Altera Video and Image Processing Suite User Manual
Page 228
Figure 17-2: Lanczos 2 Function at Various Phases
The figure below shows how a 2-lobe Lanczos-windowed sinc function (usually referred to as Lanczos 2)
is sampled for a 4-tap vertical filter.
Note: The two lobes refer to the number of times the function changes direction on each side of the
central maxima, including the maxima itself.
0
1
2
3
−0.2
0
0. 2
0. 4
0. 6
0. 8
1
1. 2
phase(0)
phase( P
v
/2)
phase( P
v
−1)
The class of Lanczos N functions is defined as:
As can be seen in the figure, phase 0 centers the function over tap 1 on the x-axis. By the equation above,
this is the central tap of the filter.
• Further phases move the mid-point of the function in 1/P
v
increments towards tap 2.
• The filtering coefficients applied in a 4-tap scaler for a particular phase are samples of where the
function with that phase crosses 0, 1, 2, 3 on the x-axis.
• The preset filtering functions are always spread over the number of taps given. For example, Lanczos 2
is defined over the range –2 to +2, but with 8 taps the coefficients are shifted and spread to cover 0 to 7.
UG-VIPSUITE
2015.05.04
Choosing and Loading Coefficients
17-7
Scaler II IP Core
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