Images can be grouped in two categories, pixel oriented and vector oriented. Pixel oriented data formats are jpg, gif, png and programs to edit and view them are numerous. A disadvantage are, when zoom in, pixels appear and the picture is no more sharp. Text is no more computer readable.
A way to not have this effect is using lines (vectors) instead of pixels. The file format used in the GPL world is svg (Scalable Vector Graphics) and the program to edit such files is: inkscape
Some keyboard mouse operations are necessary to be known:
Drag mouse wheel => pan
Scroll mouse wheel => pans up and down
Shift scroll mouse wheel => pans left right
Click mouse wheel => zooms in
Shift click mouse wheel => zooms out
svg is xml that is directly supported by many programs and all modern web browsers. Since it is xml, inkscape allows to go in xml mode where xml data can be observed and modified.
Inkscape uses an "inkscape" version of svg that supports enhanced features as layers. Inkscape can save the work as plain svg, where those features disappear.
When opening a inkscape svg file with an other tool as image viewer or web browser it can happen that text s not visible. The inkscape svg version supports "free-flow text box" that is a feature that was/is in discussion to be added to plain svg. The solution is to convert it to regular text: Text => Convert to Text.
When the xml editor is opened also the drawing window stays open (maybe it get hidden by the full screen xml editor). Now in both windows can be worked and both windows are synchronized. This is a good way exploring svg and the drawing.
To have your personalized settings store an inkscape file under:
~/.inkscape/templates/default.svg
Inkscape is pure XML data, therefore external programs (as java scripts) can easily modify inkscape drawings.
As File=>New shows, inkscape can be used for many things as cd-covers, business cards, fontforge-glyph, Open A4 landscape and turning on the grid gives something ready for making a drawing on a 1mm grid (where it snaps when close) and 5mm major gridlines.
The secret of drawing straight lines is selecting Bezier curves (but pressing the Ctrl Button) when left clicking the mouse button to get a straight line between points. This can be continued until CR or mouse right click happens.
One annoying thing on technical drawing is that the line width changes when the lines are made longer or shorter. There is a setting to turn this off => Edit => Preference => Transforms => Scale stroke width