Is vertex attrubute pointer persistent in OpenGL ES?

Question

Imagine scenario when have two GLSL programms A and B that are called one after another. If to set uniform variable of the program A it's value stays the same and requires no initialization before every draw call of the program. So it can be considered as a "member" of the program (in terms of OOP).

But what about attribute values or when vertex attribute is set as pointer?

gman · Answer 1 · 2018-03-29T23:16:27.170

You tagged your question as OpenGL ES and WebGL. In both of those attributes are global. Their state is not connected to any GLSL programs.

You can think of it kind of like this

glState = {
  attributes: [
    { enabled: false, type: gl.FLOAT, size: 4, stride:0, offset: 0, buffer, null, },
    { enabled: false, type: gl.FLOAT, size: 4, stride:0, offset: 0, buffer, null, },
    { enabled: false, type: gl.FLOAT, size: 4, stride:0, offset: 0, buffer, null, },
    { enabled: false, type: gl.FLOAT, size: 4, stride:0, offset: 0, buffer, null, },
    ...
  ]
};

gl.enableVertexAttribArray, gl.disableVertexAttribArray, and gl.vertexAtrribPointer effect that global attribute state. More details here.

There is an extension on ES 2.0 and WebGL for Vertex Array Objects. That extensions allows the combine state of all attributes to be set and stored on Vertex Array Objects or "VAO"s. There is still global state as well which for the most part can be considered the default VAO.

You can consider that to work like this

glState = {
  defaultVAO = {
    attributes: [
      { enabled: false, type: gl.FLOAT, size: 4, stride:0, offset: 0, buffer, null, },
      { enabled: false, type: gl.FLOAT, size: 4, stride:0, offset: 0, buffer, null, },
      { enabled: false, type: gl.FLOAT, size: 4, stride:0, offset: 0, buffer, null, },
      { enabled: false, type: gl.FLOAT, size: 4, stride:0, offset: 0, buffer, null, },
      ...
    ]
  },
  currentVAO = defaultVAO
};

Now gl.enableVertexAttribArray, gl.disableVertexAttribArray, and gl.vertexAtrribPointer affect the attributes of currentVAO in the above example. Calling createVertexArrayOES creates a new VAO. Calling bindVertexArrayOES sets currentVAO in the above pseudo code to your new VAO. Calling bindVertexArrayOES with null sets currentVAO back to the default one.

In ES 3.0 VAOs are always available (in other words they are no longer an extension, they're part of the basic set of features.)

NOTE: VAOs are easy to emulate. There's an emulation library here so you can use VAOs anywhere in WebGL. If they are supported the emulation library will use the native ones. If they aren't it will emulate them.

The link for the OESVertexArray.js is invalid, and says not to hotlink files. — Sri Harsha Chilakapati, Jun 02 '15 at 15:57
This is the new link: https://github.com/KhronosGroup/WebGL/blob/master/sdk/demos/google/resources/OESVertexArrayObject.js — Sri Harsha Chilakapati, Jun 02 '15 at 15:59
in your pseudocode what is the difference between glState and gl ? — oceanGermanique, May 29 '21 at 12:02

score 0 · Answer 2 · answered Dec 05 '14 at 13:12

In OpenGL prior to 3.2 Core, attribute bindings are handled by the underlying state machine and are global. This means that they stay the same as long as noone modifies them.

Beginning with OpenGL 3.2 Core Profile these attribute settings are no longer global, but get stored in a VAO (Vertex Array Object). Again each VAO updates it's state when it is active, and stores the last know state when it gets deactivated.

In both cases, attribute bindings do not have any connection to the currently bound shader.

Is vertex attrubute pointer persistent in OpenGL ES?

2 Answers2