Unexpected result when using PaperHeight and MetaFUun

[Keith McKay <mckaymeister@gmail.com>]

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Dear All

I have problem understanding what is happening in the MWE below which is 
different from what I expected.

I have created a shaded background using MetaFun with dimensions 
PaperWidth by PaperHeight which mimics \setuppapersize[A4,landscape} as 
expected.  I then within \starttext \startMPpage... \stopMPpage 
\stoptext created a stripe of the shaded background, xscaled 2cm y 
scaled PaperHeight, but reversed, expecting it to cover the background 
top to bottom. However the top of the stripe seems to start at 10.5cm 
which is suspiciously half the height of A4, Landscape i.e. 21cm. I 
confirmed this by changing the shifted command to, shifted 
(19cm,10.5cm), and the stripe is positioned as expected.

Am I missing something obvious in how MetaFun handles coordinates? I 
have asumed that in MetaFun, (0,0) is the bottom left corner. Or is it 
how I applied the shifted command?

I'm using LuaMetaTex Version 2.05.01 ConTeXt ver: 2020.04.03 10:31 MKIV 
beta fmt: 2020.4.6 int: english/english

Thanks and stay safe.

Best Wishes

Keith McKay

%%%%%%%%%%% MWE %%%%%%%%%%%%%%% \setuppapersize[A4,landscape]

\definelayer[mybg]

[x=0mm, y=0mm,

width=\paperwidth, height=\paperheight] % let the layer cover the full paper

\startuniqueMPgraphic{blueShade}

draw lmt_shade [

path = fullsquare xyscaled(PaperWidth,PaperHeight),

direction = "up",

alternative = "linear",

colors = { "red", "blue" },

];

\stopuniqueMPgraphic

\setlayer[mybg]{\uniqueMPgraphic{blueShade}}

\setupbackgrounds[page][background={mybg}]

\starttext

\startMPpage

StartPage ;

%Result Left

draw lmt_shade [

path = fullsquare xscaled 2cm yscaled PaperHeight,

direction = "down",

alternative = "linear",

colors = { "red", "blue" },

]shifted (12cm,0cm);

%Result Right

draw lmt_shade [

path = fullsquare xscaled 2cm yscaled PaperHeight,

direction = "down",

alternative = "linear",

colors = { "red", "blue" },

] shifted (19cm,10.5cm);

StopPage ;

\stopMPpage

\stoptext


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