CODEWITHSUNDEEP
opengl-es
Matt N.
Matt N.

Reputation: 1239

Vertex buffers in open gl es 1.X

I am teaching myself about open gl es and vertex buffer (VBO) and I have written code and it is supposed to draw one red triangle but instead it colours the screen black:

- (void)drawRect:(CGRect)rect {

// Draw a red triangle in the middle of the screen:
glColor4f(1.0f, 0.0f, 0.0f, 1.0f);
// Setup the vertex data:
typedef struct {
    float x;
    float y;
} Vertex;
const Vertex vertices[] = {{50,50}, {50,150}, {150,50}};
const short indices[3] = {0,1,2};
glGenBuffers(1, &vertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
NSLog(@"drawrect");
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, 0);
// The following line does the actual drawing to the render buffer:
glDrawElements(GL_TRIANGLE_STRIP, 3, GL_UNSIGNED_SHORT,  indices);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, framebuffer);
[eAGLcontext presentRenderbuffer:GL_RENDERBUFFER_OES];
}

Here vertexBuffer is of type GLuint. What is going wrong? Thanks for your help.

Upvotes: 0

Views: 67

Answers (2)

Matt N.
Matt N.

Reputation: 1239

You need to add 

glViewport(0, 0, 320, 480);

where you create the frame buffer and set up the context.

And replace your call to glDrawElements with

glDrawArrays(GL_TRIANGLE_STRIP, ...);

Upvotes: 0

Fonix
Fonix

Reputation: 11597

Your vertices dont have a Z component, try {{50,50,-100}, {50,150,-100}, {150,50,-100}}; (your camera by default looks down the Z axis so putting it in the -Z should put it on screen) if you cant see it still try smaller numbers, im not sure what your near and far draw cutoff distance is, and if its not even set i dont know what the default is. This might not be the only issue but its the only one i can see by just looking quickly at it.

Upvotes: 1

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