// It appears to render from and to the main canvas, just not gray

​

function setup() {

  createCanvas(400, 400, WEBGL)

  pg = new p5.Graphics(width, height, WEBGL, _renderer._pInst)

  pg.noStroke()

  noStroke()

  background('RED')

  circle(50,50,50)

  s = createShader(vert, frag)

  pg.shader(s)

  // shader(s)  // for testing on only main canvas

  s.setUniform('tex0', _renderer)

  pg.rect(0,0,width,height)

​

  texture(pg)

  plane(width, height)

  // rect(0,0,width,height)  // for testing on only main canvas

}

​

vert = `attribute vec3 aPosition;

attribute vec2 aTexCoord;

​

varying vec2 vTexCoord;

​

void main() {

  vTexCoord = aTexCoord;

  vec4 positionVec4 = vec4(aPosition, 1.0);

  positionVec4.xy = positionVec4.xy * 2.0 - 1.0;

  gl_Position = positionVec4;

}`

​

frag = `precision mediump float;

varying mediump vec2 vTexCoord;

​

// passed manually from sketch

uniform sampler2D tex0;

​

float luma(vec3 color) {

  return dot(color, vec3(0.299, 0.587, 0.114));

}

​

void main() {

  vec2 uv = vTexCoord;

  // gets flipped vertically

  uv.y = 1.0 - uv.y;

  vec4 sampledColor = texture2D(tex0, uv);

  float gray = luma(sampledColor.rgb);

  gl_FragColor = vec4(gray, gray, gray, 1);

}`

​