Shaders in GLSL

Question 1

What is not allowed in a fragment shader?

Answer 1

No access to adjacent fragments. Cannot change fragment position.

Question 2

What are the conversion rules for GLSL?

Answer 2

There is no implicit conversion.

Question 3

What are the basic types for GLSL?

Answer 3

void bool int float vec2 vec3 vec4 bvec2 bvec3 bvec4 ivec2 ivec3 ivec4 mat2 mat3 mat4 sampler2D samplerCube

Question 4

What are the non-type keywords in GLSL?

Answer 4

attribute const uniform varying break continue do for while if else in out inout true false lowp mediump highp precision invariant discard return struct

Question 5

What preprocessor directives are defined in GLSL?

Answer 5

#
#define
#undef
#if
#ifdef
#ifndef
#else
#elif
#endif
#error
#pragma
#extension
#version
#line

Question 6

What is the operator precedence for GLSL?

Answer 6

()
defined + - ~ !
* / %
\+ -
<< >>
< > =
== !=
&
^
|
&&
||

Question 7

What is a sampler type?

Answer 7

Sampler2D and SamplerCube are opaque handlers to textures.

Question 8

What are the usage restrictions on sampler types?

Answer 8

They can only be declared as function parameters or uniforms. Their use in expressions is limited. The cannot be used as out or inout function parameters.

Question 9

What is an anonymous structure?

Answer 9

This:
struct S {
   int i;
};
S myStruct;
// Illegal in GLSL, need "struct S myStruct"

Question 10

What is an embedded structure definition?

Answer 10

struct S {
  struct T {
  }
}
// Illegal in GLSL, but this is OK:
struct T {
}
struct S {
  T foo;
}

Question 11

How are arrays initialized at declaration in GLSL?

Answer 11

This is impossible.

Question 12

How are variables scoped in GLSL?

Answer 12

By a system of statically nested scopes, as in C or Java { { } }.

Question 13

What is the only shared global allowed in GLSL?

Answer 13

uniform

Question 14

How is const used in GLSL?

Answer 14

Compile-time constants and read-only function parameters.

Question 15

What is a GLSL attribute?

Answer 15

A linkage between a vertex shader and OpenGL ES per-vertex data

Question 16

What is a GLSL uniform?

Answer 16

Uniforms form the linkage between a fragment shader, OpenGL ES, and the application. This value does not change across the primitive being processed.

Question 17

What is a GLSL varying?

Answer 17

A linkage between a vertex shader and a fragment shader through interpolation.

Question 18

What are the storage qualifiers and the rules for when they are allowed?

Answer 18

default, const, attribute, varying, or uniform.
Local variables may only be default.
Function parameters can only use const.
Function return types and structures do not use qualifiers.

Question 19

What kind of data is used to communicate between runs of a shader?

Answer 19

None are allowed. This would defeat the SIMD nature of the GPU.

Question 20

What is “static use” of a variable?

Answer 20

It is when the code contains a compiled reference to the variable, even if the code is never executed.

Question 21

What do precision qualifiers do?

Answer 21

Specify minimum storage requirements for variables.

Question 22

What is the rule for precision of operations?

Answer 22

Operations preserve precision of their highest-precision operand, with the exception of a few computationally-intense built-in functions, such as atan().

Question 23

What does the precision keyword do for GLSL?

Answer 23

Creates a default precision for a storage type.

Question 24

What does variance refer to in shaders?

Answer 24

In general, the same expression may give different values in different shaders. Shaders are compiled independently, so different shaders may have different values for the same expression.

Question 25

gl_Position

Answer 25

The ultimate job of a vertex shader is to set gl_Position, a vec4 final position of the vertex.

Question 26

gl_FragColor

Answer 26

The ultimate job of a fragment shader is to set or discard gl_FragColor.

Question 27

Which precisions are supported by which shaders?

Answer 27

Vertex shaders support highp, mediump, and lowp. Fragment shaders support mediump and lowp, with highp being optional.

Question 28

What does this do?

vec3 foo = vec3(1.2);

Answer 28

This constructor initializes foo to (1.2, 1.2, 1.2).

Question 29

What does this do?

mat3 foo = mat3(3.4);

Answer 29

This constructor initializes the diagonal of foo to 3.4, and all other entries to 0.0.

Question 30

What does this do?
vec4 a = vec4(1.0,2.0,3.0,4.0);
vec3 b = vec3(a);

Answer 30

Shortens a by dropping 4.0 and assigns result to b.

Question 31

What is a structure constructor?

Answer 31

A structure constructor is created at structure declaration time. It matches the order and type of members as such:
struct light {
float a;
vec3 b;
};
light newLight = light(1.0,(0.1,0.2,0.3));

Question 32

Name all of the vector component aliases.

Answer 32

{x, y, z, w} // useful for vertex coordinates
{r, g, b, a} // useful for colors
{s, t, p, q} // useful for texture coordinates

Question 33

What is the type of

v4.xy

Answer 33

vec2

Question 34

Which element?

m[1][0]

Answer 34

e10, or the second row, first column.

Question 35

How many bits (typically) in lowp?

Answer 35

10 bits

Question 36

How many bits (typically) in medium?

Answer 36

16 bits

Question 37

How many bits (typically) in high?

Answer 37

24 bits

Question 38

How many uniforms are typically permitted in the system?

Answer 38

128

Question 39

What is the intrinsic declaration of gl_Position?

Answer 39

highp vec4 gl_Position

Question 40

What is the intrinsic declaration of gl_PointSize?

Answer 40

mediump float gl_Pointsize

Question 41

VNT

Answer 41

Vertex, Normal, Texture Coordinate. A vertex buffer format.

Question 42

What is the difference between static procedural deformation and dynamic procedural deformation?

Answer 42

Static procedural deformation creates time-independent effects, such as bumps on a metal surface from impact. Dynamic procedural deformation creates time-dependent effects, such as a flag waving or a creature breathing.

Question 43

How can vertex shaders aid in implementing environmental effects?

Answer 43

Effects like smoke or fog which depend on location or elevation can be calculated in a vertex shader.

Question 44

How is morphing implemented by a vertex shader?

Answer 44

Two different versions of an object are made available to the vertex shader. The shader blends the position of each vertex in these models.