Global Variable

SCNShaderModifierEntryPointGeometry

Use this entry point to deform a geometry’s surface or alter its vertex attributes.

Declaration

const SCNShaderModifierEntryPoint SCNShaderModifierEntryPointGeometry;

Discussion

Shader modifiers for this entry point execute in the vertex processing stage.

The geometry entry point declares the following structure:

struct SCNShaderGeometry {
   vec3 position;
   vec3 normal;
   vec4 tangent;
   vec2 texcoords[kSCNTexcoordCount];
} _geometry;

Your shader modifier reads from this structure and writes new values to the same structure to alter the geometric properties of each vertex in a geometry.

The position, normal, and tangent fields are expressed in model space. You can use SceneKit’s uniforms (such as u_modelViewTransform) to operate in a different coordinate space, but you must convert back to model space before writing results.

The kSCNTexcoordCount variable is a constant integer corresponding to the geometry’s number of texture coordinate sources. Each set of coordinates in the texcoords field contains raw values from the geometry—SceneKit applies the contentsTransform transformation (if any) after the geometry shader modifier completes.

The below shader modifier produces an animated sinusoidal deformation:

uniform float Amplitude = 0.1;
 
_geometry.position +=
    _geometry.normal *
    (Amplitude*_geometry.position.y*_geometry.position.x) *
    sin(1.0 * u_time);