Question

I’ve been researching this problem I have and I can’t seem to understand it well enough to solve it so I thought I might as well throw it out there and the intelligent bunch might have some ideas. 😛

Basically I have been working on a iPhone project for a while where I have the luxury to use all the newest frameworks and target 5.1. So I’ve been using GLKit and the GLKBaseEffect which have been working just fine for me. The reason I started out with GLKBaseEffect rather then writing my own shaders is that I don’t know glsl well. However the requirements have become more precise and the base effect just doesn’t seem to cut it any longer.

Since I am already doing all my transforms using the base effect I would prefer if I could keep my base effect intact but add glsl-type shaders on top if that makes any sense.

My old approach look something like this (this is in a loop rendering all objects, where an object contain such things as transforms, a mesh and some other less important things for this problem, such as textures, materials and so on)

ObjectBase *obj = [ResourceManager.shared getObjectNamed:name inScene:sceneName];
GLKMatrix4 modelview = effect.transform.modelviewMatrix;
effect.transform.modelviewMatrix = GLKMatrix4Multiply(effect.transform.modelviewMatrix, obj.transform);

[effect prepareToDraw];
[obj render];

effect.transform.modelviewMatrix = modelView;

Here we fetch an object to render and transform (i.e. translate, rotate and scale) the object then we render it, where the rendering itself fetches the mesh for the object, bind the buffers and render it.

So far so good.

What I would like to do however is that during the [obj render]; call I would like the object to also do something like glUseProgram(someProgram); adding more specialized shader code.

I guess one could argue that I am trying to use the base effect for my vertex shaders and want to use “normal” shaders for my fragment shaders. At least that’s what I think I want to do.

I have been trying some things.

I tried to create just the fragment shaders and glUseProgramon it, however it said that I need one vertex and one fragment shader when setting up and compiling it. I’ve also tried to create an empty vertex shader, which didn’t turn out very well, I don’t know what happens with that, but I am guessing that it overrule the base effect.

I am leaning toward, in the end, accept that it’s probably best to throw out the base effects and just write my own shaders all the way. I just feel like it’s a lot of work out the window, so I wanted to see how much of it I can save.

I do understand that my understanding of shaders are the part that gives me the most problems, so please be patient with that fact.

Answer 1

Just as you discovered, you can’t just use a fragment shader and leave behind the vertex shader. This is because both have different tasks. Vertex shaders deal with the per-vertex aspects: calculating the vertex data, texture (uv’s) etc and finally drawing the faces (triangles). Fragment shaders deal with what exactly will be drawn at each pixel on the screen (or in the viewport). When you provide only a fragment shader, you are not telling what your vertex data is, rather you are only telling OpenGL to do something on the pixels. And these pixels hold nothing/gibberish (I am not sure which) since your vertex shader did not do anything.

When using GLKEffect, a call to the [yourEffect prepareToDraw] method takes care of the shaders etc.

If you just wish to use a stock shader pair, why not use the one provided in the XCode OpenGL game template? When you run it, it has two cubes, one rendered using GLKit, and other the normal way. Though I think it will not be enough for most effects. In case you wish to know more about shaders, you can have a look at the NeHe GLSL introduction article. It is about GLSL and how you can write and use shaders in your code. You might want to have a look at Diney Bomfim’s All About Shaders articles and this page.

Using GLKit is nice in most cases, since it saves you from writing lots of useless, repetitive code. For example, you do not have to go through so many image formats with different color codings and bits per pixel (per format) and all when you can just use GLKTextureLoader.