GeometryShader

Geometry shader demonstration for IOS, Swift and Metal

update Jan 14,2020: XCode 11.3.1, Swfit 5

Metal does not offer a geometry shader, so this class supplies one.

Given an array of seed points, this class creates the vertices and indices required to render a multi-segmented tower based at each point.

Class Geometry offers three functions:

update() = copy the GUI control data into the control buffer and call calc..
calcGeometry() = call the geometry shader to produce the vertices & indices
ender() = draw the created geometry

GeometryPoint is provided for each seed point.

struct GeometryPoint {
vector_float3 pos; // 3D coordinate of the base of the tower
vector_float2 angle; // initial direction as and X and Y rotation
float stiffness; // controls how quickly the tower responds to angle changes
};

GeometryControl provides global data that all towers share.

struct GeometryControl {
int nSides; // number of sides of a tower
int nLevels; // number of levels in a tower
int pCount; // number of seed points provided\

vector_float2 deltaAngle;	// how much the tower rotates at each level
vector_float2 desiredAngle; // the angle all towers should be moving to (stiffness affects response time)
float radius;		// the width of the bottom of the tower
float deltaRadius;		// how the width is altered at each level
float dist;			// the vertical distance between tower levels
float deltaDist;		// amount the distance is altered at each level

};

The Geometry shader uses this data to generate the tower vertices and indices:\

kernel void calcGeometryShader
(
device TVertex *vertices [[ buffer(0) ]], // where to store the vertices
device ushort *indices [[ buffer(1) ]], // where to store the indices
device atomic_uint &vcounter [[ buffer(2) ]], // where to store the number of vertices created
device atomic_uint &icounter [[ buffer(3) ]], // where to store the number of indices created
device GeometryPoint *pnt [[ buffer(4) ]], // the seed data for each tower
constant GeometryControl &ctrl [[ buffer(5) ]], // the global control data shared by all towers
uint p [[thread_position_in_grid]])
{

Note: this shader also alters the angle field of each seed point (that's why *pnt is not constant)

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Using the GUI:
All the control widgets work the same way:
Press and hold to either side of center to affect the parameters in the specified direction and speed.

Pinch/drag the screen to control position and rotation.