Bug 16805 – Text on a path is all wonky

• Component: SVG
• Status: RESOLVED FIXED

[Oliver Hunt]

Scaled textPath text is very jaggedly positioned.  I would suspect it were a CG bug, but it seems to effect SVG fonts as well.

[Oliver Hunt]

Created attachment 18353 [details]
Simple minimised text case.

[Nikolas Zimmermann]

Hey Oliver,

this is a known bug since ages. The root lies in Path.cpp, in following function:
static void pathLengthApplierFunction(void* info, const PathElement* element)

quoting parts of the code here:

if ...   (traversalState.m_totalLength >= traversalState.m_desiredLength)) {
        FloatSize change = traversalState.m_current - traversalState.m_previous;
        float slope = atan2f(change.height(), change.width());

        if (traversalState.m_action == PathTraversalState::TraversalPointAtLength) {
            float offset = traversalState.m_desiredLength - traversalState.m_totalLength;
            traversalState.m_current.move(offset * cosf(slope), offset * sinf(slope));

I can't exactly remember what the problem was, but it's related to the slope calculation. We're really only doing a bad approimation of the actual "point at length" (ie. gimme the path point at 30% of the path length). If the path consisted of _one_ large curve segment, we're approximating _the whole_ segment using a line, as you can see above - this doesn't make too much sense and results in whacky layout....

Please fix :-)

Greetings,
Niko

[Eric Seidel (no email)]

Hum... I'm pretty sure I wrote the path point finding code.  Perhaps i'll get a chance to fix it some day. :)

[Nikolas Zimmermann]

Works in trunk, the root of the problem was missing subpixel positioning.