In general, without a fairly new GPU feature called variable rasterization rate in Metal or variable rate shading elsewhere, you can’t make one object in a scene run its fragment shader at a different resolution than the rest of the scene.
For this case, depending on what your setup is, you might be able to use SCNTechnique to render the plane in a separate pass at a different resolution, then composite that back into your scene, in the same way some game engines render particles at a lower resolution to save on fill rate. Here’s an example.
First, you’ll need a Metal file in your project (if you already have one, just add to it), containing the following:
#include <SceneKit/scn_metal>
struct QuadVertexIn {
float3 position [[ attribute(SCNVertexSemanticPosition) ]];
float2 uv [[ attribute(SCNVertexSemanticTexcoord0) ]];
};
struct QuadVertexOut {
float4 position [[ position ]];
float2 uv;
};
vertex QuadVertexOut quadVertex(QuadVertexIn v [[ stage_in ]]) {
QuadVertexOut o;
o.position = float4(v.position.x, -v.position.y, 1, 1);
o.uv = v.uv;
return o;
}
constexpr sampler compositingSampler(coord::normalized, address::clamp_to_edge, filter::linear);
fragment half4 compositeFragment(QuadVertexOut v [[ stage_in ]], texture2d<half, access::sample> compositeInput [[ texture(0) ]]) {
return compositeInput.sample(compositingSampler, v.uv);
}
Then, in your SceneKit code, you can set up and apply the technique like this:
let technique = SCNTechnique(dictionary: [
"passes": ["drawLowResStuff":
["draw": "DRAW_SCENE",
// only draw nodes that are in this category
"includeCategoryMask": 2,
"colorStates": ["clear": true, "clearColor": "0.0"],
"outputs": ["color": "lowResStuff"]],
"drawScene":
["draw": "DRAW_SCENE",
// don’t draw nodes that are in the low-res-stuff category
"excludeCategoryMask": 2,
"colorStates": ["clear": true, "clearColor": "sceneBackground"],
"outputs": ["color": "COLOR"]],
"composite":
["draw": "DRAW_QUAD",
"metalVertexShader": "quadVertex",
"metalFragmentShader": "compositeFragment",
// don’t clear what’s currently there (the rest of the scene)
"colorStates": ["clear": false],
// use alpha blending
"blendStates": ["enable": true, "colorSrc": "srcAlpha", "colorDst": "oneMinusSrcAlpha"],
// supply the lowResStuff render target to the fragment shader
"inputs": ["compositeInput": "lowResStuff"],
// draw into the main color render target
"outputs": ["color": "COLOR"]]
],
"sequence": ["drawLowResStuff", "drawScene", "composite"],
"targets": ["lowResStuff": ["type": "color", "scaleFactor": 0.5]]
])
// mark the plane node as belonging to the category of stuff that gets drawn in the low-res pass
myPlaneNode.categoryBitMask = 2
// apply the technique to the scene view
mySceneView.technique = technique
With a test scene consisting of two spheres with the same texture, and the scaleFactor set to 0.25 instead of 0.5 to exaggerate the effect, the result looks like this.
If you’d prefer sharp pixelation instead of the blurrier resizing depicted above, change filter::linear to filter::nearest in the Metal code. Also, note that the low-res content being composited in is not taking into account the depth buffer, so if your plane is supposed to appear “behind” other objects then you’ll have to do some more work in the compositing function to fix that.
