CODEWITHSUNDEEP
openglglslvertex-shader
danglingPointer
danglingPointer

Reputation: 916

GLSL - A do-nothing vertex shader?

So I have an opengl program that draws a group on objects. When I draw these objects I want to use my shader program is a vertex shader and a vertex shader exclusively. Basically, I am aiming to adjust the height of the model inside the vertex shader depending on a texture calculation. And that is it. Otherwise I want the object to be drawn as if using naked openGL (no shaders). I do not want to implement a fragment shader.

However I haven't been able to find how to make it so I can have a shader program with only a vertex shader and nothing else. Forgetting the part about adjust my model's height, so far I have:

gl_FrontColor = gl_Color;
gl_Position = modelViewProjectionMain * Position;

It transforms the object to the correct position alright, however when I do this I loose texture coordinates and also lighting information (normals are lost). What am I missing? How do I write a "do-nothing" vertex shader? That is, a vertex shader you could turn off and on when drawing a textured .obj with normals, and there would be no difference?

Upvotes: 1

Views: 1640

Answers (2)

Hugh Fisher
Hugh Fisher

Reputation: 2516

To expand a little on the entirely correct answer by Abhishek Bansal, what you want to do would be nice but is not actually possible. You're going to have to write your own vertex and fragment shaders.

From your post, by "naked OpenGL" you mean the fixed-function pipeline in OpenGL 1 and 2, which included built-in lighting and texturing. Shaders in OpenGL entirely replace the fixed-function pipeline rather than extending it. And in OpenGL 3+ the old functionality has been removed, so now they're compulsory.

The good news is that vertex/fragment shaders to perform the same function as the original OpenGL lighting and texturing are easy to find and easy to modify for your purpose. The OpenGL Shading Language book by Rost, Licea-Kane, etc has a whole chapter "Emulating OpenGL Fixed Functionality" Or you could get a copy of the 5th edition OpenGL SuperBible book and code (not the 6th edition) which came with a bunch of useful predefined shaders. Or if you prefer online resources to books, there are the NeHe tutorials.

Writing shaders seems a bit daunting at first, but it's easier than you might think, and the extra flexibility is well worth it.

Upvotes: 1

Abhishek Bansal
Abhishek Bansal

Reputation: 5335

You can't write a shader with partial implementation. Either you do everything in a shader or completely rely on fixed functionality(deprecated) for a given object.

What you can do is this:

glUseProgram(handle)
 // draw objects with shader
glUseProgram(0)
// draw objects with fixed functionality

Upvotes: 2

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