coin3d-full-screen-quad-pipeline

name	Coin3D Full-Screen Quad Pipeline
description	Reference for implementing full-screen quad raymarchers in Coin3D bypassing the standard view/projection matrices. Required reading before implementing R-006.

Coin3D Full-Screen Quad Pipeline

To render an SDF using a full-screen quad instead of proxy geometry, we draw a single rectangle spanning the viewport and use the camera matrices in the fragment shader to unproject screen coordinates into 3D world rays.

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1. Proxy Geometry (Full-Screen Quad)

We need exactly 4 vertices spanning [-1, 1] in XY. Because Coin3D manages matrices globally, we intercept the vertex shader to place these vertices directly in clip space.

coords = coin.SoCoordinate3()
coords.point.setValues(0, 4, [
    (-1, -1, 0),
    ( 1, -1, 0),
    ( 1,  1, 0),
    (-1,  1, 0)
])

# Use an IndexedFaceSet for two triangles forming the quad
faces = coin.SoIndexedFaceSet()
faces.coordIndex.setValues(0, 7, [0, 1, 2, -1, 0, 2, 3, -1])

---

2. Vertex Shader (Bypassing Matrices)

The vertex shader completely ignores gl_ModelViewProjectionMatrix. It takes the gl_Vertex.xy (which is already [-1, 1]) and passes it directly to gl_Position. It also passes the UV coordinate to the fragment shader.

varying vec2 v_uv;

void main() {
    v_uv = gl_Vertex.xy;
    gl_Position = vec4(gl_Vertex.xy, 0.0, 1.0);
}

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3. Fragment Shader (Unprojection)

In the fragment shader, v_uv gives us the exact Normalized Device Coordinate (NDC) of the pixel [-1, 1]. We use gl_ModelViewProjectionMatrixInverse to convert NDC points into 3D world space.

varying vec2 v_uv;
// ... uniforms ...

void main() {
    // 1. Unproject near plane point (z = -1.0 in NDC is the near plane)
    vec4 ndc_near = vec4(v_uv, -1.0, 1.0);
    vec4 world_near = gl_ModelViewProjectionMatrixInverse * ndc_near;
    world_near /= world_near.w; 

    // 2. Unproject far plane point (z = 1.0 in NDC is the far plane)
    vec4 ndc_far = vec4(v_uv, 1.0, 1.0);
    vec4 world_far = gl_ModelViewProjectionMatrixInverse * ndc_far;
    world_far /= world_far.w;

    // 3. Compute Ray Origin and Direction
    vec3 ro = world_near.xyz;
    vec3 rd = normalize(world_far.xyz - world_near.xyz);

    // ... standard raymarching loop using 'ro' and 'rd' ...

    // Important: Raymarching must check the bounding box intersecting with the ray
    // because the quad itself is infinite. The SDF evaluation loop should still 
    // break early if `p` exits `u_bbox_min` and `u_bbox_max`.
}

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4. Disabling Culling

Make sure to disable backface/frontface culling just in case vertex winding gets flipped.

hints = coin.SoShapeHints()
hints.vertexOrdering.setValue(coin.SoShapeHints.UNKNOWN_ORDERING)

5. Viewport / Camera Context

Because we rely on gl_ModelViewProjectionMatrixInverse, the shader must be placed in a scene graph location where FreeCAD's active camera matrices apply. Returning the SoSeparator containing this setup to vobj.RootNode.addChild() naturally inherits FreeCAD's camera matrices, so no manual camera uniform passing is strictly needed.