[LUAJit] ffi and pointer math

[Kaj Eijlers <bizziboi@...>]

Hi,

God I love LuaJit. Sliced bread can finally retire.

However.....

 

I am trying to do some openGL, which works like a charm, but I've ran into a problem. I'm sure it's me but I can't figure it out.

 

I need to pass in a pointer to an array of structs, which poses no problem,

 

  typedef struct
  {
   float u;
   float v;
  } TexCoords;
  typedef struct
  {
   float x;
   float y;
  } Positions;
  typedef unsigned int Colors;
  typedef struct
  {
   Positions Position;
   TexCoords TexCoord;
   Colors Color;
  } PTC;
 
Creating an array of those with

 

local verts = ffi.new("PTC[3]")

local vertOffs = verts

 

sending the pointer off:

  gl.glVertexPointer(2, gl.GL_FLOAT, ffi.sizeof("PTC"), vertoffs);

Works like a charm, the sizeof is the stride, vertoffs the pointer (although I am not sure if I created a copy when I assigned to 'verts')

 

Now I need to pass a pointer to a member inside PTC, in this case TexCoords which would be at offset 8.

Try as I might I can't create a pointer to it.

 

I tried

local tex = verts + 8 but that doesn't work

I tried

local ptr = ffi.new("unsigned char*")

ptr = verts

ptr = ptr + 8

 

But it doesn't do what I expect.

 

So my question...if I have a pointer to data (as verts is if I understand correctly), how do I add a value to the pointer?

 

Any pointers (har har) appreciated.

 

Kaj