Hi.
I've had some surprising results when saving images out of ParaView. I
haven't had time to thoroughly examine results by looking at difference
images until recently. Here's what I found.
When I carefully size the ParaView 3D view to, say 1280x960, and then
"File->Save Screenshot", the view looks the same. Comparing this with a
screenshot taken from gimp (with the rest of the gui carefully cropped
away), I find that the view is the same. There are some pixel
differences along thin edges, but this should be acceptable, because the
way the saved image is rendered might not be the same way as the screen
image is rendered. But the view is the same.
I do encounter problems, however, when I adjust the pixel resolution for
saving a screenshot. If I double the resolution to 2560x1080, and then
average that back down and compare it with the 1280x960 saved image, I
would expect pixel differences where thin lines average down to slightly
different antialiased colors. However, the actual view changes as well
(zooms in), so What I See Isn't What I get. The view changes further if
I save out at 3x the original resolution (doesn't match 1x or 2x).
Again, I'd expect to see pixel differences from different anti aliasing,
but the view itself shouldn't change (no zoom/pan/rotate).
So then I investigated saving an animation frame. Although I wouldn't
expect this to match the gimp screenshot for the same reasons that the
1x "save screenshot" image didn't, I would expect it to match the 1x
screenshot. But it doesn't. Furthermore, animation frames at 2x, and
3x also don't match 1x in that the view changes (zoom is different).
The change in zoom seems consistent with the change in zoom from "save
screenshot" for 1x, 2x, and 3x. I think that the correct thing would be
to have all zooms match each other, regardless of pixel resolution (at
least, for the same aspect ratio images).
It makes sense to me that the zoom changes would be consistent between
"save screenshot" and "save animation", because I would expect the same
routine to do the saving. However, I was suprised to find pixel
differences between the 1x versions, the 2x versions, and the 3x
versions. I would expect these to be, pixel-for-pixel strictly
identical. To my eye, I haven't seen any obvious differences. But
theoretically, shouldn't each 1x pair be identical? Same for 2x with
each other, and 3x with each other?
Another odd thing is that "save screenshot" defaults to the resolution
of the 3d view. But, for some reason, "save animation" for a 1280x960
3d view defaults to 1259x944. I can understand automatic tweaking of
animation frame resolutions so that they're nice multiples of 16 or 32
or whatever makes video codecs happiest. But in this case, 1280x960 is
already a perfect multiple.
It would be really nice if all methods of saving images were consistent
with respect to view (zoom/pan/rotate). It's frustrating to try piecing
movies and transitions together from animations and screenshots, only to
have them not align due to these inconsistencies. It's also tedious to
constantly have to fix the pixel resolution when saving animations
because the default doesn't match the 3d view and the save screenshot
resolution. It's also frustrating to have a 3d view with the correct
aspect ratio, compose a nice scene, save it out, have it approved, and
then come back to generate a high resolution version and find the edges
of the model cropped off because the zoom changed out from under me.
All the above behavior was in 3.11.0, 64 bit, checked out a couple days
ago, and compiled on Fedora 14. I apologize for so much text, but
because these seem related to me, I figured it made sense to keep them
together.
Any thoughts?
Thanks.
Greg
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