Where is source code for LANCZOS etc located?

[klappec-bsci]

I found Image.resize() starting on line 1922 of https://github.com/python-pillow/Pillow/blob/bcded33ceead2ae69dad602ce95d8f892ffb46df/src/PIL/Image.py

but .resize() doesn't contain the 'logic' that actually does the calculations for each pixel. I can't find anything resize() calls that actually does contain that logic for executing the math behind the resizing. Is it in some file I missed?

I need to reimplement PIL's exact lanczos algorithm in tensorflow for reasons. (The opencv2 and tf native lanczos algorithms are slightly different)

Any insight?

[nulano]

The internal C function for resize

Pillow/src/_imaging.c

Line 1786 in 44bb3e6

_resize(ImagingObject *self, PyObject *args) {

calls ImagingResample in Resample.c which resizes the image using a Lanczos kernel computed in

Pillow/src/libImaging/Resample.c

Lines 73 to 80 in 44bb3e6

	static inline double
	lanczos_filter(double x) {
	/* truncated sinc */
	if (-3.0 <= x && x < 3.0) {
	return sinc_filter(x) * sinc_filter(x / 3);
	}
	return 0.0;
	}

[radarhere]

You might be interested in looking at #6200

[radarhere]

@klappec-bsci did that help?

[klappec-bsci]

@radarhere yes! kind of. so thank you.I did find the source code thanks to you. My goal was to re-implement PIL's exact specific LANCZOS algorithm in Tensorflow. but I think this might be beyond my current skills (:

[radarhere]

I can appreciate that you might have expected to find Python code and instead found C code.

An alternate solution to your situation might be to convert images between Pillow and Tensorflow, but perhaps you've already considered that or it's not applicable.

[klappec-bsci]

we have a neural network trained using PIL resize, but resizing on the gpu would be faster. PIL resize is too slow for our needs. deploying the AI model, we need the same resize. tf.image.resize lanczos3 is close but not quite the same.

[radarhere]

Ok. Let us know if there is anything else we can do.

[klappec-bsci]

Thanks so much you guys are amazing. is there a textbook or a reference you used when implementing the lanczos algorithm? something you followed as a reference?

[radarhere]

It has been present since before Pillow was forked from PIL (just called ANTIALIAS then), so I can't answer that unfortunately.

There have been naturally been changes to the code since, like #1881 improving speed.

I think https://uploadcare.com/blog/the-fastest-image-resize/#convolution-based-resampling is a good long form explanation of what is happening.

[klappec-bsci]

Thank you SO much. This helped a lot, I never expected this level of support. Closed.