transform feedback。WebGL2で頂点シェーダの出力するvaryingをarrayBufferとして見るだけ。

transformFeedback.html

<canvas hidden></canvas><br>
<textarea id="LG" cols="30" rows="15"></textarea>
<script>"use strict";
const vsrc=`#version 300 es\nin vec4 i;	out vec4 o;	void main(){o=i*2.;}`;
const fsrc=`#version 300 es\nvoid main(){}`;
let cx=document.querySelector("canvas").getContext("webgl2");
let pg=cx.createProgram();const vout=["o"];{
	let vs=cx.createShader(cx.VERTEX_SHADER);cx.attachShader(pg,vs);
	let fs=cx.createShader(cx.FRAGMENT_SHADER);cx.attachShader(pg,fs);
	cx.transformFeedbackVaryings(pg,vout,cx.SEPARATE_ATTRIBS);
	cx.shaderSource(vs,vsrc);cx.compileShader(vs);
	cx.shaderSource(fs,fsrc);cx.compileShader(fs);cx.linkProgram(pg);
	LG.value+=`pg:${cx.getProgramInfoLog(pg)}\n`;
	LG.value+=`vs:${cx.getShaderInfoLog(vs)}\n`;
	LG.value+=`fs:${cx.getShaderInfoLog(fs)}\n`;
}
let bf=[cx.createBuffer(),cx.createBuffer()];{
	cx.bindBuffer(cx.ARRAY_BUFFER,bf[0]);{
		cx.bufferData(cx.ARRAY_BUFFER,8*4*4,cx.STATIC_DRAW);
		let data=new Float32Array(8*4);for(let i=0;i<8*4;i++)data[i]=i;
		cx.bufferSubData(cx.ARRAY_BUFFER,0,data);
		cx.vertexAttribPointer(0,4,cx.FLOAT,false,0,0);
	};cx.bindBuffer(cx.ARRAY_BUFFER,null);
	cx.bindBuffer(cx.ARRAY_BUFFER,bf[1]);{
		cx.bufferData(cx.ARRAY_BUFFER,8*4*4,cx.STATIC_DRAW);
	};cx.bindBuffer(cx.ARRAY_BUFFER,null);
}
let tf=cx.createTransformFeedback();{
	cx.bindTransformFeedback(cx.TRANSFORM_FEEDBACK,tf);{
		cx.bindBufferBase(cx.TRANSFORM_FEEDBACK_BUFFER,0,bf[1]);
		cx.useProgram(pg);cx.beginTransformFeedback(cx.POINTS);{
			cx.enableVertexAttribArray(0);
			cx.enable(cx.RASTERIZER_DISCARD);{
				cx.drawArrays(0,0,8);
			};cx.disable(cx.RASTERIZER_DISCARD);
		};cx.endTransformFeedback();
	};cx.bindTransformFeedback(cx.TRANSFORM_FEEDBACK,null);
}
cx.bindBuffer(cx.ARRAY_BUFFER,bf[1]);{
	let data=new Float32Array(8*4);cx.getBufferSubData(cx.ARRAY_BUFFER,0,data);
	for(let i=0;i<8*4;i+=4){
		for(let j=0;j<4;j++){LG.value+=`${data[i+j]},`};LG.value+=`\n`;
	};LG.value+=`\n`;
};cx.bindBuffer(cx.ARRAY_BUFFER,null);
</script>