A simple SceneKit shader modifier snippet for triplanar texture mapping

SceneKit Triplanar Mapping – Surface Shader Modifier

// Be sure to to set the WrapS and WrapT properties of your texture to .repeat
float4 worldPos = scn_frame.inverseViewTransform * float4(_surface.position, 1.0);
float4 worldNormal = scn_frame.inverseViewTransform * float4(_surface.normal, 0.0);

float3 blending = abs(worldNormal).xyz;
blending = normalize(max(blending, 0.00001)); // Force weights to sum to 1.0
float b = (blending.x + blending.y + blending.z);
blending /= float3(b, b, b);

float scale = 2.0;

float4 xAxisTexture = u_diffuseTexture.sample(u_diffuseTextureSampler, worldPos.yz * scale);
float4 yAxisTexture = u_diffuseTexture.sample(u_diffuseTextureSampler, worldPos.xz * scale);
float4 zAxisTexture = u_diffuseTexture.sample(u_diffuseTextureSampler, worldPos.xy * scale);

float4 triplanarTexture = xAxisTexture * blending.x + yAxisTexture * blending.y + zAxisTexture * blending.z;
_surface.diffuse = triplanarTexture;