opengl - glBegin/glEnd to glDrawElements() -


i've been trying port immediate mode(glbegin/glend) code direct mode(vas) rendering plane. please let me know if direct mode code work immediate mode code.

note: consider 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 structure 2 floating point values holding x , y

update after correcting glvertexpointer() 2-d co-ordinates. observed triangulation happening following way:

with glbegin()-glend():

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

with gldrawelements():

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

sorry alignment issues in illustration. can see, index array , gldrawelements(), number of triangles increased. how can modify index array match winding similar results of glbegin()/glend()?

since have 2d coordinates, glvertexpointer call wrong. 

glvertexpointer(3, gl_float, sizeof(2dpoint), mesh);

this line tell opengl read 3 floats per vertex, if have 2 of them have change to:

glvertexpointer(2, gl_float, sizeof(2dpoint), mesh);                 ^

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