I have no idea where to start.
Start simple, because it is actually relatively simple.
You've autotraced the Louis Vitton decal. Autotracing is quick, but bad. Manually tracing (Hand tracing) is better, but more time consuming. Creating stuff from scratch is basically fully fledged graphic design (at a point), which is a professional skill, so don't beat yourself up if it doesn't come easily.
You need to understand that a vector file (SVG) is a list of commands that allows a system (a browser, GT7, other graphics packages etc) to re-draw your decal using those commands. Inkscape et al save what you've drawn as that list of commands - it's literally text, called XML. The more of that text (XML) there is, the bigger the file. The goal of efficient files in GT7 is to have as little XML as possible.
Different packages write the XML differently. Some are more efficient than others.
In whatever package you've got most decals can be created using a set of fairly simple set of primary tools:
- A line tool
- A poly line tool
- An arc
- A rectangle
- An ellipsis
These will get you a shape by writing a path. You then use the following to colour the shape/path:
- Stroke colour (the outline)
- Fill colour
There are then secondary functions that act on the shape/path:
- Add & Subtract functions (i.e. Weld, front minus back, intersect etc.)
- Application specific things such as contour, fillet/scallop/bevel etc.
And there are more creative options for the fill such as
- Gradient fill (linear gradients work in GT7, simple radial gradients do, other gradients fills don't)
- Partial Transparency (Again, iffy as to what works in GT)
These things all translate directly back to those commands written in the SVG file. Most things are referred to using coordinates. The more coordinates there are, the bigger the file. The more specific the coordinates are, the bigger the file. The more fills you have, the bigger the file, the more gradients you have, the bigger the file.
Since a lot of these things use decimal numbers (e.g. 123.456) they can be simplified by rounding. 123 is mostly the same as 123.456...(it's less then half a percent different) but instead of needing 7 characters of XML in the SVG file, it needs 3... this means a much smaller file. SVGOMG takes car of rounding most of the numbers in your file (as well as cutting out unused stuff), which is how it saves you most of the file size it does.
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This is the basis of all decal work you see in GT. The more familiar with it you get, the more complex decals you can make.
edit: decals are only half the equation, but making good quality decals from scratch makes the end livery better and easier.. I'm not a livery expert though, I like to keep mine simple
