Raster resampling happens before map canvas rotation

[qgib]

Author Name: Johannes Kroeger (Johannes Kroeger)
Original Redmine Issue: 15679
Affected QGIS version: 2.16.3
Redmine category:unknown

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If you use QGIS' canvas rotation feature and have raster images in your map, they will look much worse than intended.

It seems like the resampling is done before the rotation. The rotation of course leads to different neighborhood relationships between pixels so the quality degrades. Would it be possible to resample after rotating?

Attached is an example of the same area with Nearest Neighbor resampling as well as Average with 0° and 10° rotation applied. As you can see, the image quality in the 10° Average image looks not very good.

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• filtering.png (Johannes Kroeger)
• no resampling on reprojection.png (Johannes Kroeger)

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• easy_fix was configured as 0
• regression was configured as 0

[qgib]

Author Name: Jürgen Fischer (@jef-n)

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• category_id was configured as Unknown

[qgib]

Author Name: Johannes Kroeger (Johannes Kroeger)

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This also affects OTF reprojection as shown in this image. I would update the title if I could.

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• 12403 was configured as no resampling on reprojection.png

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• no resampling on reprojection.png (Johannes Kroeger)

[qgib]

Author Name: Giovanni Manghi (@gioman)

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End of life notice: QGIS 2.18 LTR

Source:
http://blog.qgis.org/2019/03/09/end-of-life-notice-qgis-2-18-ltr/

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• resolution was changed from to end of life
• status_id was changed from Open to Closed

[kannes]

This is still the case in 3.8, please re-open.

Maybe there is no filtering applied to the map canvas in general after rotation?

[Pedro-Murteira]

This is still valid on QGIS 3.22.3.

[AlisterH]

Here is another sample showing that this is really bad if people want to bring raster (perhaps scanned) maps to QGIS.
I sometimes "export map to image" to use in CAD (yes, to render the text it has to be very high resolution, but this is better than bringing in vast amounts of vector data to CAD).
If I have the map rotated in QGIS when I export, I get a rotated geotif. For reference, this is what it looks like in an image viewer (which does not know anything about the rotation):

If I wanted to load a rotated image like this into QGIS, it would be illegible (regardless of the rotation I set for the QGIS window):


If I wanted to bring in scanned maps or something with some rotation, I would need to upsample them significantly for legibility.

[elpaso]

I am afraid there is nothing we can do: the exported image is rotated and GDAL loads it correctly.

You can try different resampling algorithms to get a slightly better image.

[kannes]

The issue is that this resampling is done before the canvas rotation. The "resampling" of the rotation is not filtered and leads to jarring aliasing.

A reduction in visual quality when rotating rasters is to be expected but it could be drastically reduced if it wasn't a nearest neighbor operation (I guess).

[elpaso]

The issue is that this resampling is done before the canvas rotation. The "resampling" of the rotation is not filtered and leads to jarring aliasing.

Rotation of the canvas in QGIS is not involved at all in this issue.

After you load a raster which was exported from a rotated image the issue appears also if the canvas is not rotated. It is the original raster which was rotated when exported, GDAL knows that from the world file and applies the appropriate transformation.

[kannes]

Ah, I missed that @AlisterH's comment was only tangentially really related to my bug report. @AlisterH maybe make a feature request to make raster export of a rotated canvas store a resampled image instead of "just" an image with its rotation defined in the metadata (if that is what happens).

[AlisterH]

No, I'm not complaining about export of rotated maps, I'm talking about display of rasters with rotated geotransform. I think it is #46262, which has been marked as duplicate of this (although it was also addressed in another old ticket). So maybe that one should be reopened if we want to keep the issues separate?

[alexbruy]

Does activating "Early resampling" checkbox in the recent QGIS versions improves situation?

[AlisterH]

There is almost no improvement in my example of rasters with rotated geotransform.
I don't see any improvement in the case of a rotated canvas and a raster without rotated geotransform - but perhaps there is a minimal improvement and I just can't see it.