ios - MTLBuffer の部分を描画しますか?

Question

この呼び出しでバッファからポイントフラグメントをレンダリングしています:

renderEncoder.drawPrimitives(type: .point,
                             vertexStart: 0,
                             vertexCount: 1,
                             instanceCount: emitter.currentParticles)

emitter.currentParticlesバッファ内の粒子の総数です。どうにかしてバッファの一部だけを描画することはできますか?

私はこれを試しましたが、バッファの前半を描画します:

renderEncoder.drawPrimitives(type: .point,
                             vertexStart: emitter.currentParticles / 2,
                             vertexCount: 1,
                             instanceCount: emitter.currentParticles / 2)

実際、vertexStart効果はないようです。任意の値に設定できるように見えますが、それでも 0 から始まります。

編集：

パイプライン構成:

private func buildParticlePipelineStates() {
    do {
        guard let library = Renderer.device.makeDefaultLibrary(),
        let function = library.makeFunction(name: "compute") else { return }

        // particle update pipeline state
        particlesPipelineState = try Renderer.device.makeComputePipelineState(function: function)

        // render pipeline state
        let vertexFunction = library.makeFunction(name: "vertex_particle")
        let fragmentFunction = library.makeFunction(name: "fragment_particle")
        let descriptor = MTLRenderPipelineDescriptor()
        descriptor.vertexFunction = vertexFunction
        descriptor.fragmentFunction = fragmentFunction

        descriptor.colorAttachments[0].pixelFormat = renderPixelFormat
        descriptor.colorAttachments[0].isBlendingEnabled = true
        descriptor.colorAttachments[0].rgbBlendOperation = .add
        descriptor.colorAttachments[0].alphaBlendOperation = .add
        descriptor.colorAttachments[0].sourceRGBBlendFactor = .sourceAlpha
        descriptor.colorAttachments[0].sourceAlphaBlendFactor = .sourceAlpha
        descriptor.colorAttachments[0].destinationRGBBlendFactor = .oneMinusSourceAlpha
        descriptor.colorAttachments[0].destinationAlphaBlendFactor = .oneMinusSourceAlpha

        renderPipelineState = try
        Renderer.device.makeRenderPipelineState(descriptor: descriptor)
        renderPipelineState = try Renderer.device.makeRenderPipelineState(descriptor: descriptor)
    } catch let error {
        print(error.localizedDescription)
    }
}

頂点シェーダー:

struct VertexOut {
    float4 position   [[ position ]];
    float  point_size [[ point_size ]];
    float4 color;
};


vertex VertexOut vertex_particle(constant float2 &size [[buffer(0)]],
                             device Particle *particles [[buffer(1)]],
                             constant float2 &emitterPosition [[ buffer(2) ]],
                             uint instance [[instance_id]])
{
    VertexOut out;

    float2 position = particles[instance].position + emitterPosition;
    out.position.xy = position.xy / size * 2.0 - 1.0;
    out.position.z = 0;
    out.position.w = 1;
    out.point_size = particles[instance].size * particles[instance].scale;
    out.color = particles[instance].color;
    return out;
}

fragment float4 fragment_particle(VertexOut in [[ stage_in ]],
                              texture2d<float> particleTexture [[ texture(0) ]],
                              float2 point [[ point_coord ]]) {
    constexpr sampler default_sampler;

    float4 color = particleTexture.sample(default_sampler, point);

    if ((color.a < 0.01) || (in.color.a < 0.01)) {
        discard_fragment();
    }
    color = float4(in.color.xyz, 0.2 * color.a * in.color.a);
    return color;
}