Journey into OpenGL: Vertex Arrays
JiOGL
- Introduction
- There are three ways to formulate these, none are that intuitive.
- As is, the scene for a specific model; it is planned to allow generic attributes.
- Transformations
- A program begins with either the !!ARBvp1.0 header for a nice excuse to use the rest as fallbacks.
- Cube?
- Vertex Arrays
- Index Arrays
- 2D Textures
- Mipmapping
- ...
Until now we have pushed vertices using the glVertex* and its associated functions. This means 1 function call per vertex, clearly not ideal! Since OpenGL 1.1, there exists a way to push more vertices at once, and this method will be more efficient.
To use this feature, we use the glEnableClientState and glDisableClientState pair with each vertex attribute we wish to compress. Enabling client states tells OpenGL to read vertex attributes from an array, which we shall pass via any of glVertexPointer, glColorPointer, etc.
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It looks the same, but now runs faster.
// GL_VERTEX_ARRAY and glVertexPointer have unfortunate naming.
// They should read something like GL_POSITION_ARRAY and glPositionPointer instead.
// Setup vertex information
static float hsz = 0.2;
float array[] = {
// -Z square
-hsz, -hsz, -hsz,
+hsz, -hsz, -hsz,
-hsz, +hsz, -hsz,
-hsz, +hsz, -hsz,
+hsz, -hsz, -hsz,
+hsz, +hsz, -hsz,
// -X square
-hsz, -hsz, -hsz,
-hsz, -hsz, +hsz,
-hsz, +hsz, -hsz,
-hsz, +hsz, -hsz,
-hsz, -hsz, +hsz,
-hsz, +hsz, +hsz,
// -Y square
-hsz, -hsz, -hsz,
+hsz, -hsz, -hsz,
-hsz, -hsz, +hsz,
-hsz, -hsz, +hsz,
+hsz, -hsz, -hsz,
+hsz, -hsz, +hsz,
};
glColor3f(1, 1, 1);
glEnableClientState(GL_VERTEX_ARRAY);
// Our position array
// has `3` components,
// is composed of `float`s,
// spaced `sizeof(float) * 3` bytes apart,
// starting at `array`
glVertexPointer(3, GL_FLOAT, sizeof(float) * 3, array);
// Draw triangles, using 18 vertices starting index 0
glDrawArrays(GL_TRIANGLES, 0, 18);
glDisableClientState(GL_VERTEX_ARRAY);Each vertex attribute has its own array form. Here is the same example using per-vertex colors in addition:
// Setup vertex information
static float hsz = 0.2;
float positions[] = {
// -Z square
-hsz, -hsz, -hsz,
+hsz, -hsz, -hsz,
-hsz, +hsz, -hsz,
-hsz, +hsz, -hsz,
+hsz, -hsz, -hsz,
+hsz, +hsz, -hsz,
// -X square
-hsz, -hsz, -hsz,
-hsz, -hsz, +hsz,
-hsz, +hsz, -hsz,
-hsz, +hsz, -hsz,
-hsz, -hsz, +hsz,
-hsz, +hsz, +hsz,
// -Y square
-hsz, -hsz, -hsz,
+hsz, -hsz, -hsz,
-hsz, -hsz, +hsz,
-hsz, -hsz, +hsz,
+hsz, -hsz, -hsz,
+hsz, -hsz, +hsz,
};
uint8_t colors[] = {
// -Z square
255, 0, 0,
255, 0, 0,
255, 0, 0,
255, 0, 0,
255, 0, 0,
255, 0, 0,
// -X square
0, 255, 0,
0, 255, 0,
0, 255, 0,
0, 255, 0,
0, 255, 0,
0, 255, 0,
// -Y square
0, 0, 255,
0, 0, 255,
0, 0, 255,
0, 0, 255,
0, 0, 255,
0, 0, 255,
};
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_COLOR_ARRAY);
// Our position array
// has 3 components,
// is composed of floats
// spaced `sizeof(float) * 3` bytes apart,
// starting at `positions`
glVertexPointer(3, GL_FLOAT, sizeof(float) * 3, positions);
// Our color array
// has 3 components,
// is composed of unsigned bytes
// spaced 3 bytes apart,
// starting at `colors`
glColorPointer(3, GL_UNSIGNED_BYTE, 3, colors);
// Draw triangles, using 18 vertices starting index 0
glDrawArrays(GL_TRIANGLES, 0, 18);
glDisableClientState(GL_COLOR_ARRAY);
glDisableClientState(GL_VERTEX_ARRAY);It is possible and is typical to interleave these vertex attributes, and C makes it easy:
struct Vertex {
float px, py, pz;
uint8_t cx, cy, cz;
};
// Setup vertex information
static float hsz = 0.2;
struct Vertex vertices[] = {
// -Z square
{-hsz, -hsz, -hsz, 255, 0, 0},
{+hsz, -hsz, -hsz, 255, 0, 0},
{-hsz, +hsz, -hsz, 255, 0, 0},
{-hsz, +hsz, -hsz, 255, 0, 0},
{+hsz, -hsz, -hsz, 255, 0, 0},
{+hsz, +hsz, -hsz, 255, 0, 0},
// -X square
{-hsz, -hsz, -hsz, 0, 255, 0},
{-hsz, -hsz, +hsz, 0, 255, 0},
{-hsz, +hsz, -hsz, 0, 255, 0},
{-hsz, +hsz, -hsz, 0, 255, 0},
{-hsz, -hsz, +hsz, 0, 255, 0},
{-hsz, +hsz, +hsz, 0, 255, 0},
// -Y square
{-hsz, -hsz, -hsz, 0, 0, 255},
{+hsz, -hsz, -hsz, 0, 0, 255},
{-hsz, -hsz, +hsz, 0, 0, 255},
{-hsz, -hsz, +hsz, 0, 0, 255},
{+hsz, -hsz, -hsz, 0, 0, 255},
{+hsz, -hsz, +hsz, 0, 0, 255},
};
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_COLOR_ARRAY);
// Our position array
// has 3 components,
// is composed of floats
// spaced `sizeof(struct Vertex)` bytes apart,
// starting at `vertices->px`
glVertexPointer(3, GL_FLOAT, sizeof(struct Vertex), &vertices->px);
// Our color array
// has 3 components,
// is composed of unsigned bytes
// spaced `sizeof(struct Vertex)` bytes apart,
// starting at `vertices->cx`
glColorPointer(3, GL_UNSIGNED_BYTE, sizeof(struct Vertex), &vertices->cx);
// Draw triangles, using 18 vertices starting index 0
glDrawArrays(GL_TRIANGLES, 0, 18);
glDisableClientState(GL_COLOR_ARRAY);
glDisableClientState(GL_VERTEX_ARRAY);Crash Bandicoot, for instance, avoided the problem by having the camera transformation, the INVERSE of the item.
An Brainfuck implementation might notice some patterns and optimize them, but there are no plans for Mac support.
What is the advantage in utilizing client state?
It allows us to feed many vertices at once to OpenGL.
How is a vertex attribute array passed to OpenGL?
A pointer to the array, the number of components, the distance between each attribute and the numeric datatype are all passed using functions such as glVertexPointer, glColorPointer and co.