Progress on #118 (ignore aspect ratio)

[skedastik]

Happy to say I'm making good progress on feature request #118. Everything is working as intended and all tests are passing. There is one complication, however.

In ResizeWorker::Execute, sharp applies a Gaussian blur prior to large affine reductions. To do so, it generates a square Gaussian image based on the standard deviation of the residual. This is fine if the aspect ratio is preserved, but I suspect a square kernel is not suitable for "non-aspect" reductions.

I'd love to hear ideas on how to proceed. Thinking out loud:

• Generate a Gaussian image based on the smaller residual (x or y), and box-shrink it to match the new aspect?
• The kernel should probably have odd dimensions after the shrink.
• If I'm not mistaken, libvips' separable convolution won't work for a non-square kernel, meaning non-aspect reductions won't be as optimized.

Also, I have to confess ignorance as to why a blur is applied prior to large affine reductions in the first place. Feel free to clue this image-processing newbie in.

[lovell]

Hi Alaric, good to hear you're making progress.

The Gaussian blur is to reduce the effects of aliasing when performing a large affine reduction. I suspect you'll still need this if either x or y residuals are < 1.

If you've not already tried it, a quick approach might be to use the average of x and y residual factors as the single residual input to the sigma approximation. You'd need to manually inspect a few images to see how this looks, especially as the difference between x and y factors increases.

[skedastik]

Good call on averaging the residuals. Here's the test image (512x512):

And results of down-scaling to the following non-integral dimensions (cropped and magnified):

The hard vertical edge is preserved in the first case which is perfect since no horizontal scaling occurs. The blur doesn't become perceptible until the x and y residuals reach more of a parity. These results were computed using the "nohalo" interpolator.

I'll go ahead with this approach in the provisional PR.