
}

Vertex shader:

Fragment shader:

Silhouettes: a small hitch for some objects

  • Mesh normals must be smooth, otherwise there will be gaps in the silhouette at hard edges
  • Not a problem for organic shapes, though

Silhouettes: Drawing halo as wireframe

  • Draw halo as thick wireframe
  • Solves broken outline on hard edges
  • Problem: WebGL on Windows has limited wire thickness

Standard shader (1)

Thick wireframe (5)

Result

Silhouettes: Status in Human