I recently started programming with openGL. I've done code creating basic primitives and have used shaders in webGL. I've googled the subject extensively but it's still not that clear to me. Basically, here's what I want to know. Is there anything that can be done in GLSL that can't be done in plain openGL, or does GLSL just do things more efficiently?
The short version is: OpenGL is an API for rendering graphics, while GLSL (which stands for GL shading language) is a language that gives programmers the ability to modify pipeline shaders. To put it another way, GLSL is a (small) part of the overall OpenGL framework.
To understand where GLSL fits into the big picture, consider a very simplified graphics pipeline.
Vertexes specified ---(vertex shader)---> transformed vertexes ---(primitive assembly)---> primitives ---(rasterization)---> fragments ---(fragment shader)---> output pixels
The shaders (here, just the vertex and fragment shaders) are programmable. You can do all sorts of things with them. You could just swap the red and green channels, or you could implement a bump mapping to make your surfaces appear much more detailed. Writing these shaders is an important part of graphics programming. Here's a link with some nice examples that should help you see what you can accomplish with custom shaders: http://docs.unity3d.com/Documentation/Components/SL-SurfaceShaderExamples.html.
In the not-too-distant past, the only way to program them was to use GPU assembler. In OpenGL's case, the language is known as ARB assembler. Because of the difficulty of this, the OpenGL folks gave us GLSL. GLSL is a higher-level language that can be compiled and run on graphics hardware. So to sum it all up, programmable shaders are an integral part of the OpenGL framework (or any modern graphics API), and GLSL makes it vastly easier to program them.
As also covered by Mattsills answer GL Shader Language or GLSL is a part of OpenGL that enables the creation of algorithms called shaders in/for OpenGL. Shaders run on the GPU.
Shaders make decisions about factors such as the color of parts of surfaces, and the way surfaces share information such as reflected light. Vertex Shaders, Geometry Shaders, Tesselation Shaders and Pixel Shaders are types of shader that can be written in GLSL.
Q1:
Is there anything that can be done in GLSL that can't be done in plain OpenGL?
A:
You may be able to use just OpenGL without the GLSL parts, but if you want your own surface properties you'll probably want a shader make this reasonably simple and performant, created in something like GLSL. Here are some examples:
Q2:
Or does GLSL just do things more efficiently?
A:
Pixel shaders specifically are very parallel, calculating values independently for every cell of a 2D grid, while also containing significant caveats, like not being unable to handle "if" statement like conditions very performantly, so it's a case of using different kinds of shaders to there strengths, on surfaces described and dealt with in the rest of OpenGL.
Q3:
I suspect you want to know if just using GLSL is an option, and I can only answer this with my knowledge of one kind of shader, Pixel Shaders. The rest of this answer covers "just" using GLSL as a possible option:
A:
While GLSL is a part of OpenGL, you can use the rest of OpenGL to set up the enviroment and write your program almost entirly as a pixel shader, where each element of the pixel shader colours a pixel of the whole screen.
For example:
(Note that WebGL has a tendency to hog CPU to the point of stalling the whole system, and Windows 8.1 lets it do so, Chrome seems better at viewing these links than Firefox.)
No, this is not a video clip of real water:
https://www.shadertoy.com/view/Ms2SD1
The only external resources fed to this snail some easily generatable textures:
https://www.shadertoy.com/view/ld3Gz2
Rendering using a noisy fractal clouds of points:
https://www.shadertoy.com/view/Xtc3RS
https://www.shadertoy.com/view/MsdGzl
A perfect sphere: 1 polygon, 1 surface, no edges or vertices:
https://www.shadertoy.com/view/ldS3DW
A particle system like simulation with cars on a racetrack, using a 2nd narrow but long pixel shader as table of data about car positions:
https://www.shadertoy.com/view/Md3Szj
Random values are fairly straightforward:
fract(sin(p)*10000.)
I've found the language in some respects to be hard to work with and it may or may not be particularly practical to use GLSL in this way for a large project such as a game or simulation, however as these demos show, a computer game does not have to look like a computer game and this sort of approach should be an option, perhaps used with generated content and/or external data.
As I understand it to perform reasonably Pixel Shaders in OpenGL:
Have to be loaded into a small peice of memory.
Do not support:
"if" statement like conditions.
recursion or while loop like flow control.
Are restricted to a small pool of valid instructions and data types.
Including "sin", mod, vector multiplication, floats and half precision floats.
Lack high level features like objects or lambdas.
And effectively calculate values all at once in parallel.
A consequence of all this is that code looks more like lines of closed form equations and lacks algorythms or higher level structures, using modular arithmetic for something akin to conditions.
Related
I have an art application I'm dabbling with that uses OpenGL for accelerated graphics rendering. I'd like to be able to add the ability to draw arbitrary piecewise curves - pretty much the same sort of shapes that can be defined by the SVG 'path' element.
Rather than tessellating my paths into polygons on the CPU, I thought it might be better to pass an array of values in a buffer to my shader defining the pieces of my curve and then using an in/out test to check which pixels were actually inside. In other words, I'd be iterating through a potentially large array of data describing each segment in my path.
From what I remember back when I learned shader programming years ago, GPUs handle if statements by evaluating both branches and then throwing away the branch that wasn't used. This would effectively mean that it would end up silently running through my entire buffer even if I only used a small part of it (i.e., my buffer has the capacity to handle 1024 curve segments, but the simple rectangle I'm drawing only uses the first four of them).
How do I write my code to deal with this variable data? Can modern GPUs handle conditional code like this well?
GPUs can handle arbitrary-length buffers and conditionals (or fake it convincingly). The problem is that a vertex and geometry shaders cannot generate arbitrary number of triangles from a short description.
OpenGL 4.0 added two new types of shaders: Tessellation Control shaders and Tessellation Evaluation shaders. These shaders give you the ability to tessellate curves and surfaces on the GPU.
I found this tutorial to be quite useful in showing how to tessellate Bezier curves on the GPU.
I have been researching different approaches to terrain systems in game engines for a bit now, trying to familiarize myself with the work. A number of the details seem straightforward, but I am getting hung up on a single detail.
For performance reasons many terrain solutions utilize shaders to generate parts or all of the geometry, such as vertex shaders to generate positions or tessellation shaders for LoD. At first I figured those approaches were exclusively for renders that weren't concerned about physics simulations.
The reason I say that is because as I understand shaders at the moment, the results of a shader computation generally are discarded at the end of the frame. So if you rely on shaders heavily then the geometry information will be gone before you could access it and send it off to another system (such as physics running on the CPU).
So, am I wrong about shaders? Can you store the results of them generating geometry to be accessed by other systems? Or am I forced to keep the terrain geometry on CPU and leave the shaders to the other details?
Shaders
You understand parts of the shaders correctly, that is: after a frame, the data is stored as a final composed image in the backbuffer.
BUT: Using transform feedback it is possible to capture transformed geometry into a vertex buffer and reuse it. Transform Feedback happens AFTER the vertex/geometry/tessellation shader, so you could use the geometry shader to generate a terrain (or visible parts of it once), push it through transform-feedback and store it.
This way, you potentially could use CPU collision detection with your terrain! You can even combine this with tessellation.
You will love this: A Framework for Real-Time, Deformable Terrain.
For the LOD and tessellation: LOD is not the prerequisite of tessellation. You can use tessellation to allow some more sophisticated effects such as adding a detail by recursive subdivision of rough geometry. Linking it with LOD is simply a very good optimization avoiding RAM-memory based LOD-mesh-levels, since you just have your "base mesh" and subdivide it (Although this will be an unsatisfying optimization imho).
Now some deeper info on GPU and CPU exclusive terrain.
GPU Generated Terrain (Procedural)
As written in the NVidia article Generating Complex Procedural Terrains Using the GPU:
1.2 Marching Cubes and the Density Function Conceptually, the terrain surface can be completely described by a single function, called the
density function. For any point in 3D space (x, y, z), the function
produces a single floating-point value. These values vary over
space—sometimes positive, sometimes negative. If the value is
positive, then that point in space is inside the solid terrain.
If the value is negative, then that point is located in empty space
(such as air or water). The boundary between positive and negative
values—where the density value is zero—is the surface of the terrain.
It is along this surface that we wish to construct a polygonal mesh.
Using Shaders
The density function used for generating the terrain, must be available for the collision-detection shader and you have to fill an output buffer containing the collision locations, if any...
CUDA
See: https://www.youtube.com/watch?v=kYzxf3ugcg0
Here someone used CUDA, based on the NVidia article, which however implies the same:
In CUDA, performing collision detection, the density function must be shared.
This will however make the transform feedback techniques a little harder to implement.
Both, Shaders and CUDA, imply resampling/recalculation of the density at at least one location, just for the collision detection of a single object.
CPU Terrain
Usually, this implies a RAM-memory stored set of geometry in the form of vertex/index-buffer pairs, which are regularly processed by the shader-pipeline. As you have the data available here, you will also most likely have a collision mesh, which is a simplified representation of your terrain, against which you perform collision.
Alternatively you could spend your terrain a set of colliders, marking the allowed paths, which is imho performed in the early PS1 Final Fantasy games (which actually don't really have a terrain in the sense we understand terrain today).
This short answer is neither extensively deep nor complete. I just tried to give you some insight into some concepts used in dozens of solutions.
Some more reading: http://prideout.net/blog/?tag=opengl-transform-feedback.
Is there equivalent functionality to Directx's texturing blending? http://msdn.microsoft.com/en-us/library/windows/desktop/bb206241(v=vs.85).aspx
It basically blends several textures together before applying it onto the mesh.
From scanning through that page, I believe you can do all of that in a fragment shader. You can bind multiple textures, sample them all in your shader, and combine the results at your hearts desire.
It looks similar to functionality that OpenGL used to have in the fixed function pipeline. My old version of the red book (OpenGL Programming Guide) has chapters on "Multitexturing" and "Texture Combiner Functions". This is still available if you use the compatibility profile. But IMHO, this is a great example of where squeezing certain kinds of functionality into the fixed pipeline looked very cumbersome, while doing the same thing in shaders is much easier and more flexible.
Assume that we have different shader programs for different objects in a game. For example the player model has a shader that controls skeleton system (bone matrices multiplication etc.), or a particle has a shader for sparkling effects, wall has parallax mapping etc.
But what if I want to add fog to the game that must affect every one of these objects ? For example I have a room that will have a red fog, should I change EVERY glsl program to have fog code or is there a possible way to make global filters ? Should I change every glsl program when i want to add a feature ?
The typical process for this type of thing is to use a full-screen shader in post processing using the depth buffer from your fully rendered scene, or using a z-pass, which renders only to the depth buffer. You can chain them together and create any number of effects. It typically involves some render-to-texture work, and is not a real trivial task (too much to post code here), but it's not THAT difficult either.
If you want to take a look at a decent post-processing system, take a look at the PostFx system in Torque3D:
https://github.com/GarageGames/Torque3D
And here is an example of creating fog with GLSL in post:
http://isnippets.blogspot.com/2010/10/real-time-fog-using-post-processing-in.html
While trying to get the gist of OpenGL, I eventually ran into GLSL. I have used OpenGL before for miminal things, like triangles and colors (since I haven't learnt much yet), but when I found out about deprecated functions like glBegin and glEnd, I had to unlearn the things I had just learnt.
Now, I have come across vertex buffers, vertex buffer objects, vertex and fragments shaders... One thing I never understood though is why should one use GLSL? Why use GLSL along with OpenGL? What are the things you can't do using pure OpenGL? To me integrating GLSL shaders into programs adds complexity, since you have deal with external files or you have to embed shaders into programs, which causes more work.
I have very little experience. I'd like to learn more about the subject, but because of this incomprehensible contradiction, I'm unable to progress.
So, why use GLSL along with OpenGL?
Your question is not about GLSL; it's about shaders in general. GLSL is just the sanctioned way to provide shaders in OpenGL. Your question is really, "Why use shaders along with OpenGL?"
I'm not going to get into all of the details of what shaders can do that fixed function cannot. But here are just a few of the things that you cannot do with fixed-function OpenGL:
GPU-powered vertex weighted skinning.
Dual-quaternion-based skinning.
Proper tangent-space bump mapping.
Various deferred rendering techniques.
On-GPU frustum culling for instanced rendering.
Arbitrary lighting models, including BRDFs or other complex illumination models.
Complex shadow mapping techniques.
High-dynamic range rendering (OpenGL FFP doesn't let you exceed 1.0 on any color calculations).
Arbitrary tone mapping techniques.
I can keep going (for a long time), but I think you get the point. If you actually care about little things like "visual fidelity", you should be using shaders.
The question isn't "why use shaders;" it's "why not use shaders?"