clipka <ano### [at] anonymousorg> wrote:

> In all cases, the black dot is exactly at the center.
>
> As can quite easily be seen, when interpolation is activated the image
> appears slightly shifted to the bottom-right, in the order of magnitude
> of half a pixel.

I was somewhat aware of this; I think fixing it would be a good idea. But I'm
wondering about that bottom-right shift.

I just did an interpolation test on a 'test chart' image I made last year in
Photoshop. (It's 720X540, made up of lots of different single-pixel-width lines,
shapes, letters, etc. With no P.Shop antialising--just nice clean pixels. It's
main purpose was to test out my 'motion blur post-processing' animation code,
which simply averages multiple pre-rendered images by placing
them as image_maps on a box. The chart came in handy, to make sure I was getting
a 1:1, pixel-for-pixel reproduction of the images when copying them. This method
and code *do* reproduce the image_map faithfully, edge-to-edge and
corner-to-corner.)

So I did some tests in v3.6.1, using this image_map chart (with interpolate 2)
and the same 'copying' step as mentioned above. What I see is that the 'blur' is
shifted to the bottom-LEFT. (And also downward; sort of a combination of the two
directions.) I've posted some images of the tests over at p.b.i.--I can't seem
to do it here. (The orange lines correspond to your non-moving 'center dot.')

For test #1, the image_map is re-rendered 1:1 (that is, at 720 X 540) then blown
up in P.Shop by 800%, to more clearly show the shift--which, BTW, appears to be
by a full pixel, probably(?) just a result of the test's image rez. Test #2 was
re-rendered a 8-times the original--5760 x 4320--and NOT blown up in P.Shop.
(Although I *have* included a blown-up close-up of that.) Again, it looks like a
full-pixel shift. BTW, I thought this discrepancy between our results might have
something to do with the 'once' keyword; but it doesn't.

I'm puzzled by the difference in what we're getting. I'm wondering if I've done
something stupid in my methodology, or whether our test set-ups differ in some
fundamental way. Care to describe your test procedure?

Ken

Post a reply to this message