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The main functionality is [generation][] of (2D or 3D) quads with the provided
[vertex shader][] and [geometry shader][], expected to be used with a custom
[fragment shader][].
Mostly as a convenience, [fragment shader][]s are provided for [drawing][] (2D
or 3D) textures.
Other libraries, interoperating well with this one, that may be of interest:
- [`glshader`][], easing the use of [shader][]s.
- [`glframebuffer`][], easing the use of [framebuffer object][]s.
- [`glconventions`][], documenting the conventions used.
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[generation]: #generation
[vertex shader]: https://www.khronos.org/opengl/wiki/Vertex_Shader
[geometry shader]: https://www.khronos.org/opengl/wiki/Geometry_Shader
[fragment shader]: https://www.khronos.org/opengl/wiki/Fragment_Shader
[drawing]: #drawing
[`glshader`]: https://git.rcrnstn.net/rcrnstn/glshader
[shader]: https://www.khronos.org/opengl/wiki/Shader
[`glframebuffer`]: https://git.rcrnstn.net/rcrnstn/glframebuffer
[framebuffer object]: https://www.khronos.org/opengl/wiki/Framebuffer_Object
[`glconventions`]: https://git.rcrnstn.net/rcrnstn/glconventions
## Requirements
[Geometry shader][]s require OpenGL [\>=3.2][] or the
[`ARB_geometry_shader4`][] extension.
[`ARB_geometry_shader4`]: https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_geometry_shader4.txt
## Usage
Link the shader program with:
- `quad.vert`
- `quad.geom`
The following uniforms need to be set:
- `int instance_count`
The following outputs are written by the [geometry shader][]:
- `vec3 position`, in the range `-1` to `+1`
- `vec2 tex_coord`, in the range `0` to `1`
When issuing the draw call, use the following for 2D:
```
glDrawArrays(GL_POINTS, 0, 1);
```
and the following for 3D:
```
glDrawArraysInstanced(GL_POINTS, 0, 1 instance_count);
```
Here, `instance_count` is the number of [layer][]s in the [framebuffer][]; `1`
for 2D, texture depth for 3D.
[layer]: https://www.khronos.org/opengl/wiki/Geometry_Shader#Layered_rendering
[framebuffer]: https://www.khronos.org/opengl/wiki/Framebuffer
### Generation
Vertices are generated entirely within a [geometry shader][], so no vertex
state (except for the obligatory [vertex array object][] for OpenGL [\>=3.3][])
has to be set up on the CPU (no data is even read by the [vertex shader][]),
potentially making operation transparent to other code, which can leave their
state bound.
[Instanced rendering][], with each instance rendering to a separate [layer][],
is used since it enables a large, dynamic, number of layers. [Geometry shader
instancing][] is more self-contained and requires less work by the CPU (simply
`glDrawArrays`), but is limited to a fixed, small, number of layers, and
requires OpenGL [\>=4.0][] or the [`ARB_gpu_shader5`][] extension.
[vertex array object]: https://www.khronos.org/opengl/wiki/Vertex_Specification#Vertex_Array_Object
[\>=3.3]: https://en.wikipedia.org/wiki/OpenGL#Version_history
[instanced rendering]: https://www.khronos.org/opengl/wiki/Vertex_Rendering#Instancing
[geometry shader instancing]: https://www.khronos.org/opengl/wiki/Geometry_Shader#Instancing
[\>=4.0]: https://en.wikipedia.org/wiki/OpenGL#Version_history
[`ARB_gpu_shader5`]: https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_gpu_shader5.txt
### Drawing
The following outputs are written by these fragment shaders:
- ` vec4 frag_color`
#### 2D
Link the shader program with:
- `quad2d.frag`
The following uniforms need to be set:
- `sampler2D texture0`
#### 3D
Link the shader program with:
- `quad3d.frag`
The following uniforms need to be set:
- `sampler3D texture0`
- `float tex_coord_z`, `z` coordinate with which to sample `texture0`
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## Build system
This project has support for [CMake][], and uses [`cmake-cxx`][].
There are several ways to use this project in another CMake-based project:
- Use [`FetchContent`][] to download it and import the targets automatically
as part of the configure step:
```cmake
include(FetchContent)
FetchContent_Declare(project-name
GIT_REPOSITORY https://example.com/user/project-name
)
FetchContent_MakeAvailable(
project-name
)
```
- Bundle it and import the targets with [`add_subdirectory`][].
- Use [`find_package`][] (requires that the project is [packaged](#packaging)
or [installed](#installing)).
As usual, use [`add_dependencies`][]/[`target_link_libraries`][] to declare the
dependency.
[CMake]: https://cmake.org
[`cmake-cxx`]: https://git.rcrnstn.net/rcrnstn/cmake-cxx
[`FetchContent`]: https://cmake.org/cmake/help/v3.14/module/FetchContent.html
[`add_subdirectory`]: https://cmake.org/cmake/help/v3.14/command/add_subdirectory.html
[`find_package`]: https://cmake.org/cmake/help/v3.14/command/find_package.html
[`add_dependencies`]: https://cmake.org/cmake/help/v3.14/command/add_dependencies.html
[`target_link_libraries`]: https://cmake.org/cmake/help/v3.14/command/target_link_libraries.html
### Configure and generate
To configure and generate a build tree, use `cmake`:
```sh
cmake -B build
```
To set the [`CMAKE_BUILD_TYPE`][], pass e.g. `-DCMAKE_BUILD_TYPE=Release`.
[`cmake`]: https://cmake.org/cmake/help/v3.14/manual/cmake.1.html#generate-a-project-buildsystem
[`CMAKE_BUILD_TYPE`]: https://cmake.org/cmake/help/v3.14/variable/CMAKE_BUILD_TYPE.html
### Build
To build, use [`cmake --build`][]:
```sh
cmake --build build
```
[`cmake --build`]: https://cmake.org/cmake/help/v3.14/manual/cmake.1.html#build-a-project
### Test
To run tests, use [`ctest`][]:
```sh
(cd build && ctest --output-on-failure)
```
[`ctest`]: https://cmake.org/cmake/help/v3.14/manual/ctest.1.html
### Package
To package, use [`cpack`][]:
```sh
(cd build && cpack)
```
[`cpack`]: https://cmake.org/cmake/help/v3.14/manual/cpack.1.html
### Install
To install onto the current system, use [`cmake --install`][]:
```sh
cmake --install build
```
[`cmake --install`]: https://cmake.org/cmake/help/v3.14/manual/cmake.1.html#install-a-project
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