Hello Guest

[CLOSED] glGetVertexAttribPointer

  • 1 Replies
  • 2744 Views
*

Offline Krux

  • *
  • 24
[CLOSED] glGetVertexAttribPointer
« on: July 31, 2013, 03:48:14 »
opengl:
Code: [Select]
void glGetVertexAttribPointerv(
    GLuint index,
    GLenum pname,
    GLvoid **pointer);

lwjgl:
Code: [Select]
static java.nio.ByteBuffer glGetVertexAttribPointer(int index, int pname, long result_size)

it is not obvious what to do with the parameter result_size (not documented). Also this method returns a ByteBuffer, but doesn't accept one as parameter. That breaks consistency. And most importantly this method returns null in my code that should never happen.

Sorry for being impolite or making mistakes. Worked all night. Good night.

Edit: I finally found out what the api is meant to do. But this doesn't solve my problem. I want to get the offset within the buffer. So an API like this would be required:
static int glGetVertexAttribPointer(int index, int pname)
« Last Edit: July 31, 2013, 10:22:41 by Krux »

*

Offline spasi

  • *****
  • 2132
    • WebHotelier
Re: [BUG] glGetVertexAttribPointer
« Reply #1 on: July 31, 2013, 16:51:53 »
This method is messy indeed, I'll try to explain.

LWJGL 2.x:

The version that takes the result_size parameter is meant to be used when the vertex attribute is sourced from a buffer in client memory. It retrieves the buffer pointer and creates a ByteBuffer that starts at the pointer address and has a capacity of result_size bytes. You're seeing a null return value because you use it on a vertex attrib that's being sourced from a VBO at offset 0. The native implementation interprets the 0 offset as a null pointer, hence you get a null ByteBuffer back.

The version that takes a ByteBuffer parameter is meant to be used when the vertex attribute is sourced from a VBO. The VBO offset is written at the current position() of the ByteBuffer parameter. You can use it like so:

Code: [Select]
ByteBuffer offsetBuf = BufferUtils.createByteBuffer(PointerBuffer.getPointerSize());
glGetVertexAttribPointer(attribIndex, GL_VERTEX_ATTRIB_ARRAY_POINTER, offsetBuf);
long offset = PointerBuffer.is64Bit() ? offsetBuf.getLong(0) : offsetBuf.getInt(0);

LWJGL 3.0:

The new LWJGL codebase follows the original function interfaces strictly (in general, not only for this method). It also provides lots of utilities for direct memory access, so all 3 alternatives for this method return a pointer value (or offset).

Using a ByteBuffer (cleaned-up version of the above)
Code: [Select]
ByteBuffer offsetBuf = BufferUtils.createByteBuffer(POINTER_SIZE);
glGetVertexAttribPointer(attribIndex, GL_VERTEX_ATTRIB_ARRAY_POINTER, offsetBuf);
long offset = PointerBuffer.get(offsetBuf, 0);

Using a PointerBuffer (this is how it should have been in 2.x if PointerBuffer had been implemented earlier)
Code: [Select]
PointerBuffer offsetBuf = BufferUtils.createPointerBuffer(1);
glGetVertexAttribPointer(attribIndex, GL_VERTEX_ATTRIB_ARRAY_POINTER, offsetBuf);
long offset = offsetBuf.get(0);

Using the "convenient" alternative (this uses an LWJGL-managed buffer internally)
Code: [Select]
long offset = glGetVertexAttribPointer(attribIndex, GL_VERTEX_ATTRIB_ARRAY_POINTER);