Re: [Gimp-user] How to reduce image file size while retaining width/height of image?

[Steve Kinney <admin pilobilus net>]

On 07/09/2015 08:55 PM, Dora Smith wrote:

> I'm not sure if gif offers compression or not.

Depending on the image content, GIF files may be noticeably larger
or smaller than equivalent JPG files.  PNG files will usually be
substantially larger than either.

The GIF format is an indexed format:  Every color in the image is
assigned a numerical value, and the individual pixels are referenced
to the resulting table.  In some instances, a GIF file will be
smaller than an equivalent JPG file, especially where the number of
unique colors in an image is low, i.e. line art, cartoons, logos
etc.  Photographic content, not so much.

Why nobody uses GIF any more:

GIF does a very poor job of rendering gradients, i.e. photos with
lots of clear sky or smooth shiny objects, due to its limited
storage space for the values of unique colors:  Smooth transitions
in an image are rendered as a series of bands with visible edges
when converted to GIF.  Even where this is not a problem, GIF
renderings of photographic content are usually larger than
equivalent JPG files.

The result of scaling a GIF file to a larger or smaller size, or
editing color values in one, is horrible.  The first step when
working on a "found" GIF file in the GIMP is to do Image > Mode >
RGB.  If the edited file will be exports as a GIF, the GIMP will
convert the image back to indexed format during the export process.

At one time GIF was the only browser compatible format with
transparency, but since it only has one value for transparency
(100%) it has major aliasing problems - transparent regions that are
not rectangular have jagged or stair-step edges when viewed against
a contrasting background.  PNG has a real alpha channel, enabling
partial transparency which anti-aliases transparent regions very
nicely, so these days that's what people use when transparency is
required for web graphics, or to make image content with
transparency portable across all common image editing tools.

Another GIF feature, much overlooked today, is animation:  Multi
layer image files can be saved as GIF animations, and the GIMP has a
tool for optimizing animated GIF file size by removing content that
does not change from frame to frame in the individual layers.

Internet advertisers abandoned animated GIF banners when Flash
evolved into a user tracking spyware tool, and there are not many
use cases for GIF animation in web design these days - Flash and
Javascript can do more of the stuff web designers like.  I have not
made a GIF file for a decade or so.

:o)