Textured layers randomly switch position - iphone

Question

In my project I want to create 5 textured layers.
Each layer is made out of 4 textured rectangles each. These four parts of a layer are arranged so that it looks like one big texture.
The layers are partly transparent and are arranged in front of each other to create a three dimensional look.
When I run the project with only one layer enabled, everything looks fine, but as soon as I add a second (or more) layers everything gets chaotic.
Some parts go missing, other parts have a completely wrong z-Coordinate (the value itself looks fine, but the background layer is suddenly the foremost layer). Some parts even shift their x-Coordinate (this one looks good as well on setup, if I use NSLog to output all the square coordinates).
This is my setupVBOs function where I write the object coordinates into the VBOs (I have only one Index VBO because every square is the same, but an array of 5x4 VBOs to hold the coordinates for every single part of the layers.)
- (void)setupVBOs {
glGenBuffers(1, &_indexBufferLayer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _indexBufferLayer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(IndicesLayer), IndicesLayer, GL_STATIC_DRAW);
for (int layerNo = 1; layerNo < LAYER_COUNT + 1; ++layerNo)
{
for (int layerPart = 1; layerPart < LAYER_PARTS + 1; ++layerPart)
{
glGenBuffers(1, &_vertexBufferLayer[layerNo][layerPart]);
GLfloat x = -3.0 + (2.0 * (layerPart - 1));
GLfloat z = 0.0 + (50.0 * (layerNo - 1));
NSLog(#"Layer %d, Part %d: x=%f, z=%f", layerNo, layerPart, x, z);
// Alter the Texture slightly to
// remove errors from compression (x-Coord.)
Vertex Vertices[] = {
{{x + 1.0, -1.0, z}, {0.9865, 1.0}},
{{x + 1.0, +1.0, z}, {0.9865, 0}},
{{x - 1.0, +1.0, z}, {0.01, 0}},
{{x - 1.0, -1.0, z}, {0.01, 1.0}}
};
glBindBuffer(GL_ARRAY_BUFFER, _vertexBufferLayer[layerNo][layerPart]);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
x = z = 0;
}
}
}
This is my render Function where I draw everything and add the textures.
- (void)render: (CADisplayLink*)displayLink {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
CC3GLMatrix *projection = [CC3GLMatrix matrix];
float h = 4.0f * self.frame.size.height / self.frame.size.width;
[projection populateFromFrustumLeft:-1 andRight:1 andBottom:-h/4 andTop:h/4 andNear:2 andFar:500];
CC3GLMatrix *modelView = [CC3GLMatrix matrix];
// Translate the Modelviewmatrix
[modelView populateFromTranslation:CC3VectorMake(_cameraX, _cameraY, -5.0)];
// Rotate the Modelviewmatrix
[modelView rotateBy:CC3VectorMake(_currentRotation, 0, 90)];
[modelView translateByZ:_cameraZoom];
//
// Draw all layers
//
for (int layerNo = 1; layerNo < LAYER_COUNT + 1; layerNo++)
{
GLfloat layerFactor = (LAYER_COUNT + 1 - layerNo) * 22.0;
GLfloat scaleFactor = 100.0 + layerFactor;
[modelView scaleByX:scaleFactor];
[modelView scaleByY:scaleFactor];
for (int layerPart = 1; layerPart < LAYER_PARTS + 1; layerPart++)
{
glUniformMatrix4fv(_modelViewUniform, 1, 0, modelView.glMatrix);
glUniformMatrix4fv(_projectionUniform, 1, 0, projection.glMatrix);
GLuint uniformTexture = glGetUniformLocation(programHandle, "Texture");
// Bind Buffer and Texture
glBindBuffer(GL_ARRAY_BUFFER, _vertexBufferLayer[layerNo][layerPart]);
// Activate Texturing Pipeline and Bind Texture
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, _layers[layerNo][layerPart][0]);
glUniform1i(uniformTexture, 0);
// Vertex Shader calls
glVertexAttribPointer(_positionSlot, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid*) 0);
glVertexAttribPointer(_texCoordSlot, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid*) (sizeof(float) * 3));
glEnableVertexAttribArray(_positionSlot);
glEnableVertexAttribArray(_texCoordSlot);
glDrawElements(GL_TRIANGLES, sizeof(IndicesLayer)/sizeof(IndicesLayer[0]), GL_UNSIGNED_BYTE, 0);
glDisableVertexAttribArray(_texCoordSlot);
glDisableVertexAttribArray(_positionSlot);
}
[modelView scaleByX:1/scaleFactor];
[modelView scaleByY:1/scaleFactor];
}
[_context presentRenderbuffer:GL_RENDERBUFFER];
}

Answer 1

Use:
glenable(GL_TEXTURE_2D)
And also:
glActiveTexture — select active texture unit