On Mon, Jul 29, 2024 at 11:10 AM Werner LEMBERG <w...@gnu.org> wrote:
>
>
> >> It would seem that though shifts and changes in the lengths of the
> >> staves are "common", small and relatively benign problems,
> >> rotations and scales (magnifications) should be considered major
> >> disasters, right?
> >
> > Rotations do not generally happen. Virtually all the positioning is
> > rectilinear, and scaling is also not common. What happens that
> > objects end up in different locations, and sometimes variable
> > objects (slurs, beams) have different sizes.
>
> BTW, I got an interesting reply on StackExchange; maybe you two (Luca
> and Han-Wen) want to comment on it.
>
>
> https://computergraphics.stackexchange.com/questions/14143/search-for-special-image-difference-metric/14145
AFAICT, this just does some fuzzing so that single-pixel
differences/shifts do not generate differences.
I had an idea of what one could do, but really, we first need a
representative test set of image pairs, both pairs with important
differences and spurious differences, so we can see what a new
algorithm looks like.
The idea is that we'd want to trigger less on diffuse lines of
difference (shifted staffline), but more on concentrated blobs
(disappearing symbol). Here is a suggestion
* generate images without anti-aliasing
* generate the image diff
* for each changed pixel, count the number of changed neighbor pixels.
* for each pixel, take max(changed - THRESHOLD, 0)
* then do MAE or some other metric on that image.
By setting THRESHOLD = 2, you could make single line differences
disappear. (We'd need to make sure to make the resolution such that
stafflinethickness is at least 2 pixels, or otherwise dramatic changes
in staff positioning would not trigger anything.)
Alternatively, on the diff image, you could segment the diff into
connected components, and weight the errors by the minimum of {x-size,
y-size}. Thus, a line has a diameter of only 1 pixel, but missing
notehead is much larger.
Without a test set, it's hard to see if this is really a big
improvement, though.
--
Han-Wen Nienhuys - hanw...@gmail.com - http://www.xs4all.nl/~hanwen
