glVertex3f(1.0f, -1.0f, 1.0f);
glVertex3f(1.0f, -1.0f, -1.0f);
glEnd(); // of the color cube
Indexed Vertices
Primitives often share vertices. Instead of repeatedly specifying the vertices, it is more efficient to create an index list of vertices, and use
the indexes in specifying the primitives.
For example, the following code fragment specifies a pyramid, which is formed by 5 vertices. We first define 5 vertices in an index array,
followed by their respectively color. For each of the 5 faces, we simply provide the vertex index and color index.
float[] vertices = { // 5 vertices of the pyramid in (x,y,z)
-1.0f, -1.0f, -1.0f, // 0. left-bottom-back
1.0f, -1.0f, -1.0f, // 1. right-bottom-back
1.0f, -1.0f, 1.0f, // 2. right-bottom-front
-1.0f, -1.0f, 1.0f, // 3. left-bottom-front
0.0f, 1.0f, 0.0f // 4. top
};
float[] colors = { // Colors of the 5 vertices in RGBA
0.0f, 0.0f, 1.0f, 1.0f, // 0. blue
0.0f, 1.0f, 0.0f, 1.0f, // 1. green
0.0f, 0.0f, 1.0f, 1.0f, // 2. blue
0.0f, 1.0f, 0.0f, 1.0f, // 3. green
1.0f, 0.0f, 0.0f, 1.0f // 4. red
};
byte[] indices = { // Vertex indices of the 4 Triangles
2, 4, 3, // front face (CCW)
1, 4, 2, // right face
0, 4, 1, // back face
4, 0, 3 // left face
};
// Transfer the arrays to vertex-buffer, color-buffer and index-buffer.
// Draw the primitives (triangle) from the index buffer
3.4Pixel vs. Fragment
Pixels refers to the dots on the display, which are aligned in a 2-dimensional grid of a certain rows and columns corresponding to the
display's resolution. A pixel is 2-dimensional, with a (x, y) position and a RGB color value (there is no alpha value for pixels). The purpose of
the Graphics Rendering Pipeline is to produce the color-value for all the pixels for displaying on the screen, given the input primitives.
In order to produce the grid-aligned pixels for the display, the rasterizer of the graphics rendering pipeline, as its name implied, takes each
input primitive and perform raster-scan to produce a set of grid-aligned fragments enclosed within the primitive. A fragment is 3-
dimensional, with a (x, y, z) position. The (x, y) are aligned with the 2D pixel-grid. The z-value (not grid-aligned) denotes its depth. The z-
values are needed to capture the relative depth of various primitives, so that the occluded objects can be discarded (or the alpha channel
of transparent objects processed) in the output-merging stage.
Fragments are produced via interpolation of the vertices. Hence, a fragment has all the vertex's attributes such as color, fragment-normal
and texture coordinates.
In modern GPU, vertex processing and fragment processing are programmable. The programs are called vertex shader and fragment
shader.
(Direct3D uses the term "pixel" for "fragment".)
3D Graphics with OpenGL - The Basic Theory
Page 5 of 21
http://www.ntu.edu.sg/home/ehchua/programming/opengl/CG_BasicsTheory.html
2019/5/5
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