There are several ways to decode a DVD Movie on a PC. The two main methods are Hardware decoding and Software decoding.

However, even with certain types of Hardware decoders, the CPU usage differs and several of the Software decoders can use certain acceleration functions that are made available by the Display Card being used.


Acceleration Types:

1. Dedicated decoder cards:
   These are cards with a single usage, the decoding of MPEG streams (DVD uses the MPEG2 format). These cards usually decode the entire MPEG/DVD stream in hardware requiring a far weaker CPU to achieve smooth playback.

2. Colorspace Conversion:
   This acceleration method was first enabled for MPEG-1 acceleration used in VCD titles. The MPEG format keeps the image data in a non-uniform color-space. Rather than the Red Green and Blue (RGB) color-space used by the computer Monitor and the Television, MPEG stores the image in a YUV format in order to achieve higher compression rates. By doing this conversion in hardware, a lot of CPU time is freed for other steps in the decoding process. Nearly every card released since 1995 supports this, with older Matrox and Permedia cards being the exception.

3. Overlay Windows:
   Overlay windows were also implemented back in 95. They provide a method of creating a separate window within the display area that ignores the limitations of the currently displayed video mode. This means, that even if your display mode is currently at 256 colors, the overlay could open a window within your screen that can be 16 million colors. Overlays require more memory from the display adaptor, but also provide hardware image scaling (stretching) using smoothing algorithms to remove the pixelization effects usually associated with scaling.

4. Motion Compensation:
   Motion Compensation was the first DVD-Specific hardware acceleration implemented into a display adaptor. It helps accelerate part of the MPEG-2 decoding process used by DVD video streams. At present time, Motion Compensation is the most popular acceleration method used by display cards. Some of the cards supporting it include: ATI, S3, NVIDIA (GeForce), SiS and Trident.

5. iDCT (Inverse Discrete Cosine Transform):
   iDCT is a further MPEG-2 acceleration, not as many cards support it, and cards that do, usually do it along side of Motion Compensation. Between Motion Compensation and iDCT acceleration, the CPU usage is reduced by around 40%-70%. Some of the cards supporting it include: ATI (Rage128 and newer cards such as the Radeon), SiS.