Re: Gimp: Erased sections of images print in some cases

From: Elio Grieco (grieco@egx.com)
Date: 10/30/02


Date: Tue, 29 Oct 2002 17:30:02 -0700
To: Clark Mills <c.mills@auckland.ac.nz>
From: Elio Grieco <grieco@egx.com>

I saved the image from the link provided and opened it in several
programs, here are the results:
    Apple's Preview - The erased portion was transparent as expected.
    Macromedia's Fireworks MX - The erased portion was transparent as
expected.
    Macromedia's Freehand 10 - The erased portion of the image appeared
as black.
    Lemke Software's GraphicConverter - The entire original image was
present.
    Preview directly in the OS X Finder - The entire original image was
present.

I'd suggest actually "erasing" the image data rather than modifying the
alpha channel. When you modify the alpha channel you are just adding
transparency information to the image rather than deleting information.
It is then up to the program that reads the file as to how it
interprets the transparency info generated.

Two other points to consider:
    1. Make sure the image is "flat", i.e. it has no layer data or only
1 layer.
    2. Make sure that the program you are using does not leak any
sensitive information into any tags within the image, e.g. EXIF data in
JPEGs. (If I remember right Fireworks stores quite a bit of data within
the PNG files it generates)

Definitely worthy of note if you are sending images to untrusted
parties or posting them publicly.

     Elio Grieco

On Tuesday, October 29, 2002, at 03:08 PM, Clark Mills wrote:

> Hi people.
>
> As part of documenting processes, I take screen captures and then chop
> stuff out that I don't want the world to see. I do this within the
> Gimp
> by setting an alpha channel on the screen capture and then _erasing_
> the
> parts of the image that I want to obscure. An example is at:
>
> http://c-mills.ctru.auckland.ac.nz/transparent.png
>
> If you view this image from Linux in Galeon 1.2.6 or Netscape/7.0 all
> looks fine however if you print the image from within these
> applications
> then the _erased_ section prints as if it was never erased.
>
> This could be a great bit of fun and a way of making print pages appear
> with extra content that wasn't visible on the web page (pr0n,
> copyright).
>
> On windows the images obscure which is great but obviously the _erased_
> information is still in the sent image.
>
> I let the Gimp people know this but they replied considering it a
> feature not a bug which could be the case in some/most circumstances,
> Bug #97167.
>
> Not an earth shattering bug but perhaps worthy of note.
>
> Cheers... Clark