CODEWITHSUNDEEP
c++opengl
Brandon
Brandon

Reputation: 23495

Something wrong with my VBO

I'm trying to emulate exactly how a game sets up a VBO and draws it to the screen. I've never set one up before and the tutorials all show how to do it with glDrawArrays but I want to use glDrawElements.

I came up with the following:

glViewport(0, 0, 765, 553);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, 765, 553, 0.0, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

xCast(ptr_glActiveTextureARB, ptr_wglGetProcAddress("glActiveTextureARB"));
xCast(ptr_glMultiTexCoord2fARB, ptr_wglGetProcAddress("glMultiTexCoord2fARB"));
xCast(ptr_glGenBuffersARB, ptr_wglGetProcAddress("glGenBuffersARB"));
xCast(ptr_glBindBufferARB, ptr_wglGetProcAddress("glBindBufferARB"));
xCast(ptr_glBufferDataARB, ptr_wglGetProcAddress("glBufferDataARB"));

struct PointInfo
{
    float Pos[3];
    float Colour[3];
};

const int NumVerts = 3, NumInds = 3;
std::vector<PointInfo> Vertices;
Vertices.push_back({{0.0f, 1.0f, 0.0f}, {1, 1, 1}}); ///top left;
Vertices.push_back({{0.5f, 0.0f, 0.0f}, {1, 1, 1}}); ///bottom middle;
Vertices.push_back({{1.0f, 1.0f, 0.0f}, {1, 1, 1}}); ///top right;

std::vector<std::uint32_t> Indices = {0, 1, 2};

std::uint32_t VBO = 0, IBO = 0;
ptr_glGenBuffersARB(1, &VBO);
ptr_glGenBuffersARB(1, &IBO);

///Put Vertices In.
ptr_glBindBufferARB(GL_ARRAY_BUFFER, VBO);
ptr_glBufferDataARB(GL_ARRAY_BUFFER, sizeof(PointInfo) * NumVerts, &Vertices[0], GL_STATIC_DRAW);
Log(glGetError());

///Put Indices In.
ptr_glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER, IBO);
ptr_glBufferDataARB(GL_ELEMENT_ARRAY_BUFFER, sizeof(int) * NumInds, &Indices[0], GL_STATIC_DRAW);
Log(glGetError());

I run the above only once at the start of my program. Then in my while loop, I run:

glPushMatrix();
glClearColor(0.0f, 1.0f, 1.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
Log(glGetError());

ptr_glBindBufferARB(GL_ARRAY_BUFFER, VBO);
Log(glGetError());
glVertexPointer(3, GL_FLOAT, sizeof(PointInfo), (void*) offsetof(PointInfo, Pos));
Log(glGetError());
glColorPointer(3, GL_FLOAT, sizeof(PointInfo), (void*) offsetof(PointInfo, Colour));
Log(glGetError());
ptr_glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER, IBO);
Log(glGetError());
glDrawElements(GL_TRIANGLES, NumInds, GL_UNSIGNED_INT, 0);
Log(glGetError());

glPopMatrix();
SwapBuffers(DC);
Sleep(1);

But the only thing that happens is my screen clearing. I never see my triangle at all :S I think it might be my view setup via the glOrtho but I'm not sure. Is there anything wrong with what I did? The glGetError just prints 0.. No errors :S

Upvotes: 0

Views: 102

Answers (2)

Bartek Banachewicz
Bartek Banachewicz

Reputation: 39390

You should use glVertexAttribPointer. The functions you are using are deprecated. Perhaps you could get this code to work, but if you aren't forced to use such an ancient OpenGL, chances are you'd save yourself a lot of trouble.

Oh also manually loading function pointers is extremely cumbersome. I suggest you looked at libraries such as GLload.

A specialized debugger such as CodeXL or gDebugger can be very helpful in solving issues like that.

As for the problems in this code, your triangle is simply too small.

Upvotes: 1

Andreas Haferburg
Andreas Haferburg

Reputation: 5520

The triangle coordinates you specified are very small. The triangle occupies only half of a pixel at the top left corner of the screen. Try scaling it by 100.

Also I think you're missing calls to glEnableClientState with GL_VERTEX_ARRAY and GL_COLOR_ARRAY.

As a general approach I would suggest to take things one step at a time. Start with immediate mode glVertex to make sure you got the coordinates and camera setup right. Then add shaders. Then convert to a position-only VBO with DrawArrays. Then add vertex colors. Then convert to DrawElements. That way you have a better sense of where problems might lie.

You might also be interested in the glload library here to get rid of these ptr_ prefixes.

Upvotes: 2

Related Questions