How do you render a game without using DirectX or OpenGL?

For example in some games there are 3 different display mode there are

1. OpenGL
2. DirectX
3. Software

What is this software mode? Like, how do programmers ma开发者_开发技巧ke game engine that generates images without using OpenGL or DirectX are there classes in C++ that generates frames?

Software means exactly that: software.

All rendering is is coloring pixels via some algorithm. That algorithm can be done by dedicated hardware, but you could simply implement those functions yourself in actual code. Now, that doesn't mean it's particularly fast; it takes a great deal of skill to implement a triangle rasterizer that has decent speed.

Software Mode can mean two things:

1. A System-provided Emulation layer. For example DX11 provides the WARP-device where you, as the application programmer, just specify "I want to use WARP" and the rest is done by DirectX. The Emulation Layer basically does Option Number 2:

2. Do it all by hand. Essentially a hardware accelerated GFX-card mostly only draws triangles. You can write a function that draws the pixels of a textured triangle directly into the screen-memory of the graphics-card. It's not very fast nowadays (that's why hardware-accelerated gfx-cards exist), but that's how it was done in the 80s and 90s when no such cards existed yet.

For a rough examplanation how a texture mapper works just look into the wikpedia article: https://en.wikipedia.org/wiki/Texture_mapping

I'm not aware of any gfx-libs that provide an own software layer, but i'm sure they exist somewhere.

As an example, directx has a layered setup, there is the code interface, which interacts with the HAL, or hardware abstraction layer. Depending on the capabilities of the underlying hardware, the HAL might run some peices of code on the CPU because the drivers reported the GPU doesn't support that feature. (Yes I know this a gross oversimplification)

see: http://msdn.microsoft.com/en-us/library/gg426101(v=vs.85).aspx

and: http://www.codeproject.com/KB/graphics/DirectX_Lessons_2_.aspx