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

[Jehan Pagès <jehan marmottard gmail com>]

Hi again!

On Wed, Oct 4, 2017 at 1:01 PM, Elle Stone
<ellestone ninedegreesbelow com> wrote:
> On 10/03/2017 09:33 PM, Jehan Pagès wrote:
> > I don't know how this info is computed in GIMP code (i.e. does it
> > check for actual memory size or does it do some basic maths as I am
> > about to do), but assuming your image is 8 bits per channel, and all 7
> > layers are the size of the image and have no alpha, that makes:
> > 4000*6000*7*24 = 4 032 000 000 / 1024^3  ~ 3.8GB!:-)
>
>
> Hi,
>
> 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.

Hopefully I didn't make too many errors this time. :P
You also said that there were a few extra channels. Selection is a
channel as well, so if you have one, it must be taken into account.
This said, I am wondering if the channels are all 3-bytes precision in
a 32-bit image?

Jehan

> As an experiment, I followed Simon's suggestion and made a new document,
> same size, 7 layers, and filled each layer with random LCH noise. This
> brought the %m value up to 4.8 GB.
>
> Removing all operations from the Undo history brought the %m value down to
> 2.9 GB. Saved to disk, the size is 1.6 GB. I guess random noise also
> compresses?
>
> Best,
> Elle



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