CHAPTER 1. INTRODUCTION
Vertex buffer
Uniform Variables
Attributes gl_Position
Varying variables
Vertex Shader
Assembler
Figure 1.2: Vertices are stored in a vertex buffer. When a drawcallisissuedeach
of the vertices passes through the vertex shader. On input to the vertex shader, each
vertex (black) has associated attributes. On output, each vertex (cyan) has a value for
gl
Position and for its varying variables.
vertices that we will use to determine their ultimate appearances. For example, we may
associate a color (using 3 numbers representing amounts of red, green and blue) with
each vertex. Other attributes might be used to represent relevant material properties
describing, say, how shiny the surface at the vertex is.
Transmitting the vertex data from the CPU to the graphics hardware (the GPU)
is an expensive process, so it is typically done as infrequently as possible. There are
specific API calls to transfer vertex data over to OpenGL whichstoresthisdataina
vertex buffer
Once the vertex data has been given to OpenGL, at any subsequent time we can
send a draw call to OpenGL. This commands OpenGL to walk down the appropriate
vertex buffers and draw each vertex triplet as a triangle.
Once the OpenGL draw call has been issued, each vertex (i.e., all of its attributes)
gets processed independently by your vertex shader (See Figure 1.2). Besides the
attribute data, the shader also has access to things called uniform variables.Theseare
variables that are set by your program, but you can only set them in between OpenGL
draw calls, and not per vertex.
The vertex shader is your own program, and you can put whateveryouwantin
it. The most typical use of the vertex shader is to determine the final position of the
vertices on the screen. For example, a vertex can have its own abstract 3D position
stored as an attribute. Meanwhile, a uniform variable can be used to describe a virtual
camera that maps abstract 3D coordinates to the actual 2D screen. We will cover the
details of this kind of computation in Chapters 2- 6 and Chapter 10.
Once the vertex shader has computed the final position of the vertex on the screen, it
assigns this value to the the reserved output variable called gl
Position.Thex and
y coordinates of this variable are interpreted as positions within the drawing window.
The lower left corner of the window has coordinates (−1, −1),andtheupperright
corner has coordinates (1, 1).Coordinatesoutsideofthissquarerepresentlocations
Foundations of 3D Computer Graphics
S.J. Gortler
MIT Press, 2012
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