The screen capture entry is a special capture "driver" within VirtualDub that allows the video screen to be used as the capture source instead of an external source.

Selecting the region to capture

Use the Video > Set Custom Format... menu option to choose the size of image to capture. Use 32-bit RGB as the format.

By default, the image capture will occur at the top-left corner of the screen. The Video > Source... menu option exposes a couple of additional options to control the origin: it can be centered on the mouse cursor, to the current active top-level window, or both. When both options are active, VirtualDub will pan around the bounds of the active window according to the cursor position.

Starting the capture and performance implications

You will usually want to set hotkey shortcuts for starting and stopping the capture via Capture > Preferences, so as to avoid capturing the VirtualDub window itself.

Capturing from the screen is very CPU-intensive and depends greatly on the speed of your CPU and video card. The size of the region captured greatly influences the CPU usage, so if you have problems, consider capturing a smaller size. More modern video cards and video drivers also tend to be more efficient at screen capture.

Enabling OpenGL acceleration

If you have a video card with 3D acceleration, you may be able to enable OpenGL acceleration mode, which enables additional features and also speeds up the screen capture process.

The amount of acceleration possible depends on your video card's features.

• Basic OpenGL support allows 32-bit capture with hardware accelerated resizing.
• Advanced blending support permits hardware accelerated conversion to YCbCr formats, including YUY2, UYVY, and YV12. For cases where some color bleeding can be tolerated, this drops the raw data bandwidth requirements by 50-60%, and also speeds up real-time video compression if used. You need at least an NVIDIA GeForce video card (NV_register_combiners OpenGL extension) for hardware YCbCr conversion.
• Pixel (fragment) shader support lowers the load on the video card. To take advantage of pixel shader support, you need at least an NVIDIA GeForce 3 (NV_register_combiners2 extension) or an ATI RADEON 8500 (ATI_fragment_shader extension).
• Occlusion query support allows duplicate frames to be removed on the video card with neglegible CPU load. Occlusion query support requires the NV_occlusion_query OpenGL extension (this is generally supported on 3D accelerators from both NVIDIA and ATI).