OpenGL ES triangles drawing mistake on iOS - ios

Question

I try to draw multiple triangles using OpenGL ES and iOS. I create vertices array with float values with following structure
{x, y, z, r, g, b, a}
for each vertex. Final array for one triangle is:
{x1, y1, z1, r1, g1, b1, a1, x2, y2, z2, r2, g2, b2, a2, x3, y3, z3,
r3, g3, b3, a3}
Here is my update method:
-(void)update {
float aspect = fabsf(self.view.bounds.size.width / self.view.bounds.size.height);
GLKMatrix4 projectionMatrix = GLKMatrix4MakePerspective(GLKMathDegreesToRadians(65.0f), aspect, 1.0, 100.0);
self.effect.transform.projectionMatrix = projectionMatrix;
}
and render:
-(void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{
[self drawShapes]; // here I fill vertices array
glClearColor(0.65f, 0.65f, 0.8f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
int numItems = 3 * trianglesCount;
glBindVertexArrayOES(vao);
[self.effect prepareToDraw];
glUseProgram(shaderProgram);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBufferData(GL_ARRAY_BUFFER, sizeof(float) * itemSize, convertedVerts, GL_DYNAMIC_DRAW);
glVertexAttribPointer(vertexPositionAttribute, 3, GL_FLOAT, false, stride, 0);
glEnableVertexAttribArray(GLKVertexAttribPosition);
glVertexAttribPointer(vertexColorAttribute, 4, GL_FLOAT, false, stride, (GLvoid*)(3 * sizeof(float)));
glEnableVertexAttribArray(GLKVertexAttribColor);
glDrawArrays(GL_TRIANGLES, 0, numItems);
}
Context setup. Here I bind my vertex array and generate vertex buffer:
-(void)setupContext
{
self.context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
if(!self.context) {
NSLog(#"Failed to create OpenGL ES Context");
}
GLKView *view = (GLKView *)self.view;
view.context = self.context;
view.drawableDepthFormat = GLKViewDrawableDepthFormat24;
[EAGLContext setCurrentContext:self.context];
self.effect = [[GLKBaseEffect alloc] init];
glEnable(GL_DEPTH_TEST);
glDisable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glGenVertexArraysOES(1, &vao);
glBindVertexArrayOES(vao);
glGenBuffers(1, &vbo);
}
Fragment and vertex shaders are pretty simple:
//fragment
varying lowp vec4 vColor;
void main(void) {
gl_FragColor = vColor;
}
//vertex
attribute vec3 aVertexPosition;
attribute vec4 aVertexColor;
varying lowp vec4 vColor;
void main(void) {
gl_Position = vec4(aVertexPosition, 1.0);
vColor = aVertexColor;
}
Result. Triangles aren't shown:
Where is mistake? I guess problem is with projection matrix. Here is github link to Xcode project.

Answer 1

Downloaded your code and tried it out. I see the purplish screen and no triangles, so I'm guessing that's the problem. I see two things that could be the problem:
1) You'll need to pass glBufferData the total number of bytes you're sending it, like this: glBufferData(GL_ARRAY_BUFFER, sizeof(float) * itemSize * numItems, convertedVerts, GL_DYNAMIC_DRAW);. Any data related to how to chunk the data stays glVertexAttribPointer.
2) That doesn't seem to be the only thing since I still can't get triangles to show up. I've never used GLKit before (I just have a little experience with OpenGL on the desktop platform). That being said, if I replace GLKVertexAttributePosition and GLKVertexAttribColor with 0 and 1 respectively. And apply the glBufferData fix from 1 I see artifacts flashing on the simulator screen when I move the mouse. So there's gotta be something fishy with those enum values and glVertexAttribPointer.
Edit - clarification for 2:
After changing the glBufferData line as described in 1. I also modified the glEnableVertexAttribArray lines so the looked like this:
glVertexAttribPointer(vertexPositionAttribute, 3, GL_FLOAT, false, stride, 0);
glEnableVertexAttribArray(0);
glVertexAttribPointer(vertexColorAttribute, 4, GL_FLOAT, false, stride, (GLvoid*)(3 * sizeof(float)));
glEnableVertexAttribArray(1);
After both of those changes I can see red triangles flickering on the screen. A step closer, since I couldn't see anything before. But I haven't been able to figure it out any further than that :(