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Merge pull request #575 from JesseTG/jtg/manual-section-2
Proofread section 2 #575
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Christophe
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manual.md
121
manual.md
@ -12,8 +12,8 @@
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+ [1.2. Faster compilation](#section1_2)
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+ [1.3. Example usage](#section1_3)
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+ [1.4. Dependencies](#section1_4)
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+ [2. Swizzle operators](#section2)
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+ [2.1. Standard C++98 implementation](#section2_1)
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+ [2. Swizzling](#section2)
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+ [2.1. Default C++98 implementation](#section2_1)
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+ [2.2. Anonynous union member implementation](#section2_2)
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+ [3. Preprocessor options](#section3)
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+ [3.1. Default precision](#section3_1)
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@ -213,29 +213,47 @@ glm::mat4 transform(glm::vec2 const& Orientation, glm::vec3 const& Translate, gl
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GLM does not depend on external libraries or headers such as `<GL/gl.h>`, [`<GL/glcorearb.h>`](http://www.opengl.org/registry/api/GL/glcorearb.h), `<GLES3/gl3.h>`, `<GL/glu.h>`, or `<windows.h>`. However, if we include `<boost/static_assert.hpp>`, then [`Boost.StaticAssert`](http://www.boost.org/doc/libs/release/libs/static_assert) will be used to provide compile-time errors. Otherwise, if using a C++11 compiler, the standard `static_assert` will be used instead. If neither is available, GLM will use its own implementation of `static_assert`.
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---
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## <a name="section2"></a> 2. Swizzle operators
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## <a name="section2"></a> 2. Swizzling
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A common feature of shader languages like GLSL is the swizzle operators. Those allow selecting multiple components of a vector and change their order. For example, “variable.x”, “variable.xzy” and “variable.zxyy”
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form respectively a scalar, a three components vector and a four components vector. With GLSL, swizzle operators can be both R-values and L-values. Finally, vector components can be accessed using “xyzw”,
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“rgba” or “stpq”.
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Shader languages like GLSL often feature so-called swizzle expressions, which may be used to freely select and arrange a vector's components. For example, `variable.x`, `variable.xzy` and `variable.zxyy` respectively form a scalar, a 3D vector and a 4D vector. The result of a swizzle expression in GLSL can be either an R-value or an L-value. Swizzle expressions can be written with characters from exactly one of `xyzw` (usually for positions), `rgba` (usually for colors), and `stpq` (usually for texture coordinates).
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```cpp
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```glsl
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vec4 A;
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vec2 B;
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...
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B.yx = A.wy;
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B = A.xx;
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vec3 C = A.bgr;
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vec3 D = B.rsz; // Invalid, won't compile
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```
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GLM supports a subset of this functionality as described in the following sub-sections. Swizzle operators are disabled by default. To enable them GLM\_SWIZZLE must be defined before any inclusion of
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<glm/glm.hpp>. Enabling swizzle operators will massively increase the size of compiled files and the compilation time.
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GLM optionally supports some of this functionality via the methods described in the following sections. Swizzling can be enabled by defining `GLM_SWIZZLE` before including any GLM header files, or as part of a project's build process.
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### <a name="section2_1"></a> 2.1. Standard C++98 implementation
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*Note that enabling swizzle expressions will massively increase the size of your binaries and the time it takes to compile them!*
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### <a name="section2_1"></a> 2.1. Default C++98 implementation
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When compiling GLM as C++98, R-value swizzle expressions are simulated through member functions of each vector type.
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```cpp
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#define GLM_SWIZZLE // Or defined when building (e.g. -DGLM_SWIZZLE)
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#include <glm/glm.hpp>
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void foo()
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{
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glm::vec4 ColorRGBA(1.0f, 0.5f, 0.0f, 1.0f);
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glm::vec3 ColorBGR = ColorRGBA.bgr();
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glm::vec3 PositionA(1.0f, 0.5f, 0.0f, 1.0f);
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glm::vec3 PositionB = PositionXYZ.xyz() * 2.0f;
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glm::vec2 TexcoordST(1.0f, 0.5f);
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glm::vec4 TexcoordSTPQ = TexcoordST.stst();
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}
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```
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Swizzle operators return a **copy** of the component values, and thus *can't* be used as L-values to change a vector's values.
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The C++98 implementation exposes the R-value swizzle operators as member functions of vector types.
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```cpp
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#define GLM_SWIZZLE
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@ -243,84 +261,59 @@ The C++98 implementation exposes the R-value swizzle operators as member functio
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void foo()
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{
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glm::vec4 ColorRGBA(1.0f, 0.5f, 0.0f, 1.0f);
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glm::vec3 ColorBGR = ColorRGBA.bgr();
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...
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glm::vec3 A(1.0f, 0.5f, 0.0f);
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glm::vec3 PositionA(1.0f, 0.5f, 0.0f, 1.0f);
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glm::vec3 PositionB = PositionXYZ.xyz() \* 2.0f;
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...
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glm::vec2 TexcoordST(1.0f, 0.5f);
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glm::vec4 TexcoordSTPQ = TexcoordST.stst();
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...
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}
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```
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Swizzle operators return a copy of the component values hence they can’t be used as L-values to change the value of the variables.
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```cpp
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#define GLM_FORCE_SWIZZLE
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#include <glm/glm.hpp>
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void foo()
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{
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glm::vec3 A(1.0f, 0.5f, 0.0f);
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// /!\\ No compiler error but A is not affected
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// This code modify the components of an anonymous copy.
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A.bgr() = glm::vec3(2.0f, 1.5f, 1.0f); // A is not modified!
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...
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// No compiler error, but A is not modified.
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// An anonymous copy is being modified (and then discarded).
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A.bgr() = glm::vec3(2.0f, 1.5f, 1.0f); // A is not modified!
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}
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```
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### <a name="section2_2"></a> 2.2. Anonymous union member implementation
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Visual C++ supports anonymous structures in union, which is a non-standard language extension, but it enables a very powerful implementation of swizzle operators on Windows supporting both L-value
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swizzle operators and a syntax that doesn’t require parentheses in some cases. This implementation is only enabled when the language extension is enabled and GLM\_SWIZZLE is defined.
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Visual C++ supports, as a _non-standard language extension_, anonymous `struct`s as `union` members. This permits a powerful swizzling implementation that both allows L-value swizzle expressions and GLSL-like syntax. To use this feature, the language extension must be enabled by a supporting compiler and `GLM_SWIZZLE` must be `#define`d.
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```cpp
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#define GLM_FORCE_SWIZZLE
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#define GLM_SWIZZLE
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#include <glm/glm.hpp>
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// Only guaranteed to work with Visual C++!
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// Some compilers that support Microsoft extensions may compile this.
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void foo()
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{
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glm::vec4 ColorRGBA(1.0f, 0.5f, 0.0f, 1.0f);
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glm::vec4 ColorRGBA(1.0f, 0.5f, 0.0f, 1.0f);
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// l-value:
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glm::vec4 ColorBGRA = ColorRGBA.bgra;
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// l-value:
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glm::vec4 ColorBGRA = ColorRGBA.bgra;
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// r-value:
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ColorRGBA.bgra = ColorRGBA;
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// r-value:
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ColorRGBA.bgra = ColorRGBA;
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// Both l-value and r-value
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ColorRGBA.bgra = ColorRGBA.rgba;
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...
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// Both l-value and r-value
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ColorRGBA.bgra = ColorRGBA.rgba;
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}
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```
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Anonymous union member swizzle operators don’t return vector types (glm::vec2, glm::vec3 and glm::vec4) but implementation specific objects that can be automatically interpreted by other swizzle operators and
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vector constructors. Unfortunately, those can’t be interpreted by GLM functions so that the programmer must convert a swizzle operators to a vector type or call the () operator on a swizzle objects to pass it to another C++ functions.
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This version returns implementation-specific objects that _implicitly convert_ to their respective vector types. As a consequence of this design, these extra types **can't be directly used** by GLM functions; they must be converted through constructors or `operator()`.
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```cpp
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#define GLM_FORCE_SWIZZLE
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#define GLM_SWIZZLE
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#include <glm/glm.hpp>
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using glm::vec4;
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void foo()
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{
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glm::vec4 Color(1.0f, 0.5f, 0.0f, 1.0f);
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...
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vec4 Color(1.0f, 0.5f, 0.0f, 1.0f);
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// Generates compiler errors. Color.rgba is not a vector type.
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glm::vec4 ClampedA = glm::clamp(Color.rgba, 0.f, 1.f); // ERROR
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// Generates compiler errors. Color.rgba is not a vector type.
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vec4 ClampedA = glm::clamp(Color.rgba, 0.f, 1.f); // ERROR
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// We need to cast the swizzle operator into glm::vec4
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// With by using a constructor
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glm::vec4 ClampedB = glm::clamp(glm::vec4(Color.rgba), 0.f, 1.f); // OK
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// Explicit conversion through a constructor
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vec4 ClampedB = glm::clamp(vec4(Color.rgba), 0.f, 1.f); // OK
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// Or by using the () operator
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glm::vec4 ClampedC = glm::clamp(Color.rgba(), 0.f, 1.f); // OK
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...
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// Explicit conversion through operator()
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vec4 ClampedC = glm::clamp(Color.rgba(), 0.f, 1.f); // OK
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}
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```
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