How color attributes work in VBO?
- by Jayesh
I am coding to OpenGL ES 2.0 (Webgl). I am using VBOs to draw primitives. I have vertex array, color array and array of indices. I have looked at sample codes, books and tutorial, but one thing I don't get - if color is defined per vertex how does it affect the polygonal surfaces adjacent to those vertices? (I am a newbie to OpenGL(ES))
I will explain with an example. I have a cube to draw. From what I read in OpenGLES book, the color is defined as an vertex attribute. In that case, if I want to draw 6 faces of the cube with 6 different colors how should I define the colors. The source of my confusion is: each vertex is common to 3 faces, then how will it help defining a color per vertex? (Or should the color be defined per index?). The fact that we need to subdivide these faces into triangles, makes it harder for me to understand how this relationship works. The same confusion goes for edges. Instead of drawing triangles, let's say I want to draw edges using LINES primitives. Each edge of different color. How am I supposed to define color attributes in that case?
I have seen few working examples. Specifically this tutorial: http://learningwebgl.com/blog/?p=370
I see how color array is defined in the above example to draw a cube with 6 different colored faces, but I don't understand why is defined that way. (Why is each color copied 4 times into unpackedColors for instance?)
Can someone explain how color attributes work in VBO?
[The link above seems inaccessible, so I will post the relevant code here]
cubeVertexPositionBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexPositionBuffer);
vertices = [
// Front face
-1.0, -1.0, 1.0,
1.0, -1.0, 1.0,
1.0, 1.0, 1.0,
-1.0, 1.0, 1.0,
// Back face
-1.0, -1.0, -1.0,
-1.0, 1.0, -1.0,
1.0, 1.0, -1.0,
1.0, -1.0, -1.0,
// Top face
-1.0, 1.0, -1.0,
-1.0, 1.0, 1.0,
1.0, 1.0, 1.0,
1.0, 1.0, -1.0,
// Bottom face
-1.0, -1.0, -1.0,
1.0, -1.0, -1.0,
1.0, -1.0, 1.0,
-1.0, -1.0, 1.0,
// Right face
1.0, -1.0, -1.0,
1.0, 1.0, -1.0,
1.0, 1.0, 1.0,
1.0, -1.0, 1.0,
// Left face
-1.0, -1.0, -1.0,
-1.0, -1.0, 1.0,
-1.0, 1.0, 1.0,
-1.0, 1.0, -1.0,
];
gl.bufferData(gl.ARRAY_BUFFER, new WebGLFloatArray(vertices), gl.STATIC_DRAW);
cubeVertexPositionBuffer.itemSize = 3;
cubeVertexPositionBuffer.numItems = 24;
cubeVertexColorBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, cubeVertexColorBuffer);
var colors = [
[1.0, 0.0, 0.0, 1.0], // Front face
[1.0, 1.0, 0.0, 1.0], // Back face
[0.0, 1.0, 0.0, 1.0], // Top face
[1.0, 0.5, 0.5, 1.0], // Bottom face
[1.0, 0.0, 1.0, 1.0], // Right face
[0.0, 0.0, 1.0, 1.0], // Left face
];
var unpackedColors = []
for (var i in colors) {
var color = colors[i];
for (var j=0; j < 4; j++) {
unpackedColors = unpackedColors.concat(color);
}
}
gl.bufferData(gl.ARRAY_BUFFER, new WebGLFloatArray(unpackedColors), gl.STATIC_DRAW);
cubeVertexColorBuffer.itemSize = 4;
cubeVertexColorBuffer.numItems = 24;
cubeVertexIndexBuffer = gl.createBuffer();
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, cubeVertexIndexBuffer);
var cubeVertexIndices = [
0, 1, 2, 0, 2, 3, // Front face
4, 5, 6, 4, 6, 7, // Back face
8, 9, 10, 8, 10, 11, // Top face
12, 13, 14, 12, 14, 15, // Bottom face
16, 17, 18, 16, 18, 19, // Right face
20, 21, 22, 20, 22, 23 // Left face
]
gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new WebGLUnsignedShortArray(cubeVertexIndices), gl.STATIC_DRAW);
cubeVertexIndexBuffer.itemSize = 1;
cubeVertexIndexBuffer.numItems = 36;
