Working with Shaders

Shaders replace OpenGL’s fixed function pipeline that has been deprecated with OpenGL 3.0. To use shaders, you have to load their source files and compile them. Then you have to attach them to a program object and link this object. Finally, you have to tell OpenGL to use that program to process your input.

In OpenGLAda, the Shader and Object types are objects derived from GL_Object, and thus need to be initialized before use by calling Initialize_Id. See the code example below for usage. More type-specific details about these objects are provided in the sections below.

The package GL.Objects.Shaders

There are multiple types of shaders. Unlike buffer objects, a shader always is of one specific type and cannot be re-used as a shader of another type. The available shader types are contained in the enumeration Shader_Type. When you create a Shader object, you have to set its type to the discriminant Kind.

Usually, your shader source is available as a text file. There is a helper procedure in the package GL.Files named Load_Shader_Source_From_File which loads a shader file directly to OpenGL. This is more efficient than the procedure Set_Source, as you do not have to load the file into a String manually. Mind that Load_Shader_Source_From_File uses the current directory as base path to interpret the given file path.

Compile does not raise an exception when the compilation fails. Instead, you have to query the status of the compilation with Compile_Status. It returns False if an error occured during the compilation. You can query the compilation log with Info_Log.

The package GL.Objects.Programs

After you attached your shaders with Attach, linking the program works similar to compiling a shader. Call Use_Program to use the program for processing in OpenGL.

The program object also gives you access to the uniform and attribute parameters of your shaders. use Uniform_Location to query the internal name of a uniform parameter (which is and integer value). You can then interact with the parameter with the procedures in GL.Uniforms.

Interaction with attribute parameters is not fully implemented yet.

Basic example

<?prettify lang=ada?>

declare
   Vertex_Shader   : GL.Objects.Shaders.Shader
     (Kind => GL.Objects.Shaders.Vertex_Shader);
   Fragment_Shader : GL.Objects.Shaders.Shader
     (Kind => GL.Objects.Shaders.Fragment_Shader);
   Program         : GL.Objects.Programs.Program;
begin

   -- initialize objects
   Vertex_Shader.Initialize_Id;
   Fragment_Shader.Initialize_Id;
   Program.Initialize_Id;

   -- load and compile shaders
   GL.Files.Load_Shader_Source_From_File (Vertex_Shader, "vertex.glsl");
   GL.Files.Load_Shader_Source_From_File (Fragment_Shader, "fragment.glsl");

   Vertex_Shader.Compile;
   Fragment_Shader.Compile;

   if not Vertex_Shader.Compile_Status then
      Ada.Text_IO.Put_Line ("Compilation of vertex shader failed. log:");
      Ada.Text_IO.Put_Line (Vertex_Shader.Info_Log);
   end if;
   if not Fragment_Shader.Compile_Status then
      Ada.Text_IO.Put_Line ("Compilation of fragment shader failed. log:");
      Ada.Text_IO.Put_Line (Fragment_Shader.Info_Log);
   end if;

   -- set up program
   Program.Attach (Vertex_Shader);
   Program.Attach (Fragment_Shader);
   Program.Link;
   if not Program.Link_Status then
      Ada.Text_IO.Put_Line ("Program linking failed. Log:");
      Ada.Text_IO.Put_Line (Program.Info_Log);
      return;
   end if;
   Program.Use_Program;

   -- do something useful here
end;