[gdal-dev] Interpolation questions

[Daniele Romagnoli]

Hi,
While investigating on some rendering issues we have with java JAI based
operations involving interpolations, I have found the topic of bicubic
interpolation introducing values outside of the input dynamic, as well as
negative values and overshooting.

The problem we had was with a grayscale GeoTIFF with noData=0.
For some very dark area (pixel near to zero) the output of the
interpolation was a negative value being clamped to zero (which is noData).
So it was introducing noData in regions where all pixels were valid.

Therefore I did a test with gdal_translate -r cubic and I got same result:


​
on the left side: original image
on the right side: cubic interpolation
weird white shapes you see on the top right are zero/nodata pixels.

I got different result once using cubic spline instead.
What is the difference between the two algorithms?

Second question:
I did a gdal_translate -outsize 800% 800% from a small pixellated image.
is it expected to have output pixels exceeding the original shape of the
valid input pixels?
I was thinking the interpolated image pixels should stay within the
original pixel edges.

Let me provide an image to clarify:

​
​
Left image: original
Right image: cubic spline.
See how the 2 edges are different, having the cubic spline producing some
kind of buffering on the edge (note the white-missing pixels on the borders
are nodata).

Please, let me know.
Best Regards,
Daniele



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