Understanding Polyphase Scaler Taps

Hi Kieron,

Thanks for the reply! I have read the VIP user manual recommendation, but wanted to know why and what was happening "behind the scenes". I have been playing around with an 8-tap filter and various coefficient sets and I think I understand the process better now. For downscaling, where you want the anti-aliasing filter, the Lanczos-2 coefficients seems to provide good quality. But I do not agree 100% with the way Altera (and the UDX4 software) calculates the coefficients for the 8-tap filter.

Altera does it as follows:

C(k) = lanczos2(k/2+p), where lanczos2(x)=sinc(pi*x)*sinc(pi*x/2) and k is the tap [-3..3] and p in the phase fraction [0..15/16].

The effect is a Lanczos-2 filter spread across the interval [-3..3], but which is shifted across two taps by the phase increment. See the attached figure Altera_Lanczos2_8tap.jpg for a picture of phase 0 and phase 15.

As far as I understand, the phase increments should shift the coefficients between a single pair of taps (pixels). Maybe for downscaling this does not matter since the Lanczos-2 already filters image, but if you use this for up-scaling, the results are terrible. See the attached file Altera_upscale_result.jpg. Note the extreme "blockiness" (also note the picture is a photo taken of the LCD screen with a camera - hence noisy background as you see the individual pixels on the screen).

I have compiled my own set of coefficients with the following function:

C(k) = lanczos2((k+p)/2)

See the attached file Custom_Lanczos2_8tap.jpg. The phase is shifted by one tap and not by two taps. For downscaling I cannot see a difference between my coefficients and the Altera coefficients. For upscaling, my coefficients produce a much better result (filtered, as expected). See Custom_upscale_result.jpg.

For equal and up-scaling I am using a Lanczos-4 8-tap coefficients (since I already have an 8-tap filter, might as well use it). See Lanczos4_8tap.jpg. This has a phase 0 coefficient set with only one non-zero tap, so if the input and output resolution are the same, there is no change to the video and the up-scaled video is sharper than with the 8-tap Lanczos2. I do not have bicubic coefficients (does anybody maybe have a set of bicubic coefficients I could try?). Would you expect a bicubic filter to give better results than the Lanczos-4, 8tap filter?

Currently my system automatically switches between the two sets of coefficients (My custom Lanczos2 for downscaling, Lanczos4 for equal and upscaling).

Sorry for the long posting!

Regards,

Niki