Why do my pyramids fade black and then back to colour again [closed]

Question

I have the following vertecies and norms

GLfloat verts[36] = {

-0.5, 0, 0.5,
 0, 0, -0.5,
0.5, 0, 0.5,

0, 0, -0.5,
0.5, 0, 0.5,
0, 1, 0,

-0.5, 0, 0.5,
0, 0, -0.5,
0, 1, 0,

0.5, 0, 0.5,
-0.5, 0, 0.5,
0, 1, 0

};

GLfloat norms[36] = {

0, -1, 0,
0, -1, 0,
0, -1, 0,

-1, 0.25, 0.5,
-1, 0.25, 0.5,
-1, 0.25, 0.5,

1, 0.25, -0.5,
1, 0.25, -0.5,
1, 0.25, -0.5,

0, -0.5, -1,
0, -0.5, -1,
0, -0.5, -1 
};

I am writing my fists Open GL game, But I need to know for sure if my Normals are correct as the colours aren't rendering correctly. my Pyramids are coloured then fade to black every half rotation then back again.

---

My app so far is based on the boiler plate code provided by apple. heres my modified setUp Method

 [EAGLContext setCurrentContext:self.context];

[self loadShaders];

self.effect = [[GLKBaseEffect alloc] init];
self.effect.light0.enabled = GL_TRUE;
self.effect.light0.diffuseColor = GLKVector4Make(1.0f, 0.4f, 0.4f, 1.0f);

glEnable(GL_DEPTH_TEST);


glGenVertexArraysOES(1, &_vertexArray); //create vertex array
glBindVertexArrayOES(_vertexArray);

glGenBuffers(1, &_vertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, _vertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(verts) + sizeof(norms), NULL, GL_STATIC_DRAW); //create vertex buffer big enough for both verts and norms and pass NULL as data..
uint8_t *ptr = (uint8_t *)glMapBufferOES(GL_ARRAY_BUFFER, GL_WRITE_ONLY_OES); //map buffer to pass data to it
memcpy(ptr, verts, sizeof(verts)); //copy verts
memcpy(ptr+sizeof(verts), norms, sizeof(norms)); //copy norms to position after verts
glUnmapBufferOES(GL_ARRAY_BUFFER);
glEnableVertexAttribArray(GLKVertexAttribPosition);
glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(0)); //tell GL where verts are in buffer
glEnableVertexAttribArray(GLKVertexAttribNormal);
glVertexAttribPointer(GLKVertexAttribNormal, 3, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(sizeof(verts))); //tell GL where norms are in buffer
glBindVertexArrayOES(0);

And the update method.

- (void)update
{

float aspect = fabsf(self.view.bounds.size.width / self.view.bounds.size.height);
GLKMatrix4 projectionMatrix = GLKMatrix4MakePerspective(GLKMathDegreesToRadians(65.0f), aspect, 0.1f, 100.0f);

self.effect.transform.projectionMatrix = projectionMatrix;

GLKMatrix4 baseModelViewMatrix = GLKMatrix4MakeTranslation(0.0f, 0.0f, -4.0f);
baseModelViewMatrix = GLKMatrix4Rotate(baseModelViewMatrix, _rotation, 0.0f, 1.0f, 0.0f);

// Compute the model view matrix for the object rendered with GLKit
GLKMatrix4 modelViewMatrix = GLKMatrix4MakeTranslation(0.0f, 0.0f, -1.5f);
modelViewMatrix = GLKMatrix4Rotate(modelViewMatrix, _rotation, 1.0f, 1.0f, 1.0f);
modelViewMatrix = GLKMatrix4Multiply(baseModelViewMatrix, modelViewMatrix);

self.effect.transform.modelviewMatrix = modelViewMatrix;

// Compute the model view matrix for the object rendered with ES2
modelViewMatrix = GLKMatrix4MakeTranslation(0.0f, 0.0f, 1.5f);
modelViewMatrix = GLKMatrix4Rotate(modelViewMatrix, _rotation, 1.0f, 1.0f, 1.0f);
modelViewMatrix = GLKMatrix4Multiply(baseModelViewMatrix, modelViewMatrix);

_normalMatrix = GLKMatrix3InvertAndTranspose(GLKMatrix4GetMatrix3(modelViewMatrix), NULL);

_modelViewProjectionMatrix = GLKMatrix4Multiply(projectionMatrix, modelViewMatrix);

_rotation += self.timeSinceLastUpdate * 0.5f;
}

But providing I understand this correct one pyramid is using the GLKit base effect shaders and the other the shaders which are included in the project. So for both of them to have the same error, I thought it would be the Norms?

Answer 1

Found the answer: it was the Normals:

GLfloat norms[36] = {

0, -1, 0,
0, -1, 0,
0, -1, 0,

1, 0.25, -0.5,
1, 0.25, -0.5,
1, 0.25, -0.5,

-1, 0.25, -0.5,
-1, 0.25, -0.5,
-1, 0.25, -0.5,

0, 0.5, 1,
0, 0.5, 1,
0, 0.5, 1
};

These are the correct ones. For some reason the maths I used produced some numbers that were negative when they shouldn't have been. thanks for all the help! GC