如何根据GLSL/WebGL中的某些条件获取像素数？

var framebuffer = gl.createFramebuffer();
gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);

var texture = gl.createTexture();
gl.bindTexture(gl.TEXTURE_2D, texture);
gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, width, height, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, texture, 0);

gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);

void main() {
    if (condition) {
        gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0); // 设置像素颜色为红色
    } else {
        gl_FragColor = vec4(0.0, 0.0, 1.0, 1.0); // 设置像素颜色为蓝色
    }
}

var pixels = new Uint8Array(width * height * 4);
gl.readPixels(0, 0, width, height, gl.RGBA, gl.UNSIGNED_BYTE, pixels);