Using the same vertex array object for different shader programs-CodePudding

my goal is to pack all mesh data into a C class, with the possibility of using an object of such a class with more than one GLSL shader program.

I've stuck with this problem: if I understand it well the way of how in GLSL vertex data are passed from CPU to GPU is based on using Vertex Array Objects which:

are made of Vertex Buffer Objects which store raw vertex data, and
Vertex Attribute Pointers which tells what shader variable location to assign and the format of the data passed to VBO above
additionally one can make an Element Buffer Object to store indices
another extra elements necessary to draw an object are textures which are made separately

I's able to make all that data and store only a VAO in my mesh class and only pass the VAO handler to my shader program to draw it. It works, but, for my goal of using multiple shader programs, the VAP stands in my way. It is the only element in my class that rely on a specific shader program's property (namely the location of input variables).

I wonder if I'm forced to remake all VAOs every time I'm using my shaders when I want to draw something. I'm afraid that the efficiency of my drawing code will suffer drastically.

So, I'm asking if I should forget of making my universal mesh class and instead make a separate objects for each shader program I'm using in my application. I would rather not, as I want to avoid unnecessary copying of my mesh data. On the other hand if this means my drawing routine will slow down because of all that remaking VAOs every milisecond during drawing then I have to rethink the design of my code :(

EDIT: OK I've misunderstood that VAOs store bindings to other objects, not the data itself. But there is one thing still left: to make an VAP I have to provide information about the location of an input variable from a shader and also the layout of data in VBO from a mesh.

What I'm trying to avoid is to make an separate object that stores that VAP which relies both on a mesh object and a shader object. I might solve my problem if all shader programs use the same location for the same thing, but at this moment I'm not sure if this is the case.

CodePudding user response：

additionally one can make an Element Buffer Object to store indices

another extra elements necessary to draw an object are textures which are made separately

Just to be clear, that's you being lazy and/or uninformed. OpenGL strongly encourages you to use a single buffer for both vertex and index data. Vulkan goes a step further by limiting the number of object names (vao, vbo, textures, shaders, everything) you can create and actively punishes you for not doing that.

As to the problem you're facing.. It would help if you used correct nomenclature, or at least full names. "VAP" is yet another sign of laziness that hides what your actual issue is.

I will mention that VAOs are separate from VBOs, and you can have multiple VAOs linked to the same VBO (VBOs are just buffers of raw data -- again, Vulkan goes a step further here and its buffers store literally everything from vertex data, element indices, textures, shader constants etc).

You can also have multiple programs use the same VAOs, you just bind whatever VAO you want active (glBindVertexArray) once, then use your programs and call your render functions as needed. Here's an example from one of my projects that uses the same terrain data to render both the shaded terrain and a grid on top of them:

        chunk.VertexIndexBuffer.Bind();

        // pass 1, render the terrain 
        terrainShader.ProgramUniform("selectedPrimitiveID", selectedPrimitiveId);
        terrainShader.Use();
        roadsTexture.Bind();
        GL.DrawArrays(PrimitiveType.Triangles, 0, chunk.VertexIndexBuffer.VertexCount);

        // pass 2, render the grid
        gridShader.Use();
        GL.DrawElements(BeginMode.Lines, chunk.VertexIndexBuffer.IndexCount, DrawElementsType.UnsignedShort, 0);