how can I access buffer memory on vertex shader using openl-es 3.0?

1
votes

I've heard the vertex shader access user's own buffer data(Texture buffer object) using over OpenGL 3.x (using TexelFetch method)

So recently I'v tried to apply TPB technic on OpenglES 3.0 vertex shader with IOS7 but I could not use TBO bcz OpenGLES 3.0 cant supply it.

My vertex shader have to access TBO and use it's data that such as velocities, positions and forces.

I wanna use similer TBO techinc on OpenglES 3.0.

If i use pixel buffer object can I access them using "texelFetch()" method on shader?

How can I figure my work out? Is anybody know a good way?

1
opengl-esvertex-shaderopengl-es-3.0

1 Answers

0
votes

I don't believe you can sample directly from a Pixel Buffer Object.

One obvious option is to use a regular texture instead of a Texture Buffer Object. The maximum texture size of ES 3.0 compatible iOS devices is 4096 (source: https://developer.apple.com/library/iOS/documentation/DeviceInformation/Reference/iOSDeviceCompatibility/OpenGLESPlatforms/OpenGLESPlatforms.html). There are a few sub-cases depending on how big your data is. With n being the number of texels:

  • If n is at most 4096, you can store it in a 2D texture that has size n x 1.
  • If n is more than 4096, you can store it in a 2D texture of size 4096 x ((n + 4095) / 4096).

In both cases, you can still use texelFetch() to get the data from the texture. In the first case, you sample (i, 0) to get value i. In the second case, you sample (i % 4096, i / 4096).

If you already have the data in a buffer, you can store it the texture by binding the buffer as GL_PIXEL_UNPACK_BUFFER before calling glTexImage2D(), which will then source the data from the buffer.

Another option to consider are Uniform Buffer Objects. This allows you to bind the content of a buffer to a uniform block, which then gives you access to the values in the shader. Look up glBindBuffer(), glBindBufferBase(), glBindBufferRange() with the GL_UNIFORM_BUFFER target for details. The maximum size of a uniform block in bytes is given by GL_MAX_UNIFORM_BLOCK_SIZE, and is 16,384 on iOS/A7 devices.