Converting GLSL to Rebel Shaders

This document explains the differences between Rebel Shading Language and GLSL and gives practical advice on how to migrate shaders from other sources, such as Shadertoy and The Book of Shaders, into Rebel Shaders.

For detailed information on Rebel Shading Language, please refer to the Rebel Shading Language reference.

GLSL

Rebel Shading Language is based on GLSL with the addition of a few quality-of-life features. Accordingly, most features available in GLSL are available in Rebel Shading Language.

Shader programs

In GLSL, each shader uses a separate program. You have one program for the vertex shader and one for the fragment shader. In Rebel Engine, you have a single shader that contains a vertex and/or a fragment function. If you only choose to write one, Rebel Engine will supply the other.

Rebel Shading Language allows uniform variables and functions to be shared by defining the fragment and vertex shaders in one file. In GLSL, the vertex and fragment programs cannot share variables except when varyings are used.

Vertex attributes

In GLSL, you can pass in per-vertex information using attributes and have the flexibility to pass in as much or as little as you want. In Rebel Shading Language, you have a set number of input attributes, including VERTEX (position), COLOR, UV, UV2, NORMAL. For a complete list, see the Shading language reference.

gl_Position

gl_Position receives the final position of a vertex specified in the vertex shader. It is specified by the user in clip space. Typically, in GLSL, the model space vertex position is passed in using a vertex attribute called position and you handle the conversion from model space to clip space manually.

In Rebel Shading Language, VERTEX specifies the vertex position in model space at the beginning of the vertex function. Rebel Engine also handles the final conversion to clip space after the user-defined vertex function is run. If you want to skip the conversion from model to view space, you can set the render_mode to skip_vertex_transform. If you want to skip all transforms, set render_mode to skip_vertex_transform and set the PROJECTION_MATRIX to mat4(1.0) in order to nullify the final transform from view space to clip space.

Varyings

Varyings are a type of variable that can be passed from the vertex shader to the fragment shader. In modern GLSL (3.0 and up), varyings are defined with the in and out keywords. A variable going out of the vertex shader is defined with out in the vertex shader and in inside the fragment shader.

Main

In GLSL, each shader program looks like a self-contained C-style program. Accordingly, the main entry point is main. If you are copying a vertex shader, rename main to vertex and if you are copying a fragment shader, rename main to fragment.

Constants

Global array constants are not supported in Rebel Shading Language. You can fake the functionality by using a uniform initialized to the value, but you will not benefit from the increased speed from using a constant.

Macros

In keeping with its similarity to C, GLSL lets you use macros. Commonly #define is used to define constants or small functions. There is no straightforward way to translate defines to Rebel Shading Language. If it is a function that is defined, then replace with a function, and if it is a constant, then replace with a uniform. For other macros (#if, #ifdef, etc.), there is no equivalent because they run during the pre-processing stage of compilation.

Variables

GLSL has many built-in variables that are hard-coded. These variables are not uniforms, so they are not editable from the main program.

Variable	Type	Equivalent	Description
gl_FragColor	out vec4	COLOR	Output color for each pixel.
gl_FragCoord	vec4	FRAGCOORD	For full screen quads. For smaller quads, use UV.
gl_Position	vec4	VERTEX	Position of Vertex, output from Vertex Shader.
gl_PointSize	float	POINT_SIZE	Size of Point primitive.
gl_PointCoord	vec2	POINT_COORD	Position on point when drawing Point primitives.
gl_FrontFacing	bool	FRONT_FACING	True if front face of primitive.

Coordinates

gl_FragCoord in GLSL and FRAGCOORD in the Rebel Shading Language use the same coordinate system. If using UV in Rebel Shading Language, the y-coordinate will be flipped upside down.

Precision

In GLSL, you can define the precision of a given type (float or int) at the top of the shader with the precision keyword. In Rebel Shading Language, you can set the precision of individual variables as you need by placing precision qualifiers lowp, mediump, and highp before the type when defining the variable. For more information, see the Shading Language reference.

Shadertoy

Shadertoy is a website that makes it easy to write fragment shaders and create pure magic.

Shadertoy does not give the user full control over the shader. It handles all the input and uniforms and only lets the user write the fragment shader.

Shadertoy types

Shadertoy uses the webgl spec, so it runs a slightly different version of GLSL. However, it still has the regular types, including constants and macros.

Shadertoy entry point

The main point of entry to a Shadertoy shader is the mainImage function. mainImage has two parameters, fragColor and fragCoord, which correspond to COLOR and FRAGCOORD in Rebel Shading Language, respectively. These parameters are handled automatically in Rebel Engine, so you do not need to include them as parameters yourself. Anything in the mainImage function should be copied into the fragment function when porting to Rebel Shading Language.

Shadertoy variables

In order to make writing fragment shaders straightforward and easy, Shadertoy handles passing a lot of helpful information from the main program into the fragment shader for you. A few of these have no equivalents in Rebel Shading Language because Rebel has chosen not to make them available by default. This is okay because Rebel Shading Language gives you the ability to make your own uniforms. For variables whose equivalents are listed as "Provide with Uniform", users are responsible for creating that uniform themselves. The description gives the reader a hint about what they can pass in as a substitute.

Variable	Type	Equivalent	Description
fragColor	out vec4	COLOR	Output color for each pixel.
fragCoord	vec2	FRAGCOORD.xy	For full screen quads. For smaller quads, use UV.
iResolution	vec3	1.0 / SCREEN_PIXEL_SIZE	Can also pass in manually.
iTime	float	TIME	Time since shader started.
iTimeDelta	float	Provide with Uniform	Time to render previous frame.
iFrame	float	Provide with Uniform	Frame number.
iChannelTime[4]	float	Provide with Uniform	Time since that particular texture started.
iMouse	vec4	Provide with Uniform	Mouse position in pixel coordinates.
iDate	vec4	Provide with Uniform	Current date, expressed in seconds.
iChannelResolution[4]	vec3	1.0 / TEXTURE_PIXEL_SIZE	Resolution of particular texture.
iChanneli	Sampler2D	TEXTURE	One built-in; the game developer can make more.

Shadertoy coordinates

fragCoord behaves the same as gl_FragCoord in GLSL and FRAGCOORD in Rebel Shading Language.

The Book of Shaders

Similar to Shadertoy, The Book of Shaders provides access to a fragment shader in the web browser, with which the user may interact. The user is restricted to writing fragment shader code with a set list of uniforms passed in and with no ability to add additional uniforms.

For further help on porting shaders to various frameworks generally, The Book of Shaders provides a page on running shaders in various frameworks.

The Book of Shaders types

The Book of Shaders uses the webgl spec, so it runs a slightly different version of GLSL. However, it still has the regular types, including constants and macros.

The Book of Shaders entry point

The entry point for a Book of Shaders fragment shader is main, just like in GLSL. Everything written in a Book of Shaders main function should be copied into Rebel Shading Language's fragment function.

The Book of Shaders variables

The Book of Shaders sticks closer to plain GLSL than Shadertoy does. It also implements fewer uniforms than Shadertoy.

Variable	Type	Equivalent	Description
gl_FragColor	out vec4	COLOR	Output color for each pixel.
gl_FragCoord	vec4	FRAGCOORD	For full screen quads. For smaller quads, use UV.
u_resolution	vec2	1.0 / SCREEN_PIXEL_SIZE	Can also pass in manually.
u_time	float	TIME	Time since shader started.
u_mouse	vec2	Provide with Uniform	Mouse position in pixel coordinates.

The Book of Shaders coordinates

The Book of Shaders uses the same coordinate system as GLSL.