想要在矩形棱镜opengl es ios的一面画出正方形

Question

I have rendered a rectangular prism using opengl es for ios in Xcode. I want to draw a square on one of the faces of the rectangular prism (front face). I cant use a different data structure to hold the vertices, color co-ordinates and texture because the rectangular prism rotates with touch, and if I use a separate datastructure then the square wont rotate with the rectangular prism. So in the same datastructure I want to make a square. When I add the co-ordinates to the data structure and specify the points of the triangles it does not render the new square. Here is the code...

typedef struct {
  float Position[3];
  float Color[4];
  float TexCoord[2];
} Vertex;



const Vertex Vertices[] = {
   // Front
   {{1, -2, 0.10}, {1, 1, 1, 1}, {1, 0}},
   {{1, 2, 0.10}, {1, 1, 1, 1}, {1, 1}},
   {{-1, 2, 0.10}, {1, 1, 1, 1}, {0, 1}},
   {{-1, -2, 0.10}, {1, 1, 1, 1}, {0, 0}},
   // Back
   {{1, 2, -0.10}, {1, 1, 1, 1}, {0, 1}},
   {{-1, -2, -0.10}, {1, 1, 1, 1}, {1, 0}},
   {{1, -2, -0.10}, {1, 1, 1, 1}, {0, 0}},
   {{-1, 2, -0.10}, {1, 1, 1, 1}, {1, 1}},
   // Left
   {{-1, -2, 0.10}, {1, 1, 1, 1}, {1, 0}},
   {{-1, 2, 0.10}, {1, 1, 1, 1}, {1, 1}},
   {{-1, 2, -0.10}, {1, 1, 1, 1}, {0, 1}},
   {{-1, -2, -0.10}, {1, 1, 1, 1}, {0, 0}},
   // Rightself
   {{1, -2, -0.10}, {1, 1, 1, 1}, {1, 0}},
   {{1, 2, -0.10}, {1, 1, 1, 1}, {1, 1}},
   {{1, 2, 0.10}, {1, 1, 1, 1}, {0, 1}},
   {{1, -2, 0.10}, {1, 1, 1, 1}, {0, 0}},
   // Top
   {{1, 2, 0.10}, {1, 1, 1, 1}, {1, 0}},
   {{1, 2, -0.10}, {1, 1, 1, 1}, {1, 1}},
   {{-1, 2, -0.10}, {1, 1, 1, 1}, {0, 1}},
   {{-1, 2, 0.10}, {1, 1, 1, 1}, {0, 0}},
   // Bottom
   {{1, -2, -0.10}, {1, 1, 1, 1}, {1, 0}},
   {{1, -2, 0.10}, {1, 1, 1, 1}, {1, 1}},
   {{-1, -2, 0.10}, {1, 1, 1, 1}, {0, 1}},
   {{-1, -2, -0.10}, {1, 1, 1, 1}, {0, 0}},

  //new square
  {{1, -2, -0.1}, {1, 1, 1, 1}, {1, 0}},
  {{1, -2, 0.1}, {1, 1, 1, 1}, {1, 1}},
  {{-1, -2, 0.1}, {1, 1, 1, 1}, {0, 1}},
  {{-1, -2, -0.1}, {1, 1, 1, 1}, {0, 0}},
};




 const GLubyte Indices[] = {
   // Front
   0, 1, 2,
   2, 3, 0,

   // Back
   4, 6, 5,
   4, 5, 7,
   // Left
   8, 9, 10,
   10, 11, 8,

   // Right
   12, 13, 14,
   14, 15, 12,
   // Top
   16, 17, 18,
   18, 19, 16,
   // Bottom
   20, 21, 22,
   22, 23, 20,
   //new square
   24, 25, 26,
   26, 27, 24,
};

Is the square not rendering because I messed up with my co-ordinates, or is there another method that I need to check and specify I am adding new co-ordinates. Also I am using GLKBaseEffect, so no vertex and fragment shaders.

- (void)setupGL {

[EAGLContext setCurrentContext:self.context];
glEnable(GL_CULL_FACE);

self.effect = [[GLKBaseEffect alloc] init];

NSDictionary * options = [NSDictionary dictionaryWithObjectsAndKeys:
                          [NSNumber numberWithBool:YES],
                          GLKTextureLoaderOriginBottomLeft, 
                          nil];

NSError * error;    
NSString *path = [[NSBundle mainBundle] pathForResource:@""ofType:@"png"];
GLKTextureInfo * info = [GLKTextureLoader textureWithContentsOfFile:path options:options error:&error];
if (info == nil) {
    NSLog(@"Error loading file: %@", [error localizedDescription]);
}
self.effect.texture2d0.name = info.name;
self.effect.texture2d0.enabled = true;

// New lines
glGenVertexArraysOES(1, &_vertexArray);
glBindVertexArrayOES(_vertexArray);

// Old stuff
glGenBuffers(1, &_vertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, _vertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
//glBufferData(GL_ARRAY_BUFFER, sizeof(VerticesTwo), VerticesTwo, GL_STATIC_DRAW);

glGenBuffers(1, &_indexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _indexBuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Indices), Indices, GL_STATIC_DRAW);
//glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(IndicesTwo), IndicesTwo, GL_STATIC_DRAW);


// New lines (were previously in draw)
glEnableVertexAttribArray(GLKVertexAttribPosition);        
glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (const GLvoid *) offsetof(Vertex, Position));
glEnableVertexAttribArray(GLKVertexAttribColor);
glVertexAttribPointer(GLKVertexAttribColor, 4, GL_FLOAT, GL_FALSE, sizeof(Vertex), (const GLvoid *) offsetof(Vertex, Color));
glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (const GLvoid *) offsetof(Vertex, TexCoord));

glEnableVertexAttribArray(GLKVertexAttribPosition);
//glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, sizeof(VertexTwo), (const GLvoid *) offsetof(Vertex, Position));
glEnableVertexAttribArray(GLKVertexAttribColor);
//glVertexAttribPointer(GLKVertexAttribColor, 4, GL_FLOAT, GL_FALSE, sizeof(VertexTwo), (const GLvoid *) offsetof(VertexTwo, Color));
glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
//glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, sizeof(VertexTwo), (const GLvoid *) offsetof(Vertex, TexCoord));

// New line
glBindVertexArrayOES(0);

_rotMatrix = GLKMatrix4Identity;
_quat = GLKQuaternionMake(0, 0, 0, 1);
_quatStart = GLKQuaternionMake(0, 0, 0, 1);

UITapGestureRecognizer * dtRec = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(doubleTap:)];
dtRec.numberOfTapsRequired = 2;
[self.view addGestureRecognizer:dtRec];
//[self addGestureRecognizer:dtRec];
//[self.view bringSubviewToFront: self.view];


}

Answer 1

Where is the rest of the code? How do you draw this?

If I am reading this correct you simply added extra vertices which are exactly the same as the bottom face of your shape which means you just draw the same face twice and there should be no change in the result.

No matter the rotations you may still have a square as a separate object. You simply use the same matrix as you use to rotate the prism and it will rotate with the square. You can even use a sub-matrix for the square to position it depending on the prism original coordinates and multiply the prism matrix with this matrix to get the desired result. This way you would for instance create a square that is rotating around the prism.

But to begin with you could at least change the color of the square to see if you can then find it. Even an eskimo will not se a white square over a white surface.