glBegin/glEnd to glDrawElements()

Question

I've been trying to port this immediate mode(glBegin/glEnd) code to direct mode(VAs) for rendering a plane. Please let me know if the direct mode code will exactly work as the immediate mode code.

Note: consider a 50X50 mesh Immediate mode code:

int once=0, a=0,b=0;
for(int j=0; j<50-1; j++)
{
 once=0;
 for(int i=0; i<50; i++)
 {
  a=i+j*(50);
  b=i+(j+1)*50;
  if(once)
  {
   glBegin(GL_TRIANGLE_STRIP);
   once=1;
  }
  else
  {
   glTexCoord2f(Texture[a].x, Texture[a].y);
   glVertex2f(Mesh[a].x, Mesh[a].y);
   glTexCoord2f(Texture[a].x, Texture[a].y);
   glVertex2f(Mesh[b].x, Mesh[b].y);
  }
 }
  if(once)
  {
   glEnd();
  }
}

Direct mode code:

unsigned int indexArray[50*50];
int idx=0;
for(int j=0; j<50-1; j++)
{
 for(int i=0; i<50; i++)
 {
  a=i+j*(50);
  b=i+(j+1)*50;
  indexArray[idx]=a;
  indexArray[idx+1]=b;
  idx+=2;
 }
}
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
glEnableClientState(GL_VERTEX_ARRAY);

glTexCoordPointer(2, GL_FLOAT, sizeof(2dPoint), Texture);
glVertexPointer(3, GL_FLOAT, sizeof(2dPoint), Mesh);
glDrawElements(GL_TRIANGLE_STRIP, (50-1)*(50-1)*2, GL_UNSIGNED_INT, indexArray);

glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);

Note: 2dPoint is a structure for 2 floating point values holding x and y

Update After correcting the glVertexPointer() for 2-d co-ordinates. I observed the triangulation happening the following way:

With glBegin()-glEnd():

       /\       /\          /\      /\          /
      /  \     /  \        /  \    /  \        /
\    /    \   /    \      /    \  /    \      /
 \  /      \ /      \    /      \/      \    /
  \/        \        \  /       /\       \  /
  /\       / \        \/       /  \       \/
 /  \     /   \       /\      /    \      /\
/    \   /     \     /  \    /      \    /  \
      \ /       \   /    \  /        \  /    \
\      /         \ /      \/          \/      \
 \    / \         \       /\          /\       \
  \  /   \       / \     /  \        /  \       \
   \/     \     /   \   /    \      /    \       \
   /\      \   /     \ /      \    /      \       \ 
  /  \      \ /       /        \  /        \       \
 /    \      /       / \        \/          \       \
/      \    / \     /   \       /\           \       \
        \  /   \   /     \     /  \           \       \
         \/     \ /       \   /    \           \       \
         /\      /         \ /      \           \       \

With glDrawElements():

       /\       /\          /\      /\          /
      /  \     /  \        /  \    /  \        /
\    /    \   /    \      /    \  /    \      /
 \  /      \ /      \    /      \/      \    /
--\/--------\--------\--/-------/\-------\--/
  /\       / \        \/       /  \       \/
 /  \     /   \       /\      /    \      /\
/    \   /     \     /  \    /      \    /  \
------\-/-------\---/----\--/--------\--/----\
\      /         \ /      \/          \/      \
 \    / \         \       /\          /\       \
  \  /   \       / \     /  \        /  \       \
   \/     \     /   \   /    \      /    \       \
   /\      \   /     \ /      \    /      \       \ 
  /  \      \ /       /        \  /        \       \
-/----\----- \-------/-\--------\/----------\-------\
/      \    / \     /   \       /\           \       \
        \  /   \   /     \     /  \           \       \
         \/     \ /       \   /    \           \       \
         /\      /         \ /      \           \       \

Sorry for the alignment issues in the illustration. but as you can see, with the index array and glDrawElements(), the number of triangles increased. how can i modify the index array to match the winding similar to the results of glBegin()/glEnd()?

At least glVertexPointer line looks wrong, since you only have two floats per vertex but tell OpenGL to read three of them. In general: What is the problem with this code? — BDL
Thank you so much for your response. your answer was absolutely correct. the problem was this piece of code was not rendering the entire mesh like in glBegin()/glEnd(). again thanks for the answer. this resolved the issue. — Sam
The original pattern looks strange to me, since there are non-triangular areas? — BDL
Sorry, as OpenGL ES doesn't support glPolygonMode(), I rendered using GL_LINE_STRIP to understand the pattern. — Sam

BDL BDL · Accepted Answer · 2015-09-30T10:40:53

Since you only have 2d coordinates, the glVertexPointer call is wrong.

glVertexPointer(3, GL_FLOAT, sizeof(2dPoint), Mesh);

This line tell OpenGL to always read 3 floats per vertex, so if you only have 2 of them you have to change it to:

glVertexPointer(2, GL_FLOAT, sizeof(2dPoint), Mesh);
                ^

glBegin/glEnd to glDrawElements()

1 Answers