How can I draw a triangle with a VBO? - c

Question

I am trying to get my VBO to draw, but I can't see anything. I am attempting to draw a single triangle (seems to me that a single triangle is a good start in the right direction). Everything compiles and runs without breaking.
void initGraphics(int width, int height) {
glViewport(0, 0, width, height);
glEnable(GL_TEXTURE_2D);
glEnable(GL_BLEND);
glDisable(GL_DEPTH_TEST);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glClearColor(1.0, 1.0, 1.0, 1.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0.0, width, height, 0.0);
glMatrixMode(GL_MODELVIEW);
}
GLuint vboId;
void initVbo(void) {
GLsizei dataSize;
GLfloat* vertices;
int vCount = 3;
dataSize = sizeof(GLfloat) * 3 * vCount;
vertices = (GLfloat*)malloc(dataSize);
vertices[0] = 0;
vertices[1] = 0;
vertices[2] = 0;
vertices[3] = 100;
vertices[4] = 0;
vertices[5] = 0;
vertices[6] = 100;
vertices[7] = 100;
vertices[8] = 0;
glewInit();
glGenBuffersARB(1, &vboId);
glBindBufferARB(GL_ARRAY_BUFFER_ARB, vboId);
glBufferDataARB(GL_ARRAY_BUFFER_ARB, dataSize, vertices, GL_STATIC_DRAW_ARB);
free(vertices);
//glDeleteBuffersARB(1, &vboId); // edit #1
}
unsigned int indices[] = { 0, 1, 2 }; // edit #3
void drawVbo(void) {
glClear(GL_COLOR_BUFFER_BIT);
glClearColor(1.0, 1.0, 1.0, 1.0);
glBindBufferARB(GL_ARRAY_BUFFER_ARB, vboId);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, 0);
//edit #1, #2
glDrawElements(GL_TRIANGLES, 3 /*1*/, /*GL_UNSIGNED_BYTE*/ GL_UNSIGNED_INT, &indices);
glDisableClientState(GL_VERTEX_ARRAY);
glBindBufferARB(GL_ARRAY_BUFFER_ARB, 0);
}
These are the functions that are getting called. I am running on windows and I am avoiding posting a bunch of windows related code...
EDIT #1
Modified the code to reflect genpfault's suggestions.
EDIT #2
Modified the code to reflect Nicol Bolas' suggestions.
COMMENT #1
The following code works (just to prove projections are set up correct):
glBegin(GL_TRIANGLES);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(100.0, 0.0, 0.0);
glVertex3f(100.0, 100.0, 0.0);
glEnd();
EDIT #3
Modified the code to reflect Nicol Bolas' suggestions.
UPDATE #1
The modified code now works. Although I am curious how to get glDrawArrays to work properly...my implementation looked like:
glDrawArrays(GL_TRIANGLES, 0, 0);
This doesn't seem right to me, but the spec says:
mode: Specifies what kind of primitives to render. Symbolic constants GL_POINTS, GL_LINE_STRIP, GL_LINE_LOOP, GL_LINES, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN, GL_TRIANGLES, GL_QUAD_STRIP, GL_QUADS, and GL_POLYGON are accepted.
first: Specifies the starting index in the enabled arrays.
count: Specifies the number of indices to be rendered.
Based on what Nicol Bolas was saying, since I shouldn't need indices, 0, 0 make sense as arguments. Right?

Answer 1

glGenBuffersARB(1, &vboId);
glBindBufferARB(GL_ARRAY_BUFFER_ARB, vboId);
glBufferDataARB(GL_ARRAY_BUFFER_ARB, dataSize, vertices, GL_STATIC_DRAW_ARB);
free(vertices);
glDeleteBuffersARB(1, &vboId); // wat
After the glDeleteBuffersARB() call the pointed-to VBO ID(s) are invalidated and can't be used in glBindBufferARB().
Also:
glDrawElements(GL_TRIANGLES, 1, GL_UNSIGNED_BYTE, &indices);
You've defined indices as a unsigned int so you should use GL_UNSIGNED_INT instead of GL_UNSIGNED_BYTE.

Answer 2

glDrawElements(GL_TRIANGLES, 1, GL_UNSIGNED_INT, &indices);
Triangles, as you may be aware, are made of three vertices. You are sending one (the second parameter). You can't draw a triangle from one position.