A timed stroke shader for ARKit iOS 11.4 & 12

TimedStroke.metal

//
//  TimedStroke.metal
//  Delasign
//
//  Created by Oscar De la Hera Gomez on 8/12/18.
//  Copyright © 2018 Delasign. All rights reserved.
//

#include <metal_stdlib>
using namespace metal;
#include <SceneKit/scn_metal>

struct NodeBuffer {
    float4x4 modelTransform;
    float4x4 modelViewTransform;
    float4x4 normalTransform;
    float4x4 modelViewProjectionTransform;
    float2x3 boundingBox;
    
    float4x4 viewTransform;
    float4x4 inverseViewTransform; // view space to world space
    float4x4 projectionTransform;
    float4x4 viewProjectionTransform;
    float4x4 viewToCubeTransform; // view space to cube texture space (right-handed, y-axis-up)
    float4      ambientLightingColor;
    float4      fogColor;
    float3      fogParameters; // x: -1/(end-start) y: 1-start*x z: exponent
    float       time;     // system time elapsed since first render with this shader
    float       sinTime;  // precalculated sin(time)
    float       cosTime;  // precalculated cos(time)
    float       random01; // random value between 0.0 and 1.0
};

typedef struct {
    float3 position [[ attribute(SCNVertexSemanticPosition) ]];
    float2 texCoords0 [[ attribute(SCNVertexSemanticTexcoord0) ]];
    half4 color [[ attribute(SCNVertexSemanticColor)]];
    
} VertexInput;


constant float loopTime = 15; // In Seconds
constant float startTime = 0;
constant float endTime = 10;

struct Vertex
{
    float4 position [[position]];
    float2 uv;
    half4 color;
    float2 texCoords [[user(tex_coords)]];
    float time;
};

vertex Vertex TimedStrokeVertexShader(VertexInput in [[ stage_in ]],
                                         uint vertexID [[vertex_id]],
                                         constant SCNSceneBuffer& scn_frame [[buffer(0)]],
                                         constant NodeBuffer& scn_node [[buffer(1)]])
{
    Vertex vert;
    vert.color = half4( half3(1,1,1), 0.1);
    vert.uv = in.texCoords0;
    vert.time = scn_frame.time;
    vert.position = scn_node.modelViewProjectionTransform * float4(in.position, 1.0);
    return vert;
}



fragment half4 TimedStrokeSurfaceFragment(Vertex in [[stage_in]],
                                     constant SCNSceneBuffer& scn_frame [[buffer(0)]],
                                     constant NodeBuffer& scn_node [[buffer(1)]])
{
    
//    float delayTime = 0;
    
    float currentTime = in.time;
    float timerMilestone = floor(currentTime/loopTime);
    
    float loopTimer = (currentTime - timerMilestone*loopTime);
    
    if ( loopTimer < startTime) {
        // hasnt started
        discard_fragment();
    }
    else if (loopTimer < endTime) {
        // has started
        
        float showUVsIfSmallerThan = (loopTimer-startTime)/(endTime-startTime);
        
        if ((1-in.uv.y) > showUVsIfSmallerThan) {
            // These Are Invalid, remove
            discard_fragment();
        }
        
    }
    else {
        // SHOW ALL
    }
    
    half4 fragColor;
    fragColor.rgb = half3(in.color.rgb);
    fragColor.a = in.color.a;
    return fragColor;
    
}