openglglslrendering-engine

Vertex shader attribute mapping in GLSL

I'm coding a small rendering engine with GLSL shaders:

Each Mesh (well, submesh) has a number of vertex streams (eg. position,normal,texture,tangent,etc) into one big VBO and a MaterialID.

Each Material has a set of textures and properties (eg. specular-color, diffuse-color, color-texture, normal-map,etc)

Then I have a GLSL shader, with it's uniforms and attributes. Let's say:

uniform vec3 DiffuseColor;
uniform sampler2D NormalMapTexture;
attribute vec3 Position;
attribute vec2 TexCoord;

I'm a little bit stuck in trying to design a way for the GLSL shader to define the stream mappings (semantics) for the attributes and uniforms, and then bind the vertex streams to the appropriate attributes.

Something in the lines of saying to the mesh :"put your position stream in attribute "Position" and your tex coordinates in "TexCoord". Also put your material's diffuse color in "DiffuseColor" and your material's second texture in "NormalMapTexture"

At the moment I am using hard-coded names for the attributes (ie. vertex pos is always "Position" ,etc) and checking each uniform and attribute name to understand what the shader is using it for.

I guess I'm looking for some way of creating a "vertex declaration", but including uniforms and textures too.

So I'm just wondering how people do this in large-scale rendering engines.

Edit:

Recap of suggested methods:

1. Attribute/Uniform semantic is given by the name of the variable (what I'm doing now) Using pre-defined names for each possible attribute.The GLSL binder will query the name for each attribute and link the vertex array based on the name of the variable:

//global static variable

semantics (name,normalize,offset) = {"Position",false,0} {"Normal",true,1},{"TextureUV,false,2}

 ...when linking
for (int index=0;index<allAttribs;index++)
{
   glGetActiveAttrib(program,index,bufSize,length,size[index],type[index],name);      
   semantics[index]= GetSemanticsFromGlobalHardCodedList(name);
} 
... when binding vertex arrays for render
 for (int index=0;index<allAttribs;index++)
{
    glVertexAttribPointer(index,size[index],type[index],semantics[index]->normalized,bufferStride,semantics[index]->offset);

}

2. Predefined locations for each semantic

GLSL binder will always bind the vertex arrays to the same locations.It is up to the shader to use the the appropriate names to match. (This seems awfully similar to method 1, but unless I misunderstood, this implies binding ALL available vertex data, even if the shader does not consume it)

.. when linking the program...
glBindAttribLocation(prog, 0, "mg_Position");
glBindAttribLocation(prog, 1, "mg_Color");
glBindAttribLocation(prog, 2, "mg_Normal");

3. Dictionary of available attributes from Material, Engine globals, Renderer and Mesh

Maintain list of availlable attributes published by the active Material, the Engine globals, the current Renderer and the current Scene Node.

eg:

 Material has (uniformName,value) =  {"ambientColor", (1.0,1.0,1.0)}, {"diffuseColor",(0.2,0.2,0.2)}
 Mesh has (attributeName,offset) = {"Position",0,},{"Normals",1},{"BumpBlendUV",2}

then in shader:

 uniform vec3 ambientColor,diffuseColo;
 attribute vec3 Position;

When binding the vertex data to the shader, the GLSL binder will loop over the attribs and bind to the one found (or not? ) in the dictionary:

 for (int index=0;index<allAttribs;index++)
    {
       glGetActiveAttrib(program,index,bufSize,length,size[index],type[index],name);      
      semantics[index] = Mesh->GetAttributeSemantics(name);
}

and the same with uniforms, only query active Material and globals aswell.

Solution

  • Attributes:

    Your mesh has a number of data streams. For each stream you can keep the following info: (name, type, data).

    Upon linking, you can query the GLSL program for active attributes and form an attribute dictionary for this program. Each element here is just (name, type).

    When you draw a mesh with a specified GLSL program, you go through programs attribute dictionary and bind the corresponding mesh streams (or reporting an error in case of inconsistency).

    Uniforms:

    Let the shader parameter dictionary be the set of (name, type, data link). Typically, you can have the following dictionaries:

    After linking, the GLSL program is given a set of parameter dictionaries in order to populate it's own dictionary with the following element format: (location, type, data link). This population is done by querying the list of active uniforms and matching (name, type) pair with the one in dictionaries.

    Conclusion: This method allows for any custom vertex attributes and shader uniforms to be passed, without hard-coded names/semantics in the engine. Basically only the loader and render know about particular semantics: