Rotational.Interfaces breaks color convention

[henrikt-ma]

The connector icon colors in Modelica.Mechanics.Rotational.Interfaces break a design convention I thought was very broadly affected as a de-facto standard:

• For a pair of connectors (for instance Translational.Interfaces.Flange_a and Translational.Interfaces.Flange_b), one of the icons has white as the dominant fill color, and the other icon has a non-white dominant fill color.
• The line color of the dominant graphics item – which tools are expected to pick up as the color for connection Line annotations – matches the non-white dominant fill color.

In Modelica.Mechanics.Rotational.Interfaces, the problem is that the border line color is black, while the dominant non-white fill color is the slightly dark grey {135, 135, 135}. Since it has been like this for a very long time, our ecosystem is supposedly full of connection Line annotations with black line color, not matching the dominant connector icon color. One only has to look at the diagram Modelica.Mechanics.Rotational.Examples.First to see the issue.

Since it would be a hassle to update the color of all the Line annotations out there, I suggest that we fix the issue by instead aligning the fill color of Modelica.Mechanics.Rotational.Interfaces.Flange_a with the convention, making the whole connector icon pitch black instead of grey.

Here is a related discussion on the need for some related standardization, but this discussion is about specification, not design conventions:

• Lack of standard for deriving connect-Line style from connector icon ModelicaSpecification#3380

[HansOlsson]

There are also other implicit convention that the Rotational flanges break:

• The border line color isn't specified, but relies on the default (Black). I guess that explains why it was missed.
• The Diagram and Icon-Ellipses for the Flange_a connector have slightly different colors: {135,135,135} vs {95,95,95}.

I believe the reason we haven't detected this is that anti-aliasing and similar effects mean that we don't really separate a black line and a gray line. Thus, I'm not convinced that there's an actual issue in the library, as I think that using a pitch-black connector might be too much.

But we should at least document this in a better way.

[HansOlsson]

Thinking more:
The anti-aliasing-like effects were even more visible in some old versions of Dymola that drew the lines as pixel-wide, right @DagBruck ?

Thus it seems plausible and even likely that the intent was to have gray connectors and gray lines, and people didn't notice that the thin lines were in some sense darker than expected. However, users would notice if we mix gray and black lines; which rules out a gradual change.

In terms of moving forward:

Changing the color to gray for all existing connection lines for rotational-connections in user models is as noted not realistic - so to me a possible solution would be:

• Document the issue - but don't change fill-color to black.
• Ensure that the two Flange_a ellipses have the same gray color (135).

I'm not saying that a pitch-black connector would be unacceptable - only that I think it will be a bit distracting.