"Kenneth" <kdw### [at] earthlinknet> wrote:
> "Kenneth" <kdw### [at] earthlinknet> wrote:

> Hmm, I'm wondering if something in my own 'reproduction' code is the fault here.
> Since I'm re-rendering typical 4/3 images, the box that they're applied to is
> this (with the camera at x=0,y=0 and look_at the same):
>
> box{
> pigment{image_map...}}
> translate <-.5,-.5,0>
> scale <4/3,1 1>
> }
>
> The 4/3 scaling is the only oddity I can think of.
>

Nope; I just did a far-simpler test (square box, square image_map, no
'motion-blur' code at all) and I still see the bottom-LEFT shift.  So I'm still
puzzled. Perhaps there's been some kind of change between v3.6.1 and one of the
betas?

Ken

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Am 01.05.2010 08:59, schrieb Kenneth:

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

Duh - now that you mention it, I realize that my test setup mirrors the 
test image horizontally. So yes, you're right: It is a shift to the 
bottom-LEFT with respect to the original picture.

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

> Duh - now that you mention it, I realize that my test setup mirrors the
> test image horizontally. So yes, you're right: It is a shift to the
> bottom-LEFT with respect to the original picture.

Ah, the ol' mirror-trick, eh? So I guess something *did* change since
3.6.1--'clipka's mirror.' ;-P

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"Kenneth" <kdw### [at] earthlinknet> wrote:

> ...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.

Sorry, I was mistaken about that; it *is* a 1/2-pixel-shift, in the context of
what is meant here. (I got confused about what '1 pixel vs. 1/2 pixel' meant.)

For Clipka:
Something else that occurs me to me about my posted image tests (something
obvious, and you probably realized this already) is that the 1/2 shift during
interpolation is a shift *based on the original pixels of the image_map* (NOT a
1/2 shift in the pixels of the *re-rendered* image of the image_map.) To help
explain that more clearly: My 720 X 540 test chart with its single-pixel-lines,
when re-rendered at 8X its size, creates 8-pixel-wide lines; that's perfectly
understandable. But when interpolated, it creates an 8-pixel shift to the left
and down (i.e., not 1/2 or 1 pixel as I would have expected.)  With the
interpolate 'blur' filling an area of 16 pixels--twice the width of the 8-pixel
line. I wonder if this behavior makes sense?  Don't know; I would have expected
far less blur and far less 'movement' in such a case.

What this seems to mean is that the shift that we currently see (and the
'amount' of interpolation) is based on the magnification factor when
re-rendering the image_map's pixels. For example, say the image_map is on a box
that's deep into z, so that it re-renders at a fraction of its real size. (Which
is usually the case.) In such a case, the re-rendered shift would be far less
than 1/2 pixel, to my mind. Which I *guess* is a good thing(?)

I don't quite know what to make of this; but I thought I'd mention it. It may
have a bearing on how you figure out your corrections.

Ken

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clipka <ano### [at] anonymousorg> wrote:
> 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.
>
> Normally I'd just fix that offset, but it has probably been there since
> eons - and changing it might actually break a few scenes that have
> deliberately worked around this issue (I have at least once designed
> such a scene).
>
> How do you feel about it?

Since it would probably eliminate the diagonal drifting of images for things
like ripple tank and CA sims, I'm all for it.

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