CODEWITHSUNDEEP
c++openglgraphics3d
BulBul
BulBul

Reputation: 1209

Drawing geometry in opengl

Taking the standard opengl 4.0+ functions & specifications into consideration; i've seen that geometries and shapes can be created in either two ways:

  1. making use of VAO & VBO s.

  2. using shader programs.

which one is the standard way of creating shapes?? are they consistent with each other? or they are two different ways for creating geometry and shapes?

Upvotes: 3

Views: 2024

Answers (5)

Nicol Bolas
Nicol Bolas

Reputation: 473447

Let's lay out the full facts.

Shaders need input. Without input that changes, every shader invocation will produce exactly the same values. That's how shaders work. When you issue a draw call, a number of shader invocations are launched. The only variables that will change from invocation to invocation within this draw call are in variables. So unless you use some sort of input, every shader will produce the same outputs.

However, that doesn't mean you absolutely need a VAO that actually contains things. It is perfectly legal (though there are some drivers that don't support it) to render with a VAO that doesn't have any attributes enabled (though you have to use array rendering, not indexed rendering). In which case, all user-defined inputs to the vertex shader (if any) will be filled in with context state, which will be constant.

The vertex shader does have some other, built-in per-vertex inputs generated by the system. Namely gl_VertexID. This is the index used by OpenGL to uniquely identify this particular vertex. It will be different for every vertex.

So you could, for example, fetch geometry data yourself based on this index through uniform buffers, buffer textures, or some other mechanism. Or you can procedurally generate vertex data based on the index. Or something else. You could pass that data along to tessellation shaders for them to tessellate the generated data. Or to geometry shaders to do whatever it is you want with those. However you want to turn that index into real data is up to you.

Here's an example from my tutorial series that generates vertex data from nothing more than an index.

Upvotes: 3

JAre
JAre

Reputation: 4756

Geometry is loaded into the GPU with VAO & VBO.

I strongly (like really strongly) recommend you reading this http://fgiesen.wordpress.com/2011/07/09/a-trip-through-the-graphics-pipeline-2011-index/

Upvotes: 4

datenwolf
datenwolf

Reputation: 162164

i've seen that geometries and shapes can be created in either two ways:

Not either. In modern OpenGL-4 you need both data and programs.

VBOs and VAOs do contain the raw geometry data. Shaders are the programs (usually executed on the GPU) that turn the raw data into pixels on the screen.

Upvotes: 2

Aki Suihkonen
Aki Suihkonen

Reputation: 20027

Vertex shaders can be used to displace vertices, or to generate them from a builtin formula and the vertex index, which is available as a built in attribute in later open gl versions.

The difference between vertex and geometry shaders is that vertex shader is a 1:1 mapping, while geometry shader can create more vertices -- can be utilized in automatic Level of Detail generation for e.g. NURBS or perlin noise based terrains etc.

Upvotes: 1

Bahbar
Bahbar

Reputation: 18015

VAOs and VBOs how about what geometry to draw (specifying per-vertex data). Shader programs are about how to draw them (which program gets applied to each provided vertex, each fragment and so on).

Upvotes: 4

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