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cur_image

OpenGL glDrawArrays and glDrawElements

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cur_image    100
Hello, I have a job interview the next monday and I'm reviewing some concepts of OpenGL. I have a doubt about two questions that i found on internet and I don't know how to solve them.
Question 1:
[font="Verdana, Arial, Helvetica, sans-serif"][size="2"][b]Quote:[/b] "You have this code:

GLfloat coords[4][12] = {
{ /* coords of an object (a quad) */ },
{ /* ... */ },
{ /* ... */ },
{ /* ... */ }
};

GLfloat colors[4][12] = {
{ /* colors of vertices of first object */ },
{ /* ... */ },
{ /* ... */ },
{ /* ... */ }
};

Matrix4 matrices[4] = { /* ... */ };

for (int i = 0; i < 4; ++i)
{
glPushMatrix();
glLoadMatrixf(matrices[i].pointer());
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_COLOR_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, coords[i]);
glColorPointer(3, GL_FLOAT, 0, colors[i]);
glDrawArrays(GL_QUADS, 0, 4);
glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_COLOR_ARRAY);
glPopMatrix();
}

Optimize it :
Note, Matrix4 class is used as to represent a 4x4 matrix. Its methods can be used to perform matrix operations and data manipulation."[/i][/i][/i][/size][/font]
[font="Verdana, Arial, Helvetica, sans-serif"][size="2"][i][i][i]
[/i][/i][/i][/size][/font]
[font="Verdana, Arial, Helvetica, sans-serif"][size="2"][i][i][i]I think the way to optimize it its using glDrawElements adding an index array. not?[/i][/i][/i][/size][/font]
[font="Verdana, Arial, Helvetica, sans-serif"][size="2"][i][i][i]
[/i][/i][/i][/size][/font]
[font="Verdana, Arial, Helvetica, sans-serif"][size="2"][i][i][i]Question 2:[/i][/i][/i][/size][/font]
[font="Verdana, Arial, Helvetica, sans-serif"][i][i][i][size="2"][b]Quote:[/b] "The following code

GLint vertices[9][2] = {{0, 0}, {2, 1}, {1, 2}, {2, 0}, {0, 1}, {1, 0}, {0, 2}, {1, 1}, {2, 2}};
GLubyte indices[8][3] = {{5, 1, 7}, {4, 2, 6}, {7, 1, 8}, {0, 7, 4}, {5, 3, 1}, {7, 8, 2}, {0, 5, 7}, {4, 7, 2}};
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(2, GL_INT, 0, &vertices[0][0]);
glDrawElements(GL_TRIANGLES, 24, GL_UNSIGNED_BYTE, &indices[0][0]);

draws this:
(0,2) (1,2) (2,2)
+-------+-------+
| /| /|
| / | / |
| / | / |
| / | / |
| / | / |
| / | / |
|/ (1,1)|/ |
(0,1) +-------+-------+ (2,1)
| /| /|
| / | / |
| / | / |
| / | / |
| / | / |
| / | / |
|/ |/ |
+-------+-------+
(0,0) (1,0) (2,0)
a. Describe two problems with this rendering procedure, and why they are problems.
[/size][/i][/i][/i][/font][font="Verdana, Arial, Helvetica, sans-serif"][i][i][i][size="2"]b. How can the code be improved?"[/size][/i][/i][/i][/font]
[font="Verdana, Arial, Helvetica, sans-serif"][size="2"][i]
[/i][/size][/font]
[font=Verdana, Arial, Helvetica, sans-serif][i][i][i][size="2"][size=2]Not idea of the answer for this question. [/size][/size][/i][/i][/i][/font]
[font=Verdana, Arial, Helvetica, sans-serif][i][i][i][size="2"][size=2]
[/size][/size][/i][/i][/i][/font]
[font=Verdana, Arial, Helvetica, sans-serif][i][i][i][size="2"][size=2]P[/size][/size][size="2"][size=2]lease help me to figure out with this two questions.[/size][/size][size="2"][size=2]Thanks.[/size][/size][/i][/i][/i][/font]


[font="Verdana, Arial, Helvetica, sans-serif"][size="2"][i]
[/i][/size][/font]


[font="Verdana, Arial, Helvetica, sans-serif"][i][i][i] [/i][/i][/i][/font]

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mhagain    13430
Now that it's Monday, here's the most obvious place to start with number 1....

[code]Matrix4 matrices[4] = { /* ... */ };

glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_COLOR_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, coords);
glColorPointer(3, GL_FLOAT, 0, colors);

glPushMatrix();

for (int i = 0; i < 4; ++i)
{
glLoadMatrixf(matrices.pointer());
glDrawArrays(GL_QUADS, 0, 4);
}

glPopMatrix();

glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_COLOR_ARRAY);[/code]

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