Video Scaling is used in video processing devices where the output resolution is greater than the input resolution by some rational value. The scaling ratio is sometimes greater than 2 or even 4 times the video input resolution.
The video scaler input is a progressive video sequence with resolution (Win x Hin). The output is a progressive video sequence with a higher resolution (Wout x Hout).
Vertical and Horizontal Scaler
The common approach to scale the video frame is to use polyphase filters independently in the vertical direction and then for each line of the vertically scaled frame in the horizontal direction. This can create a scintillation or flickering artifact for slow motion video. The scintillation is caused by spectra aliasing and though it is not completely removable, it can be decreased to a certain minimum level with the proper design of the polyphase filter. Another artifact connected to polyphase scaling is a ringing artifact that is visible on the flat background of nearby strong edges.
Independent usage of the cascade structure of vertical and horizontal scalers causes the severe jaggedness artifact for diagonal edges and texture elements. To avoid this artifact, a directional scaler is used. For each pixel the directional detector determines the direction, and then the directional scaler performs an interpolation along the measured direction.
Directional Cascade Scaler
The implementation of a directional scaler for any rational scaling factor is complex. One approach to reducing this complexity is to only implement directional scaling for scaling factors which are powers of two (2n for integer n). The output of this simpler directional scaler can then be scaled to any rational scaling factor using a vertical and horizontal scaler.