SoSLUG Archive

Gimp FAQ

Frequently Asked Questions (FAQ’s)

What do the file extensions, bmp, jpg (or jpeg) gif and png mean.

bmp stands for bitmap image file a device independent bitmap file (allegedly). jpg (or jpeg the same thing) stands for the Joint Photographic Experts Group who specified the format and which is the used by (all) digital cameras. It does not support transparency, so any images converted to .jpg that have transparent regions will have those areas convert to some arbitary colour, usually black. It uses a lossy compression algorithm. The png extension stands for Portable Network Graphics and is format that supports lossless data compression. PNG was created as replacement for the Graphics Interchange Format (gif), because of possible patent infringement. (those patents have now expired). Gif files support animation and use a lossless data compression technique (LZW – Lempel-Ziv-Welch) but only supports 8 bit graphics. Today it tends to get used for images that don’t require a bit depth greater than 8. This is a problem when converting a photograph that uses 24 bit colour depth, to 8 bit as colours will obviously change when the two images are put side by side. Otherwise you might struggle to tell the difference.

I have loaded a gif image and want to make the background transparent, but the colour to alpha under the colour menu is greyed out. How do I change this?

The image mode of gif images is indexed. Go to Image –> mode and change the mode from indexed to RGB. Now you change the background to alpha. If you then export the image as a gif file, and reload it you will see it has become an indexed file again, but now includes the alpha channel.

When I try to use the edge detect filter, the preview shows the effect but when its applied I just get a transparent layer. What am I doing wrong?

To use the detect filter the alpha channel must be removed first from the chosen layer. Go to Layer –>Transparency –> Remove Alpha Channel.

When I make the background colour transparent how do I stop other parts of the image becoming transparent as well?

There are two ways you can do this. The easy way is to choose a colour for the background which is absent in the part of the image you want to keep. Use the bucket fill to tool to change the background colour, then change that colour to alpha.

The second solution is to use the scissors tool for example, to select the parts of the image you want to keep. When you have done this, invert the selection and now change the background to alpha.

What is EXIF?

Exchangeable image file format provides information about camera settings when a photograph was taken and is embedded as part of the file format. Many image manipulation programs recognise, and preserve this information.

Apart from an images file extension, what other information can I find out about how it was saved?

On Linux systems open the control console and type file to get some very basic information. For jpg files this just tells you it is a .jpg file, but you already knew this from the extension. With a .gif file, you’ll get the image dimensions. Again you should know that .gif files use just 8 bits for colour information as part of its compression algorithm. For .png files you get the colour map e.g. 4 bit 8 bit etc. and the dimensions.

Author: Alan Campion - Page reference: 656
Last modified: Alan Campion - 2014-08-24