Digging Into DirectX 10

Time flies when you are having fun, and DirectX 9 has certainly given us plenty of that. We have been playing games on the DirectX 9 application programming interface (API) since December of 2002 - almost four years now. While many use the term "DirectX" when talking about graphics, the DirectX standard is actually a group of APIs for various aspects of the gaming experience, of which graphics is one. Big titles such as Half-Life 2, F.E.A.R., Battlefield 2, and Oblivion use the current version, which is 9.0c.

There are three core areas in DirectX: input devices, audio and video. The table below shows a list of the APIs in the current build.

For the remainder of this article we will be talking about what most people are most interested in: Direct3D. There are a lot of changes coming in DirectX 10. Direct3D 10 adds new features to deal with existing graphical problems, and tools to take on new rendering challenges. It also simplifies the 3D pipeline, which will help move bigger and better game titles out of the developers' studios and onto your PC.

http://www.tomshardware.com/2006/11/08/what_direct3d_10_is_all_about/

One constraint on content development is getting the artist's intent and creativity onto the screen. Humans have the ability to conjure realistic images in their minds, which contain all of the detail of opening their eyes to the real world. Computer graphics is very different, as the logic in our minds is much more dynamic than that of a computer. The silicon is set in stone and cannot be changed once it leaves the factory - at least until the advent of programmable shaders. However, programmable shaders are only one component of the rendering engine. Constraints are placed on what can and cannot be calculated, as there is limited computational horsepower.

As the needs of programmers has increased, a new standard for hardware implementation and software coding has come up to keep ahead of the demand. The need is there again, and Direct3D has upped the ante on what shaders can do.

Below is a table showing the changes in shader model constraints. Without going too deep, the main concept to pull from the table is that Shader Model 4.0 (SM 4.0) is far more advanced than any previous version. It also takes away the concept of vertex shader or pixel shader only designs. One key step, as listed in Blythe's paper and lecture, is that fixed function shaders will become a thing of the past.

Another place for open interpretation came into play with the approaches ATI and Nvidia have taken under DirectX 9. Nvidia has stuck to a fixed function design for its architectures, while ATI decided to fragment the traditional pipeline in order to provide greater functionality with the same limited resources.

There is nothing wrong with designing X vertex shaders to Y pixel shaders with Y ROPs (Raster Operation Processors), but as ATI has proven with its current DirectX 9.0c graphics processors, there are performance gains to be made with a dynamic core. Many game developers have shifted their titles to becoming pixel shader dependent with a fragmented design; ATI was able to capitalize on this as it added pixel shader units to new core designs. The layout for Direct3D 10 hardware could follow fixed function shader units, but what would be the advantage?

While Nvidia has not publicly commented on what structure its G80 processor is based upon, ATI has stated it would go with a unified shader core. Unified shaders makes the most sense, as it means shader units can change function on the fly. This means that if a frame has more vertex shader needs or more pixel shading needs, the core can designate more processing units or clusters to accomplish the task that is in most demand.

Below are a few examples of workload needs in a "traditional" processing environment (these are crude to keep it simple).

Example 1: Heavy vertex with light pixel processing needs.

In this example, the Pixel Shader is underutilized; it could be doing more in the frame, but is capped by the Vertex Shader output.

Example 2: Light vertex with heavy pixel processing needs.

In this example, the Pixel Shader is underutilized; it is capped by the Vertex Shader.

Example 3: Light vertex with heavy pixel processing needs.

In this example, the Pixel Shader can render the same amount as in Example 1, but there are 12 more vertex processors that can be utilized. This brings the graphics core to maximum capacity.

Example 4: Light vertex with heavy pixel processing needs.

In this last example, the Vertex Shader workload is the same as in example 2, but there are 6 more pixel processors that can be utilized.

The point to this demonstration is that shaders are merely programs running on floating point processors. With the new standard in Direct3D 10, it does not make sense to design fixed function shader architecture. With intelligent logic and a strong core, we should see significant yields on unified architectures, with the same number of floating point units as we would from fixed function architecture with the same total number of FPUs.