OpenGL lights, textures, etc. correct way?

Question

Until this moment I've only implemented all the effects in GLSL shaders using inputs, outputs and uniforms, except for a couple of really essential constants like gl_Position, etc. I've read several tutorials, had a lecture on computer graphics and everytime all they implement things by looking at physical model and calculating all the stuff using input values and uniforms. That is a kind of the way I thought it all works.

Now I faced the fact, that there are much more GLSL things, like glLight* API functions and gl_LightSource, gl_Texture constants in GLSL with a big set of light types and lighting models predefined. Seems to be a kind of different way of programming shaders.

I wonder if there are any advantages/disadvantages using one or other way? Did I miss something very important? It looks I'm doing a lot of redundant work.

Answer 1

All the glLight* calls you might find in both GLSL and the OpenGL API are from the old and deprecated fixed-function pipeline!

Now you must do all the calculations yourself through Shaders, as I can guess you're already doing.

Why did they "remove" all the awesome stuff?

They "removed" (deprecated) the Matrix Stack, Light calls, Immediate Mode Rendering, etc. etc. etc. and the list goes one for various reason. But the overall reason is that it's better to implement and control those things yourself.

It requires more work from our side implementing and controlling all those things, though you're in total control of everything and when you actually want to use something. Using the fixed-function pipeline OpenGL would allocate and load various things you might never even wanted to use.

Also when talking about the Matrix Stack as an example, you would usually (the lazy way) make OpenGL re-calculate the Matrix Stack each render call, using the old glPushMatrix(), glPopMatrix(), glTranslate*(), etc. functions. Now because YOU HAVE TO, you are forced to do all those calculations and handling the Matrices yourself. So now you would realize that most of the Matrices and much more could simply be allocated and calculated once, or atleast not every render call.

Of course they didn't deprecated Immediate Mode Rendering, because we need to implement that ourselves, now we simply need to use Buffers, because they are so much better in every way.

Extra

If you want a great spreadsheet that shows which function are deprecated and which are core functions, and extension functions, etc. Then take a look here, though be aware that this spreadsheet is made by people who use OpenGL and not by the Khronos Group (current developers of OpenGL) nor Silicon Graphics (the creators of OpenGL).

Answer 2

Ignore glLightXXX functions, the related gl_LightXXX variables and all the documentation associated with them. It's all deprecated and if you look closely at the docs, you'll probably that it's several years old or specifically designed for versions of OpenGL <= 2.x. Instead continue to work with your own vertex attributes and set up lighting configuration in your own uniforms however you please based on the model of lighting you want to implement. It's more work, but it's more flexible in the long run.

The OpenGL lighting model that uses glLight pre-dates the programmable shader pipeline, and represent a particular way of doing lighting in the fixed function pipeline.

Once GLSL entered the scene it was possible to use the OpenGL lighting model in conjunction with shaders. You could use the same glLight function and it's related functions to set up your lighting parameters but then write shaders that used the same information in different ways, allowing per-pixel lighting calculations.

Textures are a little more murky, because OpenGL still has a texture model and many of the GL functions relating to textures are still valid, though some are deprecated. However, any documentation that refers to GLSL variables like gl_Texture is similarly out of date. Current OpenGL uses sampler objects for texture access.

If you want to make sure you're doing it the 'modern' way, make sure you create a forward-compatible OpenGL profile of 3.3 or higher or 4.0 or higher, and make sure your shaders declare the appropriate version number as their first line like so:

#version 330

This will cause the use of any deprecated OpenGL function or deprecated shader variable to generate an error so that you know to avoid them.

Answer 3

Current graphics hardware offers an interface to customize any rendering step e.g Vertex Shading, Tesselation, Geometry shading, fragment shading and so on. GLSL is the language to programm or influence the rendering steps of the graphics hardware leveraging this interface.

The predefined function glLight, glTexture and so on belong to the deprecated fixed graphics pipeline of opengl. Modern OpenGL still supports the functions of this fixed pipeline but it ist strongly recommended to use GLSL for the different rendering steps.

The glLight function is a fixed function which just influences Vertex Processing. So you can just achieve a per vertex shading, which not looks very realistic.

When you programm the lighting on your own within the fragment shader using GLSL you can directly influence any pixel.

So to summarize the main advantage is that a programmer is more flexible and is able to influence every kind of rendering step, which enables you to achieve sophisticated and realistic 3d graphics. The main disadvantage is. You need much more knowledge and (GLSL, graphics pipeline) and much more programming effort to achieve the same result as with fixed functions.

Best regards