How To: Convert from OpenGL to GLES - opengl

Question

I have the following code...
void draw_polygon(struct vector2d* center, int num_points,
struct vector2d* points, int mode)
{
int i;
if(mode == GL_TRIANGLE_FAN)
glVertex2f(center->x, center->y);
for(i = 0; i < num_points; i++)
glVertex2f(points[i].x, points[i].y);
glVertex2f(points[0].x, points[0].y);
}
and I am trying to convert it so it is OpenGLES 1.1 compat. From other posts I thought it looked like I would need something similar to this...
void draw_polygon(struct vector2d* center, int num_points,
struct vector2d* points, int mode)
{
int i;
int offset=0;
GL_FLOAT *arr;
if(mode == GL_TRIANGLE_FAN)i{
*arr = (GL_FLOAT *)malloc(sizeof(GL_FLOAT) * ((num_points+1)*2));
arr[0]=center->x;
arr[1]=center->y;
offset = 2;
}
else{
*arr = (int *)malloc(sizeof(GL_FLOAT) * (num_points*2)) ;
}
for(i = 0; i < num_points; i++){
int g = i+offset;
arr[g]=points[i].x;
arr[g+1]=points[i].y;
i++;
}
}
But of course that doesn't compile. Can someone help me understand the proper way to handle this in GLES?

Answer 1

Firs about not compiling:
struct vector2d {
GLfloat x, y;
};
void draw_polygon(struct vector2d* center, int num_points,
struct vector2d* points, int mode)
{ //1st GL_FLOAT is not a type, use GLfloat
int i;
int offset=0;
GLfloat *arr;
if(mode == GL_TRIANGLE_FAN) { //what was that "i" doing here
arr = (GLfloat *)malloc(sizeof(GLfloat) * ((num_points+1)*2)); //no "*" before arr
arr[0]=center->x;
arr[1]=center->y;
offset = 2;
}
else{
arr = (GLfloat *)malloc(sizeof(GLfloat) * (num_points*2)) ; //no "*" before arr; Why do you typcast it to "(int *)"?
}
for(i = 0; i < num_points; i++){
int g = (i*2)+offset;//i*2
arr[g]=points[i].x;
arr[g+1]=points[i].y;
i++;
}
}
As for doing the whole code. GLES does not work with begin/end calls but you have to call the draw call. At this point with the code above what you did was create an array of vertex data, now you need to push it to GPU (or set the pointer for the data) and call glDrawArrays.
So you need to add:
glEnableClientState(GL_VERTEX_ARRAY); //this is usually set only once unless you want to disable it at some point for some very unlikelly reason
glVertexPointer(2, GL_FLOAT, 0, arr); //set the pointer (alternative is to use the VBO and copy the data directly to the GPU)
glDrawArrays(mode, 0, num_points+offset/2);
free(arr); //you need to free the allocated memory
in the same function.
To make your method look a bit less complicated you can try this:
void draw_polygon2(struct vector2d* center, int num_points, struct vector2d* points, int mode) {
if(mode == GL_TRIANGLE_FAN) {
vector2d *newBuffer = (vector2d *)malloc(sizeof(vector2d)*(num_points+1));//create a new buffer just to append center to beginning
newBuffer[0] = *center; //copy center
memcpy(newBuffer+1, points, sizeof(vector2d)*num_points); //copy all other points after center
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(2, GL_FLOAT, sizeof(vector2d), newBuffer);
glDrawArrays(mode, 0, num_points+1);
free(newBuffer);
}
else {
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(2, GL_FLOAT, sizeof(vector2d), points);
glDrawArrays(mode, 0, num_points);
}
}
Do note that since this function uses directly vector2d structure it is considered, that x and y are the first 2 parameters in the vector2d structure. Even if you added 'z' as the 3rd parameter the code should work.