Created attachment 147821 [details]
Shape with arcTo, that is wrongly rendered
Open attached document. Notice, the first slide seems to be empty. But the Navigator shows, that there is an element. The second slide has an image, how the shape is rendered in PowerPoint.
Created attachment 147822 [details]
custoomshapeproperties.cxx lines 240 and 2241 look very suspicious to me. If I comment them out, the shape is rendered correctly.
If the viewBox is forced to width=0 and height=0 here, the members nCoordWidthG and nCoordHeightG will become zero too by ApplyShapeAttributes() later on. And thus members nCoordWidth and nCoordHeight become zero too in EnhancedCustomShape2d::SetPathSize() in EnhancedCustomShape2d.cxx.
But I'm not familiar with pptx import and cannot estimate, whether such change would break something in other areas.
Confirmed using LO 184.108.40.206 / Windows 7.
The mentioned lines were added in the following commit.
author Muthu Subramanian <firstname.lastname@example.org> 2013-12-27 19:59:22 +0530
committer Muthu Subramanian <email@example.com> 2013-12-27 19:59:22 +0530
fdo#72998: Custom shapes have improper size.
Ah, then first a better solution for bug 72998 has to be found. The current "trick" to force viewBox to "0 0 0 0" is also reason for bug 76334.
Created attachment 164967 [details]
The example shape copied to docx format
Also happens if the shape is copied to Word, saved in docx format then opened in Writer. Not PPTX specific.
bug #112192 also seems to have a missing viewbox problem.
The problem in bug 112192 is different. There the angles of the arcTo command introduce only the two equations to convert the angles from 1/60000 deg in oox to degree in LO but the angles themselves are direct values in oox. Here the angles are references in oox. So the test in https://opengrok.libreoffice.org/xref/core/oox/source/drawingml/customshapeproperties.cxx?r=c70ee4a6#201 is still true even after applying the commit, which subtracts the angle conversion equations from the number of equations.
The bug here requires a rework of how LibreOffice implements the oox feature "values are interpreted in coordinate-system of current shape width/height". ODF allows for custom shapes a missing svg:viewBox which would correspond to that feature. But LO has not implemented that. Instead we have a magic svg:viewBox="0 0 0 0", which prevents rendering if not coupled with SubViewSize.