graphics libraries and GPUs

Question

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How does OpenGL work at the lowest level?

When we make a program that uses the OpenGL library, for example for the Windows platform and have a graphics card that supports OpenGL, what happens is this:

1. We developed our program in a programming language linking the graphics with OpenGL (eg Visual C++).

2. Compile and link the program for the target platform (eg Windows)

3. When you run the program, as we have a graphics card that supports OpenGL, the driver installed on the same Windows will be responsible for managing the same graphics. To do this, when the CPU will send the required data to the chip on the graphics card (eg NVIDIA GPU) sketch the results.

4. In this context, we talk about graphics acceleration and downloaded to the CPU that the work of calculating the framebuffer end of our graphic representation.

5. In this environment, when the driver of the GPU receives data, how leverages the capabilities of the GPU to accelerate the drawing? Translates instructions and data received CUDA language type to exploit parallelism capabilities? Or just copy the data received from the CPU in specific areas of the device memory? Do not quite understand this part.

6. Finally, if I had a card that supports OpenGL, does the driver installed in Windows detect the problem? Would get a CPU error or would you calculate our framebuffer?

Answer 1

You'd better work into computer gaming sites. They frequently give articles on how 3D graphics works and how "artefacts" present themselves in case of errors in games or drivers.

You can also read article on architecture of 3D libraries like Mesa or Gallium.

Overall drivers have a set of methods for implementing this or that functionality of Direct 3D or OpenGL or another standard API. When they are loading, they check the hardware. You can have cheap videocard or expensive one, recent one or one released 3 years ago... that is different hardware. So drivers are trying to map each API feature to an implementation that can be used on given computer, accelerated by GPU, accelerated by CPU like SSE4, or even some more basic implementation.

Then driver try to estimate GPU load. Sometimes function can be accelerated, yet the GPU (especially low-end ones) is alreay overloaded by other task, then it maybe would try to calculate on CPU instead of waiting for GPU time slot.

When you make mistake there is always several variants, depending on intelligence and quality of driver.

• Maybe driver would fix the error for you, ignoring your commands and running its own set of them instead.
• Maybe the driver would return to your program some error code
• Maybe the driver would execute the command as is. If you issued painting wit hred colour instead of green - that is an error, but the kind that driver can not know about. Search for "3d artefacts" on PC gaming related sites.
• In worst case your eror would interfere with error in driver and your computer would crash and reboot.

Of course all those adaptive strategies are rather complex and indeterminate, that causes 3D drivers be closed and know-how of their internals closely guarded.

Search sites dedicated to 3D gaming and perhaps also to 3D modelling - they should rate videocards "which are better to buy" and sometimes when they review new chip families they compose rather detailed essays about technical internals of all this.

Answer 2

To question 5. Some of the things that a driver does: It compiles your GPU programs (vertex,fragment, etc. shaders) to the machine instructions of the specific card, uploads the compiled programs to the appropriate area of the device memory and arranges the programs to be executed in parallel on the many many graphics cores on the card. It uploads the graphics data (vertex coordinates, textures, etc.) to the appropriate type of graphics card memory, using various hints from the programmer, for example whether the date is frequently, infrequently, or not at all updated. It may utilize special units in the graphics card for transfer of data to/from host memory, for example some nVidia card have a DMA unit (some Quadro card may have two or more), which can upload, for example, textures in parallel with the usual driver operation (other transfers, drawing, etc).