[In this Microsoft-sponsored article, part of the Gamasutra XNA microsite, the team behind graphics debugging tool PIX explains the improvements in the June 2008 release, also revealing a sneak peek at upcoming additions to the tool.]

PIX for Windows® is a comprehensive graphics debugging/optimization tool that records detailed information about an application's use of Direct3D®. PIX collects data for a single rendered frame or a continuous stream of rendered frames. It can include logs of Direct3D calls made by the application, performance counter data, screenshots, and much more.

With PIX, you can step through the function calls and shader instructions executed by your application and see exactly how the final frame image is constructed. This is a powerful way both to track down bugs and to identify inefficient usage of Direct3D.

This article focuses on the new features and improvements that were made in the June 2008 release of PIX for Windows, which you can find in the DirectX SDK, and gives a sneak peek at what is to come in future versions.

Figure 1. PIX for Windows (click for full size)

Advanced Draw Call Support

The June 2008 version of PIX improves support for draw calls in a number of key areas. The Render view, Mesh view, Pixel History, and Shader Debugger have all been updated to support Direct3D 9's hardware instancing and Direct3D 10's DrawIndexedInstanced, DrawInstanced, and DrawAuto functions.

When viewing instanced geometry in the Mesh view, note that the Post-Vertex Shader tab, Post-Geometry Shader tab, and the Viewport pane now display the vertices and primitives for all instances generated by the original draw call.

Analyzing a particular pixel using the Pixel History or Shader Debugger allows you to drill down to each instance rendered by the instanced draw calls.

Figure 2. Render view support for draw instancing (click for full size)

Predicated draw calls were introduced in Direct3D 10 as a means to skip draw calls on the GPU, without needing feedback from the CPU. For instance, you could conditionally draw some complex object based on whether a lower resolution model of that object, such as its bounding box, is visible. The occlusion test for whether the simple object is visible or not, and the conditional drawing of the complex object, both happen on the GPU without having to deal with the complex threading issues of communication between GPU and CPU. This predication mechanism has been used effectively in professional titles as a simple and powerful way to do scene graph culling on the GPU.

PIX now supports Direct3D 10's ID3D10Predicate objects during capture and when viewing geometry in the Mesh view.

Improved Performance

One of the most common ways to use PIX is in single-frame capture mode. In this mode, the user launches the target program from PIX, navigates to an area where there is a rendering glitch or performance problem, then triggers a capture of a single frame for analysis. With previous versions of PIX, the target program would run noticeably slower while in this mode. This overhead has been drastically reduced in the June 2008 version of PIX, making it much easier to get to the frame of interest. In addition, PIX's overall memory footprint has been reduced, which boosts performance and decreases the likelihood of a memory failure.

Figure 3. A single-frame capture (click for full size)