meaning of an expression in VHDLcode of JPEG2000

[farhaenis]

meaning of an expression

In the code vhdl of the JPEG2000,i dont understand what's the meaning of this expression:

FUNCTION filter_5_3_even(x, yp, yn : SIGNED(CST_BUF_DATA_BUS_WIDTH - 1 DOWNTO 0)) RETURN SIGNED IS
BEGIN
RETURN x + SHIFT_RIGHT(yp + yn + 2, 2);
END;

This function ε to the DWT bloc.

Can any one please help me?

 

[wael_sharkasy]

Re: meaning of an expression

I need the vhdl code of JPEG2000, can you send it to me?
Also we can help each others in understanding it.

 

[std_match]

It seems to be a short FIR lowpass filter, length = 3
The 3 input values: yp, x, yn.
(yp = "previous", yn = "next")

The 3 coefficients: 0.25; 1; 0.25

Result = 0.25 * yp + 1 * x + 0.25 * yn
The "+ 2" in the SHIFT_RIGHT argument creates rounding to the nearest integer.
(it is an addition of 0.5, then a truncation to integer).

This is probably a very hardware efficient filter, since no multipliers are needed.
It is only an addition of 4 values. The shifting doesn't cost anything.