[MEI-L] Coordinate system confusion

Johannes Kepper kepper at edirom.de
Tue Jul 4 15:28:48 CEST 2017

Dear Daniel,

that's a real confusion, and we need to make it clearer in the guidelines. *Pixel* coordinates are always with the origin in the top left corner. *Music* coordinates, however, are always bottom up. @ulx and so on are always in pixel units, but @vo (vertical offset) is specified in interline distances (half the distance between two staff lines, or, in other words, the vertical distance between a C4 and a D4, or any other two adjacent notes). If you want to specify that a dynamic is written above its default position, it seems more natural that values go up (i.e., @vo="3"). This means that for musical units the origin has to be bottom left. I know it's confusing in the guidelines, and we will address this at some point. If you don't mind, you're invited to prepare something on Git and submit a pull request ;-)

Hope this helps,

> Am 04.07.2017 um 14:48 schrieb Daniel Alles <DanielAlles at stud.uni-frankfurt.de>:
> Dear all,
> at the moment, I am a little bit confused about how MEI defines its coordinate system: It is possible to add the attributes @ulx, @uly, @lrx and @lry to for example a surface, as written in part 12 of the Guidelines, which places the origin of the coordinate system in the upper left corner. All the examples in that part show that behavior, ulx/uly is always 0/0. This would correspond to the coordinate systems used in SVG and DOM and (which is what I use for my work) Edirom Editor. On the other hand it is written in part 22.3, that MEI uses a coordinate system in which "the y-axis points from bottom up". That would mean, that ulx/uly could never be 0/0.
> So now my questions: Is it sufficient to use the coordinates like in the examples, with the origin in the upper left corner? Would that "override" MEIs original coordinate system? If not: Isn't the possibility to encode areas from top-left to bottom-right corners a semantic error in MEI, if the coordinate system is pointing from bottom-left to top-right?
> Best,
> Daniel
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