CODEWITHSUNDEEP
webglwebgl2
Himanshu Jotwani
Himanshu Jotwani

Reputation: 406

webGL gl_Position value saving outside shaders

I have vertex shaders for calculation something like this

 gl_Position =
      vec4(a.x * (0.5 - b.x) * v -
               a.y * b.y * u - offset * offsetMul * u,
           0, 0) +
      transform * vec4(position, 0, 1);

I need to save a particular value of gl_Position for later use outside the shaders. Is there any way to save the coordinates to be used outside the shaders?

Upvotes: 2

Views: 629

Answers (1)

user128511
user128511

Reputation:

Saving gl_Position itself is not possible AFAICT.

Saving "a particular" value doesn't sound possible either.

Saving all values is possible. You either can either render the positions

attribute vec4 position;    // your data
attribute float a_count;    // count 0, 1, 2, 3
uniform vec2 u_resolution;  // resolution of output texture

// vertex shader
varying vec4 v_position;

void main() {
  v_position =
      vec4(a.x * (0.5 - b.x) * v -
               a.y * b.y * u - offset * offsetMul * u,
           0, 0) +
      transform * vec4(position, 0, 1);
  vec2 pixelCoord = vec2(
      mod(a_count, resolution.x);
      floor(a_count / resolution.x));
  vec2 clipSpaceCoord = (pixelCoord + 0.5) / resolution * 2.0 - 1.0;

  gl_Position = vec4(clipSpaceCoord, 0, 1);
}

// fragment shdaer
precision highp float
varying vec4 v_position;
void main() {
  gl_FragColor = v_position;
}

In the case above we'd make a buffer filled with a count 0, 1, 2, 3 etc to supply data to a_count above. You'd make a floating point texture (after enabling OES_texture_float and EXT_color_buffer_float, attach to a framebuffer, then render to that framebuffer. The results would be in the texture

Note that it would probably be a lot faster to put the position data itself in a texture and do more normal GPGPU stuff. One example at the bottom here

In WebGL2 you can also use transform feedback. Transform feedback writes the output of the vertex shader to one or more buffers. In WebGL2, using shader version 300 es, you declare varyings in a vertex shader as out instead of varying. Those out values (so like above, not gl_Position itself but your own declared outputs can be written to a buffer.

Upvotes: 4

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