Hello Bruce,

Hello Hans,

Thank you very much for your help and input. I tested the proposed patch with the minimal example, and the glitch indeed disappeared.

I have to admit, I wasn’t aware that metafun could run in different instances, let alone the existence of both a double instance and a scaled instance. While your patch is quite clear, the inner workings of metapost are a mystery to me. I greatly appreciate the quick fix and the explanation.

Best regards,

Bernd

---

This version improves readability, fixes minor typos, and maintains a polite, professional tone.

On Wed, Oct 16, 2024, 22:34 Hans Hagen via ntg-context <ntg-context@ntg.nl> wrote:

Hi Bruce,

> If you change the following MWE line
>
> \connection [bt] {compiler}
>
> to
>
> \connection [br] {compiler}
>
> then the glitch goes away, so I suggest that means it is a bug in the flow chart module.

Actually not (but you're close) ... so for a moment put your calculator
(hardware) hat on (although they use decimal clipping and rounding i guess)

You noticed that coming from one directions gives a different result
than coming from the other:

metapost > message : 1.4999979972839355 1.8181943026455964
metapost > message : 1.5 2.1818056973544033

Now, why does this fail? We use a double instance here and as we
calculate intersection points we are too precise compared to scaled
where we get twice 1.5, so the points collapse in scaled mode and not in
double. Maybe I need to make some helper for clipping a given precision.

The mp graphic that does the framed loads a metafun double instance so a
later instance with the same name in the chart (lua) module takes that
one; when it is the first one it will take the scaled instance (mkiv is
scaled by default and mkxl defaults to double).

We can run these charts in their own instance (i'll do that).

Here is the patch for mp-char.mpiv that fixes the issue:

def flow_collapse_points = % this can become a core macro
begingroup ;
% remove redundant points
save n ; numeric n ;
n := 1 ;
for i=2 upto flow_xypoint :
% if not (flow_xypoints[i] = flow_xypoints[n]) :
if abs(flow_xypoints[i] - flow_xypoints[n]) > eps :
n := n + 1 ;
flow_xypoints[n] := flow_xypoints[i]
fi ;
endfor ;
flow_xypoint := n ;
% make straight lines
% if flow_xypoints[2] = flow_xypoints[flow_xypoint-1] :
if abs(flow_xypoints[2] - flow_xypoints[flow_xypoint-1]) < eps :
flow_xypoints[3] := flow_xypoints[flow_xypoint] ;
flow_xypoint := 3 ;
fi ;
endgroup ;
enddef ;

But best check this! So the bt / tb was a clue indeed.

Hans

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