What is the Direct3D? It is an API, is not it? Is it implemented by Windows or by graphics cards?
If the graphics cards implement the Direct3D API, why can not other operating systems use Direct3D resources from the graphics card?
If Direct3D is implemented by Windows, it has to use graphics card resources, such as OpenGL or OpenCL. If Direct3D calls are not directly to the graphics card, it would be slower due to intermediate calls.
Please, help me to understand what the Direct3D is.
What is the Direct3D? It is an API, is not it? Is it implemented by Windows or by graphics cards?
Yes, Direct3D is an API. It is implemented (mostly) by Windows itself. However, Windows will offload a considerable part of the actual work to the drivers of the graphics card and ultimately to the gfx card itself, so one can also say that a gfx card "implements" D3D.
If the graphics cards implement the Direct3D API, why can not other operating systems use Direct3D resources from the graphics card?
They can, and they do, but only a relatively small (but often critical part) of D3D functionality is implemented directly by the card's hardware, so a lot more work in software is required to implement D3D.
If Direct3D is implemented by Windows, it has to use graphics card resources, such as OpenGL or OpenCL.
No, that's a misunderstanding. OpenGL and OpenCL are also APIs, and only partially implemented by the gfx hardware (just like D3D). The gfx hardware usually has a (proprietary) "native" API, which is what the gfx drivers (both for D3D and OpenGL) use.
If Direct3D calls are not directly to the graphics card, it would be slower due to intermediate calls.
There are not necessarily any "intermediate calls" - the D3D driver uses the cards native API, as explained above.
Direct 3D is a graphics API created by Microsoft. Its similar in function to opengl. OpenGL is a competing 3D graphics API created as a open standard. D3D doesn't need OpenGL to function.
Card manufactorers decide which APIs they want to implement; most always include DirectX (which includes Direct3D) and possibly OpenGL.
There is indirection as it doesn't go directly to the card, the cards D3D in turn calls the drivers, but this typicially is insignificant.
Direct3D is an API developed by Microsoft designed to help developers render 3D graphics. OpenGL and Direct3D are two separate APIs, but they must both interface to the video card using the drivers developed by the companies who manufacture the video cards. Both APIs (Direct3D and OpenGL) must both go through the driver in order to access the video card, and their speeds are dependent upon their design and their implementation in the video card drivers.
OpenCL is something different -- it's designed to help developers write programs which perform general-purpose computing on the GPU (not just for graphics). OpenCL is comparable to CUDA, but the latter in only supported on NVIDIA cards. Using CUDA instead of OpenCL may have some advantages, depending on your target system, since NVIDIA can make new features available to the CUDA API before they are accepted into the OpenCL API. However, even OpenCL and CUDA must interface to the driver in order to get anything done on the GPU.
As you already know, Direct3D only works on Microsoft and on Wine (for the most part), but its structure as an API is vastly different from that of OpenGL. Direct3D makes use of structures and includes more OOP elements in its API, while OpenGL acts as a state machine, lacking any structures or OOP features. Direct3D can often progress a little faster than OpenGL in terms of the features which it claims to officially support in its API because it is not designed for maximum compatibility with a wide range of devices; on the other hand, OpenGL has typically exhibited more inertia when it has come to adopting new features because of the inherent difficulty in adding new features to its API (the Khronos Group is influenced heavily by the CAD industry as well as many others, so it must cater to a wide range of needs). The time it took for the Khronos Group to finally adopt asynchronous API calls in the OpenGL is testament to this fact, and caused many people to lose faith in OpenGL.
However, OpenGL is cross-platform, endorsed by Apple, and it works on all operating systems on which it is implemented. You can easily use it with many popular window toolkits (Qt, SDL, FreeGLUT, JogAmp, gtk, etc.) and have confidence that your application will compile on other operating systems if you wrote it properly. The OpenGL API, unlike Direct3D, is an open-source industry standard.
As far as performance goes, it's still debatable as to which one is faster: depending on how you structure your program or batch your calls, this could change. However, performance should not really be a consideration for which API you use unless you have tested your application and have evidence that the choice of API is the cause of your bottleneck.
From Wikipedia:
Direct3D is a Microsoft DirectX API subsystem component. The aim of Direct3D is to abstract the communication between a graphics application and the graphics hardware drivers. It is presented like a thin abstract layer at a level comparable to GDI (see attached diagram). Direct3D contains numerous features that GDI lacks.
Direct3D is an Immediate mode graphics API. It provides a low-level interface to every video card 3D function (transformations, clipping, lighting, materials, textures, depth buffering and so on). It also had a higher level Retained mode component, that has now been officially discontinued.
