sfcg - SFML Core Graphics Module

My attempt to implement most of SFML Graphics API in OpenGL Core profile without usage of any legacy API.

Experimental, WIP, and probably broken.

Features

My aim is to have fully-working, API-compatible and documented replacement for SFML graphics, although it's still far from that. As for now, the following features are implemented:

• Creation of render window, with given context settings
• Drawing of rectangles, circles, and convex shapes
• Sprites
• Text rendering using sf::Font

See example for usage.

Why?

I wanted to make an educational 3D pet-project with modern (core) OpenGL using SFML, used for drawing 2D UI and debugging stuff, but unfortunately, as for now, SFML Graphics module uses legacy OpenGL API, which I also couldn't run on my MacOS with M1 chip. At the same time, being a big fan of SFML clean and elegant API, I didn't want to switch to another library or introduce another dependency to my project. Therefore, I decided to write a drop-in replacement for SFML Graphics module, which would use modern OpenGL API.

AFAIK, SFML does not plan to switch to modern OpenGL in upcoming release of 3.0.0, and high chances that we will not see SFML 4.0+ for another decade or so 😁. This project should not be regarded as contribution to main FML codebase, this is mostly personal-use low-quality code project 😁

Installing

You can either build the library using CMake and link against it, or just include the source code in your project.

How to use

Most of APIs from SFML have 1-1 mapping, just change the namespace from sf:: to sfcg::.

Please note, not all classes or definitions were re-implemented, some SFML classes are completely good or independent of OpenGL context version, like sf::Vertex or sf::Vector2f. Therefore, they are not included in this library and sfcg is still dependent on sfml-graphics, so do not forget to link against it.

1. Add the library to your project
2. Include the header file: #include <sfcg/SFCG.hpp>, which will include all the necessary headers (or include them separately)
3. Create a RenderWindow with context settings, that will enable OpenGL Core profile, for example like this:

sf::ContextSettings settings;
settings.attributeFlags = sf::ContextSettings::Core;
settings.majorVersion = 4;
settings.minorVersion = 1;

sfcg::RenderWindow window(sf::VideoMode(800, 600), "App", sf::Style::Default, settings);

4. Call sfcg::init() to initialize some internal library stuff - mainly sfcg::GeometryCache which has re-usable VBOs, VAOs and shaders. This may be subject to change. Make sure you call it when OpenGL context is already created.

5. Use the library as you would use SFML Graphics module, for example:

sfcg::RectangleShape rect(sf::Vector2f(100,100));
rect.setFillColor(sf::Color::Red);
rect.setPosition(100, 100);

//in event loop, between window.clear() and window.display()
window.draw(rect);

6. Call sfcg::cleanup() before exiting the program to free all the resources.

Known Issues

First of all, keep in mind that I am relatively newbie in OpenGL and even C++, therefore the code may be broken, inefficient, or just plain wrong. I would be very grateful for any feedback, suggestions, or pull requests.

Secondly, due to fact that we have to send all the data to GPU before rendering, the codebase contains some experiments for getting a good compromise between performance and API simplicity.

For example, sfcg::RectangleShape by itself does not create any VBOs or VAOs. Instead there is sfcg::GeometryCache singleton class, which creates a unit (1x1) rectangle VBO and VAO, that are later scaled to needed size in vertex shader. This is done to avoid creating a new VBO for every rectangle, if user decides to create hundreds of them. Such approach also applies to circle shape. At the same time, this is tricky, does not work well with outlines, so it will probably be changed in future.

Due to what was said before and other circumstances, some of the features from SFML may work with issues:

• Not all APIs are fully 1-1 compatible with SFML or even implemented, see the TODO below
• Code may not work well on all platforms - for simplicity purposes, there are no checks for OpenGL extensions, assuming all needed extensions are already available
• Some approaches or code parts may be experimental
• Shapes, other than rectangles, may not work well with outlines

Long story short:

⚠️ This started as personal educational project, so no guarantees are given. Use at your own risk. ⚠️

Note on shaders

Modern OpenGL pipeline requires a shader program to render anything. To simplify the API, there is a set of default shaders in sfcg which are created and applied under the hood. This allows using this library without any creation of shaders, as in original SFML API.

sfcg comes with a default so-called base shader, which is used for rendering all the shapes, sprites, which I guess should be fine for most of the cases. There is also separate fragment shader for text rendering.

The vertex and fragment base shaders are defined in GeometryCache.cpp as follows:

#version 330 core

layout (location = 0) in vec2 position;
layout (location = 1) in vec4 color;
layout (location = 2) in vec2 texCoord;

out vec2 frag_texCoord;
out vec4 frag_vertexColor;

uniform mat4 sfcg_modelViewMatrix;
uniform mat4 sfcg_projectionMatrix;

void main()
{
    gl_Position = sfcg_projectionMatrix * sfcg_modelViewMatrix * vec4(position.x, position.y, 0.0f, 1.0f);
    frag_texCoord = texCoord;
    frag_vertexColor = color;
}

#version 330 core

layout (location = 0) out vec4 fragColor;

in vec2 frag_texCoord;

uniform vec4 sfcg_color;

uniform sampler2D sfcg_texture;

void main()
{
    fragColor = texture(sfcg_texture, frag_texCoord) * sfcg_color;
}

As you may see, there a few uniforms which are automatically set by sfcg:

• sfcg_modelViewMatrix - model-view matrix, usually it's equal to matrix of sf::Transform of currently drawn object
• sfcg_projectionMatrix - projection matrix, usually it's equal to matrix of sf::View of currently used render target
• sfcg_color - color of the object, usually the fill color of the shape or sprite
• sfcg_texture - texture of the object. To simplify shaders code, if there is NO texture bound, sfcg will bind internal texture consisting of single (1x1) white pixel before drawing. This allows writing shaders which sample the texture, without checking if it's bound or not, which is tricky in OpenGL.

If you want to use your own shaders, you can modify the sfcg::RenderStates object, passed to draw() methods, where you can use the above provided uniforms.

TODO:

• sf::Texture (use SFML variant)
• sf::Sprite -> sfcg::Sprite
• sf::RenderTexture
• sf::RenderTarget -> sfcg::RenderTarget
• sf::RenderWindow -> sfcg::RenderWindow
• sf::Shader -> sfcg::Shader
• sf::Font (use SFML variant)
• sf::Text -> sfcg::Text
• sf::VertexBuffer -> sfcg::VertexBuffer
• sf::RectangleShape -> sfcg::RectangleShape
• sf::CircleShape -> sfcg::CircleShape
• sf::ConvexShape -> sfcg::ConvexShape
• Switching between different render targets
• Caching of render states/objects in RenderTarget
• Fix CMake config, so the library can be distributed easily
• Add in-code documentation with SFML style
• Solve outlines issue (either fix the transformations or use vertex buffers)\
• Solve context-safety for sfcg::GeometryCache

Credits

• Me
• A lot of code was "borrowed" (copy-pasted) from original SFML source code, so big respects to SFML team: https://github.com/SFML/SFML
• Example contains texture from opengameart by Blarumyrran (https://opengameart.org/content/modified-32x32-treasure-chest)

License: MIT

See LICENSE file