I am using opengl shaders.
Does count of uniforms affect shader performance? If I pass 5 uniforms or 50 will it matter?
Does each shader has its own area where it working on? Or each shader can draw at any point of my application?
I often create vertex shader just to pass attributes to fragment shader. What benefit of vertex shader and why not just pass attributes in fragment?
I would guess it doesn't (and if it does, only a very minor one). But I don't have any evidence for that, so I might be wrong. This is almost certainly driver-specific.
A shader does not draw anything. A shader just processes data. In the pipeline, the rasterizer produces the fragments that are covered by your shape. And these are the fragments that you can potentially draw to. The fragment shader calculates the color (and possibly depth) and the rest of the pipeline decides what to do with the result (either updating the frame buffer, blending, or discarding it altogether). Each draw call can potentially produce a framebuffer update everywhere, not just at some specific locations.
This is perfectly fine if the application requires it. The main difference is that vertex shaders process vertices and fragment shaders process fragments. Usually, there are much more fragments than vertices, so the fragment shader is called more often than the vertex shader. Therefore, you should do as much work in the vertex shader as possible. Of course, there are things that you just cannot calculate in a vertex shader.
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I can't understand concept of smaller shaders in OpenGL. How does it work? For example: do I need to create one shader for positioning object in space and then shader another shader for lighting or what? Could someone explain this to me? Thanks in advance.
This is a very complex topic, especially since your question isn't very specific. At first, there are various shader stages (vertex shader, pixel shader, and so on). A shader program consists of different shader stages, at least a pixel and a vertex shader (except for compute shader programs, which are each single compute shaders). The vertex shader calculates the possition of the points on screen, so here the objects are being moved. The pixel shader calculates the color of each pixel, that is covered by the rendered geometry your vertex shader produced. Now, in terms of lighting, there are different ways of doing it:
Forward Shading
This is the straight-forward way, where you simply calculate the lighting in pixel shader of the same shader program, that moves to objects. This is the oldest way of calculating lighting, and the easiest one. However, it's abilities are very limited.
Deffered Shading
For ages, this is the go-to variant in games. Here, you have one shader program (vertex + pixel shader) that renders the geometrie on one (or multiple) textures (so it moves the objects, but it doesn't save the lit color, but rather things like the base color and surface normals into the texture), and then an other shader program that renders a quad on screen for each light you want to render, the pixel shader of this shader program reads the informations previously rendered in the textur by the first shader program, and uses it to render the lit objects on an other textur (which is then the final image). In constrast to forward shading, this allows (in theory) any number of lights in the scene, and allows easier usage of shadow maps
Tiled/Clustered Shading
This is a rather new and very complex way of calculating lighting, that can be build on top of deffered or forward shading. It basicly uses compute shaders to calculate an accelleration-structure on the gpu, which is then used draw huge amount of lights very fast. This allows to render thousands of lights in a scene in real time, but using shadow maps for these lights is very hard, and the algorithm is way more complex then the previous ones.
Writing smaller shaders means to separate some of your shader functionalities in another files. Then if you are writing a big shader which contains lightning algorithms, antialiasing algorithms, and any other shader computation algorithm, you can separate them in smaller shader files (light.glsl, fxaa.glsl, and so on...) and you have to link these files in your main shader file (the shader file which contains the void main() function) since in OpenGL a vertex array can only have one shader program (composition of vertex shader, fragment shader, geometry shader, etc...) during the rendering pipeline.
The way of writing smaller shader depends also on your rendering algorithm (forward rendering, deffered rendering, or forward+ rendering).
It's important to notice that writing a lot of shader will increase the shader compilation time, and also, writing a big shader with a lot of uniforms will also slow things down...
As far as I understand, the color of a pixel is determined by the fragment shader. Why do we need a vertex shader then? Is there anything a fragment shader cannot do (or cannot easily do) but a vertex shader can do (easily)?
But I still can't quite understand why it is named a "shader".
Because that's what programs executed as part of the rendering process are called. The Renderman interface specification was one of the first programmable rendering processes, and they called all of their programmable elements "shaders", even though they didn't all compute colors.
And therefore, "shader" has become the term used for describing any such program.
Vertex shaders convert vertex data, creating a 1:1 mapping from input vertices to output vertices. Fragment shaders operate on fragments. An FS invocation has no control over where it will be executed. They are generated in the location that the rasterizer says they go, and the FS has no way to affect this.
By contrast, a vertex shader has complete control over where the vertices will go.
I'm new to OpenGL, and I'm trying to understand vertex and fragment shaders. It seems you can use a vertex shader to make a gradient if you define the color you want each of the vertices to be, but it seems you can also make gradients using a fragment shader if you use the FragCoord variable, for example.
My question is, since you seem to be able to make color gradients using both kinds of shaders, which one is better to use? I'm guessing vertex shaders are faster or something since everyone seems to use them, but I just want to make sure.
... since everyone seems to use them
Using vertex and fragment shaders are mandatory in modern OpenGL for rendering absolutely everything.† So everyone uses both. It's the vertex shader responsibility to compute the color at the vertices, OpenGL's to interpolate it between them, and fragment shader's to write the interpolated value to the output color attachment.
† OK, you can also use a compute shader with imageStore, but I'm talking about the rasterization pipeline here.
In both the OpenGL and Direct3D rendering pipelines, the geometry shader is processed after the vertex shader and before the fragment/pixel shader. Now obviously processing the geometry shader after the fragment/pixel shader makes no sense, but what I'm wondering is why not put it before the vertex shader?
From a software/high-level perspective, at least, it seems to make more sense that way: first you run the geometry shader to create all the vertices you want (and dump any data only relevant to the geometry shader), then you run the vertex shader on all the vertices thus created. There's an obvious drawback in that the vertex shader now has to be run on each of the newly-created vertices, but any logic that needs to be done there would, in the current pipelines, need to be run for each vertex in the geometry shader, presumably; so there's not much of a performance hit there.
I'm assuming, since the geometry shader is in this position in both pipelines, that there's either a hardware reason, or a non-obvious pipeline reason that it makes more sense.
(I am aware that polygon linking needs to take place before running a geometry shader (possibly not if it takes single points as inputs?) but I also know it needs to run after the geometry shader as well, so wouldn't it still make sense to run the vertex shader between those stages?)
It is basically because "geometry shader" was a pretty stupid choice of words on Microsoft's part. It should have been called "primitive shader."
Geometry shaders make the primitive assembly stage programmable, and you cannot assemble primitives before you have an input stream of vertices computed. There is some overlap in functionality since you can take one input primitive type and spit out a completely different type (often requiring the calculation of extra vertices).
These extra emitted vertices do not require a trip backwards in the pipeline to the vertex shader stage - they are completely calculated during an invocation of the geometry shader. This concept should not be too foreign, because tessellation control and evaluation shaders also look very much like vertex shaders in form and function.
There are a lot of stages of vertex transform, and what we call vertex shaders are just the tip of the iceberg. In a modern application you can expect the output of a vertex shader to go through multiple additional stages before you have a finalized vertex for rasterization and pixel shading (which is also poorly named).
So I want to draw lots of quads (or even cubes), and stumbled across this lovely thing called the geometry shader.
I kinda get how it works now, and I could probably manipulte it into drawing a cube for every vertex in the vertex buffer, but I'm not sure if it's the right way to do it. The geometry shader happens between the vertex shader and the fragment shader, so it works on the vertices in screen space. But I need them in world space to do transformations.
So, is it OK to have my vertex shader simply pipe the inputs to the geometry shader, and have the geometry shader multiply by the modelviewproj matrix after creating the primitives? It should be no problem with the unified shader architecture, but I still feel queasy when making the vertex shader redundant.
Are there alternatives? Or is this really the 'right' way to do it?
It is perfectly OK.
Aside from that, consider using instanced rendering (glDrawArraysInstanced,glDrawElementsInstanced) with vertex attribute divisor (glVertexAttribDivisor). This way you can accomplish the same task without geometry shader at all.
For example, you can have a regular cube geometry bound. Then you have a special vertex attribute carrying cube positions you want for each instance. You should bind it with a divisor=1, what will make it advance for each instance drawn. Then draw the cube using glDraw*Instanced, specifying the number of instances.
You can also sample input data from textures, using gl_VertexID or gl_InstanceID for coordinates.