Re: [Gimp-developer] Size on disk vs size reported on status bar

[Elle Stone <ellestone ninedegreesbelow com>]

On 10/04/2017 09:02 AM, Jehan Pagès wrote:
> Hi!
>
> On Wed, Oct 4, 2017 at 2:28 PM, Jehan Pagès <jehan marmottard gmail com> wrote:
> > Hi again!
> >
> > On Wed, Oct 4, 2017 at 1:01 PM, Elle Stone
> > <ellestone ninedegreesbelow com> wrote:
> > > Actually the image is at 32-bit floating point precision, all the layers
> > > have alpha channels, most of the layers have masks, and there are a couple
> > > extra channels (saved masks) in the channels dialog.
> > >
> > > Ignoring the masks and the extra saved channels, and assuming I'm doing the
> > > right math, this would be:
> > >
> > > 4000*6000*7*32*4 = 21 504 000 000 / 1024^3 ~ 20 GB
> >
> > Sorry I was stupid earlier. By multiplying by 24 (or here 32), we get
> > the size in bits, we want in bytes (instead, we must multiply by 3
> > bytes for 32 bits). So a single mono-channel image should be
> > 4000*6000*3 bytes.
> > So assuming each layer has alpha and mask, that makes 5 channels per
> > layer and each channel uses 3 bytes:
> > 4000*6000*3*7*5 ~ 2.34 GB.
>
> Ok so I meant 4 bytes of course! I can't even do basic maths right anymore! :P
>
> 4000*6000*4*7*5 ~ 3.2G.

Hi Jehan,

So a byte is 8 times a bit? For people like me who can never remember 
the difference between a byte and a bit, is there a one-sentence 
explanation for why there are bytes *and* bits?

> Ok so I had a quick look at the code.
> For the size displayed in the status bar, it seems it would use the
> size as returned by each GimpObject through the get_memsize() virtual
> method: https://git.gnome.org/browse/gimp/tree/app/core/gimpobject.c#n445
>
> You can get an idea of the expected size by looking at GimpTemplate
> code which is used to guess the future memory size (to warn before
> image creation if the expected size will be big):
> https://git.gnome.org/browse/gimp/tree/app/core/gimptemplate.c#n426
>
> We can see that the expected size seems to be based on the initial
> layer + the selection + the projection (which apparently always has
> alpha and multiplies the result by 1.33:
> https://git.gnome.org/browse/gimp/tree/app/core/gimpprojection.c#n326).
> So for your image with 7 layers, alpha (and assuming all with mask),
> following this estimation algorithm:
> 4000*6000*4*7*5 + 4000*6000*4 + 4000*6000*16*1.33 ~ 3.7GB.
>
> This estimation is quite close to the actual 3.8GB GIMP gives you.
> With the few more channels you said you had, I guess that's about it.
> :-)

Thanks! That's a nice explanation! This might be useful to put in the 
user's manual, if it isn't already there.

Best,
Elle