Railway is a binary file format for vector animated pictures. This is a library for parsing, computing and rendering such pictures.
Throughout this format we use the term "Couple" which means a pair of floating point numbers. For a more exhaustive description of the file format, see the format specification.
Before drawing, railway files have a program that needs to be computed to yield results. These results are then used as coordinates for the drawing stage. A stack is used to store these results.
Couples whose values can be changed. Some have names, which implies that they're meant to be changed. Those without a name are to be seen as constant values of the program. The parameters corresponds to the initial content of the stack.
Mathematical operations which have one output and up to 3 inputs. Inputs are specified using a stack index/offset. Every instruction results in a couple which is pushed onto the stack.
Some results can be useful to the client code, for instance to overlay something precisely over the rendered picture. These are specified in the file under this section, as a name and a stack index/offset.
Once every instruction has been computed, the drawing stage can begin. It will make use of couples present in the stack, referenced by a stack index/offset.
A railway file contains a number of "layers": combinations of background triangles and masks.
Background triangles specify x/y coordinates and an RGBA color for each point.
Masks are polygons constructed with arcs and bezier curves (cubic / quadratic / linear). Bezier curves are specified using 2, 3 or 4 points. Arcs are specified using a center, two absolute angles, and two radii.
A clip specified a mask and a background triangle. Every pixel that is in both the mask and the triangle will be drawn.
A stroke is the drawing of a mask's contour in a specified color and with a specified pattern.