Shader transform controls

Teaching shaders opt into gizmos through // @control annotations. The shared binding in src/lib/utils/shaderTransforms.ts uses annotations rather than lesson IDs or fixed uniform names:

// @control default=0,0,0
uniform vec3 pivot;
// @control transform=translate pivot=pivot readonly=true
uniform mat4 translation;
// @control transform=rotate readonly=true
uniform mat4 rotation;
// @control transform=scale readonly=true
uniform mat4 scaling;

The shader owns the vertex calculation:

vec3 localPosition = (rotation * scaling * vec4(position - pivot, 1.0)).xyz;
vec4 transformedPosition = translation * vec4(pivot + localPosition, 1.0);

pivot=<name> on the translate control names a vec3 uniform or a vec3 readback and places the gizmo at translation * pivot. It only affects gizmo placement; there is no CPU-side compensation, so changing the pivot after rotating or scaling moves the mesh exactly as the shader math says.

A shader can also derive the pivot, for example from fixed choices. An int uniform with options shows a segmented toggle and holds the selected index:

// @control label="Pivot" options="Center,Right"
uniform int pivotChoice;
// @control transform=translate pivot=pivot readonly=true
uniform mat4 translation;

void main() {
  // @readback label="Pivot P"
  vec3 pivot = pivotChoice == 0 ? vec3(0.0) : vec3(1.0, 0.0, 0.0);
  // @readback visualize=point
  vec3 pivotWorld = (translation * vec4(pivot, 1.0)).xyz;
  ...
}

Add target=pivot inverse=translation to the point readback to make a pivot uniform draggable in the viewport.

Only the editor interaction keeps state on the CPU; vertex transformation stays in GLSL. Shaders without pivot annotations retain ordinary T/R/S gizmos.

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