Look up 'drawElements' in the OpenGL specification
(page 29 in http://www.opengl.org/documentation/specs/version2.0/glspec20.pdf):

The command

void DrawElements( enum mode, sizei count, enum type, void *indices );

constructs a sequence of geometric primitives using the count elements whose indices are stored in indices. type must be one of UNSIGNED BYTE, UNSIGNED SHORT, or UNSIGNED INT, indicating that the values in indices are indices of GL type ubyte, ushort, or uint respectively. Mode specifies what kind of primitives are constructed; it accepts the same token values as the mode
parameter of the Begin command. The effect of DrawElements (mode, count, type, indices); is the same as the effect of the command sequence

if (mode, count, or type is invalid )
generate appropriate error
else {
Begin(mode);
for (int i = 0; i < count ; i++)
ArrayElement(indices[i]);
End();
}

So, I think you actually want to use 'drawArrays' instead. But without seeing the full source, I'm again just guessing.

I believe 'drawElements' should be used like this (I'm writing this from the top of head, so take it with a grain of salt):

withArray [ (3*i :: GLuint) | i<-[0..count-1] ] $ \p -> drawElements Triangles count UnsignedInt p

Balazs

On Thu, Nov 18, 2010 at 2:53 AM, Ben Christy <ben.christy@gmail.com> wrote:

I am having a issue getting a seg fault with drawElements. Honestly I can not tell where the problem is. It seems as far as all I have read that it should work and being its written in haskell I am asking here first before asking in an opengl chat room.
I init my VBOs with

initModelIBO :: Int → IO BufferObject
initModelIBO listLen = do
   print "list length"
   print listLen
   print "gen ibo bytes"

   print sizeOfList
   [ibo] ← genObjectNames 1 :: IO [BufferObject]
   bindBuffer ElementArrayBuffer $= Just ibo
   tempArray2 ← newListArray (0, listLen - 1) indexList :: IO(StorableArray Int GLuint)

   withStorableArray tempArray2 (λptr ->
   bufferData ElementArrayBuffer $= ((fromIntegral sizeOfList), ptr, StaticDraw))
   bindBuffer ElementArrayBuffer $= Nothing
   return ibo

   where
   elementSize = 4
   sizeOfList = listLen * elementSize
   indexList = [i | i ← [0..(fromIntegral listLen)]] :: [GLuint]

initModelVBO :: [Vert] → IO BufferObject

initModelVBO vertexList = do
   print "list length"
   print listLen
   print "gen vbo bytes"
   print sizeOfList
   [vbo] ← genObjectNames 1 :: IO [BufferObject]

   bindBuffer ArrayBuffer $= Just vbo
   tempArray ← newListArray (0, listLen - 1) vertList :: IO(StorableArray Int GLfloat)
   withStorableArray tempArray (λptr ->
   bufferData ArrayBuffer $= ((fromIntegral sizeOfList), ptr, StaticDraw))

   bindBuffer ArrayBuffer $= Nothing
   return vbo
   where
   elementsPerVert = 10
   vertList = vertsToList vertexList
   listLen = length vertList
   elementSize = 4

   sizeOfList = listLen * elementSize

My Vert type is
data Vert = Vert {
   vertX ::GLfloat,
   vertY ::GLfloat,

   vertZ ::GLfloat,
   --normalX ::GLfloat,
   --normalY ::GLfloat,
   --normalZ ::GLfloat,
   colorR ::GLfloat,

   colorG ::GLfloat,
   colorB ::GLfloat,
   specR ::GLfloat,
   specG ::GLfloat,
   specB ::GLfloat,

   shiny ::GLfloat}
   deriving (Show)

I set vertex attributes with
setAttribPtr (Just program) = do
   print " Setting attrib pointer"

   --vertexAttribPointer (AttribLocation 1) $= (KeepIntegral, (VertexArrayDescriptor 3 Float ((4) *10) (plusPtr nullPtr (0*4))))
   GLRaw.glVertexAttribPointer 1 3 GLRaw.gl_FLOAT 0 stride (plusPtr nullPtr (0))

   vertexAttribArray (AttribLocation 1) $= Enabled
   --vertexAttribPointer (AttribLocation 2) $= (KeepIntegral, (VertexArrayDescriptor 3 Float ((4) *10) (plusPtr nullPtr (3*4))))
   GLRaw.glVertexAttribPointer 2 3 GLRaw.gl_FLOAT 0 stride (plusPtr nullPtr (12))

   vertexAttribArray (AttribLocation 2) $= Enabled
   --vertexAttribPointer (AttribLocation 3) $= (KeepIntegral, (VertexArrayDescriptor 3 Float ((4) *10) (plusPtr nullPtr (6*4))))
   GLRaw.glVertexAttribPointer 3 3 GLRaw.gl_FLOAT 0 stride (plusPtr nullPtr (24))

   vertexAttribArray (AttribLocation 3) $= Enabled

   --vertexAttribPointer (AttribLocation 4) $= (KeepIntegral, (VertexArrayDescriptor 1 Float ((4) *10) (plusPtr nullPtr (9*4))))
   GLRaw.glVertexAttribPointer 4 1 GLRaw.gl_FLOAT 0 stride (plusPtr nullPtr (36))
   vertexAttribArray (AttribLocation 4) $= Enabled
   return ()
   where
   stride = 40

I build a shader program with
buildShader vertexShader fragmentShader = do
   [vertObj] ← genObjectNames 1 ::IO [VertexShader]
   shaderSource vertObj $= [vertexShader]

   compileShader vertObj
   vsLog ← get (shaderInfoLog vertObj)
   print "vertex shader status"
   print vsLog
   [fragObj] ← genObjectNames 1 ::IO [FragmentShader]

   shaderSource fragObj $= [fragmentShader]
   compileShader fragObj
   fsLog ← get (shaderInfoLog fragObj)
   print "fragment shader status"
   print fsLog

   [programObj] ← genObjectNames 1 ::IO [Program]
   attachedShaders programObj $= ([vertObj], [fragObj])
   attribLocation programObj "position" $= AttribLocation 1
   attribLocation programObj "color" $= AttribLocation 2

   attribLocation programObj "spec" $= AttribLocation 3
   attribLocation programObj "shiny" $= AttribLocation 4
   linkProgram programObj
   progLog ← get(programInfoLog programObj)

   print "Shader Program status"
   print progLog
   return (Just programObj)

Finally my render function

instance RenderSimpleSceneGraph Model where

   render matrix (ModernModel vbo ibo shader count) = do
   clientState VertexArray $= Enabled
   version ← get (majorMinor glVersion)
   tempVBO ← vbo
   tempIBO ← ibo

   print "Render Modern Model"
   program ← shader
   currentProgram $= program
   bindBuffer ArrayBuffer $= Just tempVBO
   setAttribPtr program

   bindBuffer ElementArrayBuffer $= Just tempIBO
   print "here"
   drawElements Triangles (fromIntegral count) UnsignedInt nullPtr
   print "here1"

   resetAttribPtr program
   bindBuffer ArrayBuffer $= Nothing
   clientState VertexArray $= Disabled

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