*** General notes about rounding

Suppose a function is sampled at positions [k + o] where k is an
integer and o is a fractional offset 0 <= o < 1.

To round a value to the nearest sample, breaking ties by rounding up,
we can do this:

   round(x) = floor(x - o + 0.5) + o

That is, first subtract o to let us pretend that the samples are at
integer coordinates, then add 0.5 and floor to round to nearest
integer, then add the offset back in.

To break ties by rounding down:

    round(x) = ceil(x - o - 0.5) + o

or if we have an epsilon value:

    round(x) = floor(x - o + 0.5 - e) + o

To always round *up* to the next sample:

    round_up(x) = ceil(x - o) + o

To always round *down* to the previous sample:

    round_down(x) = floor(x - o) + o

If a set of samples is stored in an array, you get from the sample
position to an index by subtracting the position of the first sample
in the array:

    index(s) = s - first_sample


*** Application to pixman

In pixman, images are sampled with o = 0.5, that is, pixels are
located midways between integers. We usually break ties by rounding
down (i.e., "round towards north-west").


-- NEAREST filtering:

The NEAREST filter simply picks the closest pixel to the given
position:

    round(x) = floor(x - 0.5 + 0.5 - e) + 0.5 = floor (x - e) + 0.5

The first sample of a pixman image has position 0.5, so to find the
index in the pixel array, we have to subtract 0.5:

    floor (x - e) + 0.5 - 0.5 = floor (x - e).

Therefore a 16.16 fixed-point image location is turned into a pixel
value with NEAREST filtering by doing this:

    pixels[((y - e) >> 16) * stride + ((x - e) >> 16)]

where stride is the number of pixels allocated per scanline and e =
0x0001.


-- CONVOLUTION filtering:

A convolution matrix is considered a sampling of a function f at
values surrounding 0. For example, this convolution matrix:

	[a, b, c, d]

is interpreted as the values of a function f:

   	a = f(-1.5)
        b = f(-0.5)
        c = f(0.5)
        d = f(1.5)

The sample offset in this case is o = 0.5 and the first sample has
position s0 = -1.5. If the matrix is:

        [a, b, c, d, e]

the sample offset is o = 0 and the first sample has position s0 =
-2.0. In general we have 

      s0 = (- width / 2.0 + 0.5).

and

      o = frac (s0)

To evaluate f at a position between the samples, we round to the
closest sample, and then we subtract the position of the first sample
to get the index in the matrix:

	f(t) = matrix[floor(t - o + 0.5) + o - s0]

Note that in this case we break ties by rounding up.

If we write s0 = m + o, where m is an integer, this is equivalent to

        f(t) = matrix[floor(t - o + 0.5) + o - (m + o)]
	     = matrix[floor(t - o + 0.5 - m) + o - o]
	     = matrix[floor(t - s0 + 0.5)]

The convolution filter in pixman positions f such that 0 aligns with
the given position x. For a given pixel x0 in the image, the closest
sample of f is then computed by taking (x - x0) and rounding that to
the closest index:

	i = floor ((x0 - x) - s0 + 0.5)

To perform the convolution, we have to find the first pixel x0 whose
corresponding sample has index 0. We can write x0 = k + 0.5, where k
is an integer:

         0 = floor(k + 0.5 - x - s0 + 0.5)

	   = k + floor(1 - x - s0)

	   = k - ceil(x + s0 - 1)

	   = k - floor(x + s0 - e)

	   = k - floor(x - (width - 1) / 2.0 - e)

And so the final formula for the index k of x0 in the image is:

    	    k = floor(x - (width - 1) / 2.0 - e)

Computing the result is then simply a matter of convolving all the
pixels starting at k with all the samples in the matrix.


--- SEPARABLE_CONVOLUTION

For this filter, x is first rounded to one of n regularly spaced
subpixel positions. This subpixel position determines which of n
convolution matrices is being used.

Then, as in a regular convolution filter, the first pixel to be used
is determined:

    	k = floor (x - (width - 1) / 2.0 - e)

and then the image pixels starting there are convolved with the chosen
matrix. If we write x = xi + frac, where xi is an integer, we get

	k = xi + floor (frac - (width - 1) / 2.0 - e)

so the location of k relative to x is given by:

    (k + 0.5 - x) = xi + floor (frac - (width - 1) / 2.0 - e) + 0.5 - x

                  = floor (frac - (width - 1) / 2.0 - e) + 0.5 - frac

which means the contents of the matrix corresponding to (frac) should
contain width samplings of the function, with the first sample at:

       floor (frac - (width - 1) / 2.0 - e) + 0.5 - frac
     = ceil (frac - width / 2.0 - 0.5) + 0.5 - frac

This filter is called separable because each of the k x k convolution
matrices is specified with two k-wide vectors, one for each dimension,
where each entry in the matrix is computed as the product of the
corresponding entries in the vectors.