Re: [matplotlib-devel] Why are image coordinates rounded to integers?

[Michael D. <md...@st...>]

I think I agree that the rounding is necessary for raster backends, but 
is in the wrong place -- it should be in the backend itself.

It's needed for Agg to prevent "gaps" between the image and axes 
rectangle.  The axes rectangle is always drawn pixel-aligned using 
"round", so if the image is drawn by truncation to integers, there will 
be small gaps for certain values.  But yes, it makes sense to *not* do 
any rounding or truncation in the vector backends.

I'll commit something for this -- on the trunk, because it feels a bit 
too pervasive for the branch.

Mike

Jouni K. Seppänen wrote:
> Hi,
>
> Mike Fitzgerald reported bug #2832896 and has been investigating its
> causes:
>
> https://sourceforge.net/tracker/?func=detail&aid=2832896&group_id=80706&atid=560720
>
> The problem is that images are not drawn at exactly the same coordinates
> as other artists, so markers drawn on top of images are slightly off in
> pdf and eps output. Apparently this can be fixed, at least to some
> degree, by removing the round(...) operations from
>
>     renderer.draw_image(gc, round(l), round(b), im)
>
> in _AxesImageBase.draw and making a slightly more involved change in
> AxesImage.make_image (the details are in the bug report). I haven't had
> time to investigate what this does to other backends, but I imagine the
> rounding would be necessary for raster backends.
>
> Can someone who is more familiar with the image machinery comment on the
> matter? Do we need some kind of vector/raster indication in the backends,
> in addition to the get_image_magnification method?
>
>   

-- 
Michael Droettboom
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA