Howto:Shader programming in FlightGear: Difference between revisions

The ALS Landing Lights-Spotlight (we'll call it ALS Lights from now on) is a good example for showing how to incorporate a shader effect into [[Flightgear]] as it touches many parts of the visuals we see and many parts of the coding pipeline.

Some of these items may be controlled or rendered by the same effect and shader file. They might have to be accounted for individually. They may have special lighting influences that have to be accounted for. You have to take each one separately and account for all its needs.  

Preferences/[[Nasal]]/XML >> [[Property Tree]] >> Effect File >> Shader >> Rendered to Screen

Parameter entries defined in the Effect file correspond to a property tree data container (static or variable). They will contain the data needed by the shader program to perform its magic. The type of information contained in the property tree might be program control data or variable/static data that the shader program can manipulate prior to sending on to render.

In the case of ALS Lights, below is some of the data passed to, and used by, the shader program.

Some of this data may play a duel role inside the shader program. In other words it might be used to control other functions in addition to ALS Lights.

There will also be other parameter entries that have nothing to do with ALS Lights. They might be used for other actions or effects the shader is handling.  

In general, the shader program and the uniforms are defined in between the technique tags. The technique is assigned an index to distinguish one technique from another (technique n="1"). As is the case with tree.eff, sometimes the shader program and its uniforms are defined and needed in more than one technique. In the case of tree.eff it is used in technique 4 and 5. Which means in [[Flightgear]], the tree shader set to either of the the highest two shader settings still produces ALS Lights when activated.

Next comes the entry to define what shader program the parameters data is going to be passed to.

This is where you specify what shader program is to be used by the technique. ALS has the lowest techniques, with higher quality preceding lower quality.

In the case of ALS Lights, so far we only have to deal with the fragment shader.

The program section of the effect file is a nifty method used to allow users to add shaders to [[Flightgear]] without having to add code at C level language base. The C level base is programed to recognize the XML tag pair of <program></program> and thus incorporate the GLSL program files pointed to between the tags. Otherwise you would have to add the GLSL program calls in the base C requiring a completely different set of programming skills and also the necessity of compiling [[Flightgear]] everytime you want to add new shader. It can work this way because shader programs are compiled at run-time.

We'll describe the contents of the shader programs below. For now, suffice it to say tree-ALS.frag contains the main program and secondary_lights.frag has functions that are passed uniform data that is manipulated and returned to main for processing.

The uniforms section is the mechanism that feeds the parameter data to the shader program.

Note the name, "use_searchlight", which was originally defined in preferences.xml and then became an entry in parameters is now being passed to the program shader by the uniform. Below in the "Shader Program" section, we will show you how the shader receives the uniform's data.

The shader programs used in this example are "tree-ALS.frag" and "secondary_lights.frag".

Uniform data is brought into the shader in the following manner.

Variable data can be defined in the shader program. An example of variable data defined in the shader program that is needed for ALS Lights is

You don't have to use any path information or includes in the code because the GLSL compiler program takes care of linking all the shader programs as long as they are defined correctly in the "programs" section of the effect file.

The "main" function is the heart of the shader program. This is where the shader program manipulates all the data made available to it.

Some of the variable data contained in the shader program is used for other purposes and is introduced into the shader program from other property, parameter and uniform definitions not pertaining to ALS Lights.