[gdal-dev] Interpolation questions

Wed Mar 29 04:58:27 PDT 2017

Daniele,
   For your first question, you should operate with a no_data value outside the range of pixel generated by interpolation.
   So you should do a preprocessing to change no_data value outside of this range. Alternatively, if you want to keep 0 as no_data, you should clamp negative value to 1 (instead of 0).
  For your second question, it’s normal that interpolation does a kind of dissolution of your edge. If you want to keep original no_data, you should extract a band mask from your input, and integrate it on your result.

Nicolas
De : gdal-dev [mailto:gdal-dev-bounces at lists.osgeo.org] De la part de Daniele Romagnoli
Envoyé : mercredi 29 mars 2017 10:07
À : gdal-dev at lists.osgeo.org
Objet : [gdal-dev] Interpolation questions

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:

[cid:image002.jpg at 01D2A894.35516A80]

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:
[cid:image004.jpg at 01D2A894.35516A80]

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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