Ralph Shumaker wrote:
> I try to load a foto that is 1381 x 1961 pixels 24bpp. Gimp says:
>
> Image resolution is out of bounds, using the default resolution instead.
>
> Then it loads it anyway. It *seems* to be OK, but have I lost quality?
>
> (I *never* save back over the original file.)
>
> When Gimp scales an image, it gives the options None (fastest), Linear,
> and Cubic (best) Interpolation. Is this the best available?
>
> KolourPaint gives no complaints at all, but gives no interpolation options.
>
> Properties tells me that the resolution is 1381 x 1961, but even after
> Gimp is "using the default resolution instead", it still appears to be
> 1381 x 1961. What changed? Gimp reports that the image uses 20.8 MB of
> memory.
>
> How can I tell the jpg compression level used on a foto file?
>
> What is the best way to scale an image to retain the best quality?
Googling on
gimp "Image resolution is out of bounds"
brings up a message that suggests that resolution in this context means
dots per inch (dpi), and further that the image source file has dpi
metatata that gimp thinks is a mistake. So it substitutes it's own defaults.
At least, that's how I interpret the posted message. So, your message is
not about /pixel/ content which is why there's no change to the pixel
spec (which, in this context, seems to be known as /geometry/).
Your image is a jpg, I gather. What is it's origin?
Possibly interesting experiments:
convert (a prog. from the ImageMagick package)
to (say) png and back to jpg as a new name
allow gimp to save as a new name and play with that
use other progs to save as new name and see what gimp says
- - -
When scaling, I always use cubic. I believe that means it uses a cubic
interpolation algorithm for properties of inserted pixels. I don't see
any reason not to use cubic (other than speed -- but that's usually not
a real concern, unless you have a _real_ slow cpu).
- - -
use identify (another prog from ImageMagick)
identify -verbose file.jpg
compression (type & quality [level?])
- - -
Scaling down is usually not so problematic, so I assume you mean scaling up.
Also, you will see discussions about resizing vs resampling. I'm not
totally comfortable with the terminology myself, but I think the most
common intent is to change the content by adding or removing pixels.
Frankly I haven't really figured out the effect of changing the dpi in
the gimp scale image dialog. It doesn't do what I think it kinda-oughta.
(The convert program may allow more careful experiments with
resize/resample.)
==> Perhaps someone else with a better understanding will chime in here?
so for the best way to scale: there probably is more than one answer (as
usual)
- best choice may be get a better picture. ;-)
- next recommendation might be to avoid scaling _too_ much
(watchout for amplification of artifacts .. eg, from jpeg
or introduction of jaggies, etc)
- sometimes post-processing with brightness/contrast, or blurring may help.
- avoid compression until all other image processing (including
scaling) is finished, then experiment and use your own judgment. And
don't forget to consider the viewing environment. For web viewing, you
can tolerate lots of compression -- I have unscientifically settled on
compression "level" (or "quality") 60 (whatever that means) for a lot of
jpg web photos. I suppose I should do some tests and plot bytes vs
level, eh?
Regards,
..jim
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