Deconstruct a GIF file
 

Can someone deconstruct a GIF screenshot that I have edited to remove
information?

If I successively edit a GIF screenshot to remove private data and then
post the resulting GIF file on the net, is there something I should do to
test whether the data I removed is not still stuck in hidden "layers" in
the GIF file?

Deconstruct a GIF file
 

On 6/5/2017 1:27 AM, Chaya Eve wrote:
Can someone deconstruct a GIF screenshot that I have edited to remove
information?

If I successively edit a GIF screenshot to remove private data and then
post the resulting GIF file on the net, is there something I should do to
test whether the data I removed is not still stuck in hidden "layers" in
the GIF file?

Very easy: open the file in a binary editor and check for the data you
removed. If you don't see it, it's gone.

--
best regards,

Neil

Deconstruct a GIF file
 

"Chaya Eve" wrote

| Can someone deconstruct a GIF screenshot that I have edited to remove
| information?
|

GIF doesn't usually have metadata. Is that what you
mean? You can look up the file format. Aside from the file
signature and actual image data, it's mainly color
map data that details the colors. Since GIF can only
have 256 colors, those colors have to be specified.
There can be a text-based comment section, but I
don't think it's typically used. It's more like the
possibility is provided by the format.

The only issue I can think of is that if you have a
sleazy image editor. (Like Photoshop.) Some companies
like Adobe like to toot their own horn and essentially
put advertising in the header. You can check for that
with a hex editor. But I doubt even Adobe will do that
with GIFs.

In general, if you're not sure, you can save an image
as a bitmap (BMP), then open that and save it as your
final image format. A BMP consists of nothing more than a
short file header that specifies width, height, color
depth, etc, followed by the actual image bytes. It's
also a good way to work on images that you want
to save in another format later. When you save a JPG
you lose data. When you save a GIF you limit and
corrupt the colors. So those transitions should only
be done after any image editing is finished. Presumably
your screenshot started out as a BMP. If not, it
should. (PrtScr then paste will give you a plain
bitmap.)

Deconstruct a GIF file
 

On 05/06/2017 12:34, Mayayana wrote:
"Chaya Eve" wrote

| Can someone deconstruct a GIF screenshot that I have edited to remove
| information?
|

GIF doesn't usually have metadata. Is that what you
mean? You can look up the file format. Aside from the file
signature and actual image data, it's mainly color
map data that details the colors. Since GIF can only
have 256 colors, those colors have to be specified.
There can be a text-based comment section, but I
don't think it's typically used. It's more like the
possibility is provided by the format.

The only issue I can think of is that if you have a
sleazy image editor. (Like Photoshop.) Some companies
like Adobe like to toot their own horn and essentially
put advertising in the header. You can check for that
with a hex editor. But I doubt even Adobe will do that
with GIFs.

In general, if you're not sure, you can save an image
as a bitmap (BMP), then open that and save it as your
final image format. A BMP consists of nothing more than a
short file header that specifies width, height, color
depth, etc, followed by the actual image bytes. It's
also a good way to work on images that you want
to save in another format later. When you save a JPG
you lose data. When you save a GIF you limit and
corrupt the colors. So those transitions should only
be done after any image editing is finished. Presumably
your screenshot started out as a BMP. If not, it
should. (PrtScr then paste will give you a plain
bitmap.)


cough It's none of MY business, but I find things like this very
interesting! Thanks for sharing your knowledge. :-)

As an aside, Mayayana, if you've a few minutes spare, I'd welcome your
views on THIS post I've made:-

Message-ID:

TIA

--
"Do something wonderful, people may imitate it."

Deconstruct a GIF file
 

In article , Mayayana
wrote:

The only issue I can think of is that if you have a
sleazy image editor. (Like Photoshop.) Some companies
like Adobe like to toot their own horn and essentially
put advertising in the header. You can check for that
with a hex editor. But I doubt even Adobe will do that
with GIFs.

nonsense. nobody puts ads in file headers, nor is photoshop sleazy.

In general, if you're not sure, you can save an image
as a bitmap (BMP), then open that and save it as your
final image format. A BMP consists of nothing more than a
short file header that specifies width, height, color
depth, etc, followed by the actual image bytes. It's
also a good way to work on images that you want
to save in another format later.

no it isn't. bmp is a horrible choice.

When you save a JPG
you lose data. When you save a GIF you limit and
corrupt the colors. So those transitions should only
be done after any image editing is finished. Presumably
your screenshot started out as a BMP. If not, it
should. (PrtScr then paste will give you a plain
bitmap.)

bmp is very, very rarely used anymore.

Deconstruct a GIF file
 

On Mon, 5 Jun 2017 07:34:14 -0400, Mayayana
wrote:

GIF doesn't usually have metadata.

I use GIF because it doesn't have EXIF metadata!

Is that what you mean?

I'm not asking about meta data.
I'm asking about layers.

I often start with a PNG which has layers, and
I edit the PNG which has more layers and then I save
to GIF because I'm trying to get rid of the layers
that I edited out.

When you save a JPG you lose data. When you save a GIF you limit and
corrupt the colors. So those transitions should only
be done after any image editing is finished. Presumably
your screenshot started out as a BMP. If not, it
should.

They generally start out as PNG files, which have
layers which is why I want to save to some other format
which will lose those layers.

I just don't want to accidentally send out information
in layers that I thought I deleted when I cut out sections
of a screenshot.

Deconstruct a GIF file
 

On 2017-06-05 15:33:45 +0000, Chaya Eve said:

On Mon, 5 Jun 2017 07:34:14 -0400, Mayayana
wrote:

GIF doesn't usually have metadata.

I use GIF because it doesn't have EXIF metadata!

That isn't the only thing GIFs don't have.

Is that what you mean?

I'm not asking about meta data.
I'm asking about layers.

I often start with a PNG which has layers, and
I edit the PNG which has more layers and then I save
to GIF because I'm trying to get rid of the layers
that I edited out.

I guess these are not animated GIFs.

When you save a JPG you lose data. When you save a GIF you limit and
corrupt the colors. So those transitions should only
be done after any image editing is finished. Presumably
your screenshot started out as a BMP. If not, it
should.

They generally start out as PNG files, which have
layers which is why I want to save to some other format
which will lose those layers.

Only if they are created with layers. If they are as you claim, just
screenshots, there is no reason for the layers to exist. If they are
images you have lifted illicitly, that is a different issue.

I just don't want to accidentally send out information
in layers that I thought I deleted when I cut out sections
of a screenshot.

You are waaay too paranoid. Especially if all you are doing is dealing
with PNG screenshots.

--
Regards,

Savageduck

Deconstruct a GIF file
 

On 2017-06-05 11:33, Chaya Eve wrote:

I just don't want to accidentally send out information
in layers that I thought I deleted when I cut out sections
of a screenshot.

In Photoshop one could easily "flatten" the image in the layers command
(you end up with a sole layer) and then save it to any format including
..png.

Then use exiftool to verify and/or remove any (all) metadata that may be
left in there.

The "all" removal command.
exiftool =all- filename

--
"If war is God's way of teaching Americans geography, then
recession is His way of teaching everyone a little economics."
..Raj Patel, The Value of Nothing.

Deconstruct a GIF file
 

"nospam" wrote

| bmp is very, very rarely used anymore.

Of course not, on Macs. The OP seems to
be on Windows. A BMP is the most basic
bitmap format. A grid of pixel values. It's
what *all* raster graphics are. JPG, PNG,
GIF, etc all render as bitmaps. Only the
storage varies. A BMP file is only that grid
of pixel values, with a very minimal file header
of something like 22 bytes. The image you work
on in Photoshop is a bitmap. Most of the filters
you can use in Photoshop are essentially math
formulas applied to that bitmap.

So on Windows, the only sensible way to work
with non-RAW is as BMP, or as TIF if one
prefers it compressed. But in that case a TIF
is only a compressed BMP. One could work with
PNGs, but there's not much point. If one works
in JPG then each save is lossy. If one works in
GIF then the number of colors has been severely
limited. There are different ways to go to 8-bit
color from 24-bit. Getting the best GIF can
sometimes take a few tries. So there's no sense
reduing from BMP until editing is done and one
wants a finished copy as GIF.

Deconstruct a GIF file
 

"Chaya Eve" wrote

| I'm not asking about meta data.
| I'm asking about layers.
|

Layers? Are you sure you're not talking about
layers in the graphic editor? Once you save a file
to disk you're merging layers. The end result is
essentially a bitmap. I don't know of any format
that stores layers, except the custom formats
used by things like Paint Shop Pro or Photoshop
that retain info about your editing. But those are
temp formats that other people usually can't open.

If you save to GIF then you're reducing from
24-bit color to 8-bit. (16 million+ colors to 256
colors.) The only reason to use GIF is for simple
images, especially online when a small file size
is important.