Commits on Aug 23, 2023

1. lanczos3: Const-fold offset calculation to hardcoded 0.5 …

   The usefulness of this magic took a fair bit of time to understand,
   while we can trivially remove it after deducing that it always computes
   to the constant `0.5`, and gets rid of some strange bright spots in the
   center of our image compared to #26.
   
   Before:
   
   ![square_test_result](https://github.com/Traverse-Research/ispc-downsampler/assets/2325264/273556b4-6f53-43d5-9424-31fef5ca7966)
   
   After:
   
   ![square_test_result](https://github.com/Traverse-Research/ispc-downsampler/assets/2325264/e0507eee-9a58-4fd8-b9fc-7a0a3c485ee8)
   
   First, we start by knowing that `uv` is divided by `target_size` before
   it is passed to `resample_internal()`.  Hence, if we multiply it by
   `target_size` again, there should be no fractional part and
   `center_pixel` always becomes `0`.  Floating point rounding errors being
   gone now, this is what solves the bright spots in the center of the
   image mentioned above.
   
   Then we are left with:
   
       center = uv - (0.0 - 0.5) * inv_target_size
   
   Which becomes:
   
       center = uv + 0.5 * inv_target_size
   
   As a drive-by cleanup we can now see that `(inv_)target_size` is only
   used to offset `uv` by another half _target_ pixel to point to the
   center instead of the top-left.  These values were already involved in
   converting the `uv` coordinate from target pixels to normalized
   coordinates, so it reads more logical (involving less math) to factor
   this calculation into the call site and remove two extraneous function
   parameters from `resample_internal()` as a result.
   
   Now, continuing our journey, plug this into `offset` and simplify:
   
       offset = (uv - center) * target_size
       offset = (uv - (uv + 0.5 * inv_target_size)) * target_size
       offset = (-0.5 * inv_target_size) * target_size
       offset = -0.5
   
   And we have our target value.  Then, because they are subtracted when
   calling `lanczos3_filter()`, we turn this into positive `0.5`.
   
   Note that I have _zero_ clue whether this is the right value, but when
   sampling a 6x6 grid (not 7x7 as thought in #27) we only visit pixel
   positions `[-3, ..., 2]`, thus neatly retrieving weights at `[-2.5, ...,
   2.5]` and never hitting the `3.5` value which is above `3` where
   `lanczos3_filter(3.5)` returns `0.`.

   

   MarijnS95 committed Aug 23, 2023
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