Whats the difference between these two OpenGL calls (Basic OpenGL Question)? - opengl-es

Question

It's a really stupid, basic question, but: Can someone please tell me the difference between:
glBegin(GL_QUADS);
glTexCoord3f(0, 0, 0); glVertex3f(0, 1, 0);
glTexCoord3f(1, 0, 0); glVertex3f(1, 1, 0);
glTexCoord3f(1, 1, 0); glVertex3f(1, 0, 0);
glTexCoord3f(0, 1, 0); glVertex3f(0, 0, 0);
glEnd();
and
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
const GLfloat vertexData[] = {
0, 0, 0,
width, 0, 0,
0, height, 0,
width, height, 0
};
const GLfloat texCoords[] = {
0, 0,
1, 0,
0, 1,
1, 1
};
glVertexPointer(3, GL_FLOAT, 0, &vertexData);
glTexCoordPointer(2, GL_FLOAT, 0, &texCoords);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);
glDisableClientState(GL_VERTEX_ARRAY);
(width & height is the texture size in pixel)
I can't really understand why, but if I use the second code (because OpenGL ES compatible) my texture is on the y-axis inverted. What I'm doing wrong?
Edit:
I dont know, if its relevant - maybe i made a mistake at init the viewport for 2D drawing?
GLint iViewport[4];
glGetIntegerv( GL_VIEWPORT, iViewport );
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glOrtho( iViewport[0], iViewport[0]+iViewport[2], iViewport[1]+iViewport[3], iViewport[1], -1, 1 );
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();

Answer 1

Sticking to the simplest, you have 4 vertices with texcoords. Here they are put in parallel:
texcoord/vert
0, 0, / 0, 1
1, 0, / 1, 1
1, 1, / 1, 0
0, 1, / 0, 0
0, 0 / 0, 0
1, 0 / width, 0
0, 1 / 0, height
1, 1 / width, height
Now, if we reorder them by verts, you can see that the texcoords do not correspond
// sorted verts by "alphabetical" position.
0, 1, / 0, 0
0, 0, / 0, 1
1, 1, / 1, 0
1, 0, / 1, 1
0, 0 / 0, 0
0, 1 / 0, height
1, 0 / width, 0
1, 1 / width, height
As you can see, for each equivalent position, the second texture coordinate is inverted between the 2. That explains why the textures are y-flipped.
So if you want to fix it, simply flip them back on the second method:
const GLfloat texCoords[] = {
0, 1,
1, 1,
0, 0,
1, 0
};

Answer 2

Try using the same geometry in each case:
Immediate mode:
glBegin(GL_TRIANGLES);
glTexCoord2f(0, 0, 0); glVertex3f(0, 0, 0);
glTexCoord2f(1, 0, 0); glVertex3f(1, 0, 0);
glTexCoord2f(1, 1, 0); glVertex3f(1, 1, 0);
glTexCoord2f(1, 1, 0); glVertex3f(1, 1, 0);
glTexCoord2f(0, 1, 0); glVertex3f(0, 1, 0);
glTexCoord2f(0, 0, 0); glVertex3f(0, 0, 0);
glEnd();
Vertex arrays:
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
const GLfloat vertexData[] = {
0, 0, 0,
width, 0, 0,
width, height, 0
width, height, 0
0, height, 0,
0, 0, 0,
};
const GLfloat texCoords[] = {
0, 0,
1, 0,
1, 1
1, 1
0, 1,
0, 0,
};
glVertexPointer(3, GL_FLOAT, 0, &vertexData);
glTexCoordPointer(2, GL_FLOAT, 0, &texCoords);
glDrawArrays(GL_TRIANGLES, 0, 6);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);
glDisableClientState(GL_VERTEX_ARRAY);

Answer 3

Ok, try this complete GLUT program. I actually tested it this time around :)
#include <vector>
#include <GL/glut.h>
using namespace std;
size_t win_w = 0;
size_t win_h = 0;
double aspect_ratio = 0;
GLuint tex_obj;
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-10*aspect_ratio, 10*aspect_ratio, -10, 10, -1, 1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, tex_obj);
glPushMatrix();
glScalef(5,5,0);
/*
glBegin(GL_TRIANGLES);
glTexCoord2f(0, 0); glVertex3f(0, 0, 0);
glTexCoord2f(1, 0); glVertex3f(1, 0, 0);
glTexCoord2f(1, 1); glVertex3f(1, 1, 0);
glTexCoord2f(1, 1); glVertex3f(1, 1, 0);
glTexCoord2f(0, 1); glVertex3f(0, 1, 0);
glTexCoord2f(0, 0); glVertex3f(0, 0, 0);
glEnd();
*/
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
const GLfloat vertexData[] = {
0, 0, 0,
1, 0, 0,
1, 1, 0,
1, 1, 0,
0, 1, 0,
0, 0, 0,
};
const GLfloat texCoords[] = {
0, 0,
1, 0,
1, 1,
1, 1,
0, 1,
0, 0,
};
glVertexPointer(3, GL_FLOAT, 0, &vertexData);
glTexCoordPointer(2, GL_FLOAT, 0, &texCoords);
glDrawArrays(GL_TRIANGLES, 0, 6);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);
glDisableClientState(GL_VERTEX_ARRAY);
glPopMatrix();
glFlush();
glutSwapBuffers();
}
void reshape(int w, int h)
{
win_w = w;
win_h = h;
aspect_ratio = (double)win_w / (double)win_h;
glViewport(0, 0, w, h);
}
int main(int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE);
glutInitWindowSize(800,600);
glutCreateWindow("Aspect Ratio");
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glEnable(GL_TEXTURE_2D);
glGenTextures(1, &tex_obj);
glBindTexture(GL_TEXTURE_2D, tex_obj);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
vector< unsigned char > pixels(4 * 3);
pixels[3*0+0] = 255;
pixels[3*0+1] = 0;
pixels[3*0+2] = 0;
pixels[3*1+0] = 0;
pixels[3*1+1] = 255;
pixels[3*1+2] = 0;
pixels[3*2+0] = 0;
pixels[3*2+1] = 0;
pixels[3*2+2] = 255;
pixels[3*3+0] = 255;
pixels[3*3+1] = 255;
pixels[3*3+2] = 255;
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 2, 2, 0, GL_RGB, GL_UNSIGNED_BYTE, &pixels[0]);
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutMainLoop();
return 0;
}