Treat stroke-symbols as other symbols unless -W is set

[PaulWessel]

Description of the desired feature

It has annoyed me for some time that using symbols like cross, dash, etc., i.e., just line features, requires -W to set the pen thickness, making it very complicated to have variable pen widths. Also, color is not set by -G as for other symbols but taken from the pen. That color could come from a z/cpt lookup if -W+c is given. Etc. So while they are symbols they re second-class citizens in many ways.

Because of history an upgrade needs to be backwards compatible. I think this is:

• If -W is used to set pen with constant color or variable via -W+c we honour that as the current solution as backwards compatibility.
• If no -W then stroke symbol pen width is f * symbol_size, and if -G is set we set the pen to that color (same if z/cpt lookup)

This would allow -Sx14p -Gred to draw a red cross with pen width f *size and -Sx14p -Ct.cpt will do the same f * size but with pen color set via z/cpt.

So what is a reasonable f? Here are three candidates (ignore the other but reported in #7787) - I think going beyond this range gets odd: Please give your votes:

Note: As they say, different strokes for different blokes, one could also entertain a new GMT defaults MAP_STROKE_WIDTH that could default to our preferred choice but the users could override it for some project.

[PaulWessel]

Maybe @GenericMappingTools/core and @GenericMappingTools/gmt-contributors can opine on the size to pen thickness issue? FYI, I think 15% is pretty good, but others may not, hence the MAP_STROKE_WIDTH (or a better word) is proposed.

[seisman]

From what I understand:

• -Sx14p: black cross with the default pen
• -Sx14p -W1p,blue: 1p-thick blue cross (for backward compatibility)
• -Sx14p -Gred: 14*0.15-thick red cross (treat cross as a symbol)
• -Sx14p -Gred -W1p,black: 1p-thick black cross (for backward compatibility)
• -Sx14p -Gred -W1p: what will happen? 1p black cross or 1p red cross?

[PaulWessel]

There is no way to let Sx14p either be a cross with pen = default or pen = 15% of size. I know it has been like you state for 20 years but that was because we did not want users to plot nothing (no pen no fill) and be confused. I think we will have to let that go.

-Sx14p -Gred -W1p would be a red, 1p cross. Basically, we are treating all symbols the same way

[PaulWessel]

Hei, @joa-quim @anbj @Esteban82 @maxrjones @seisman @remkos : Any opinion on this - otherwise I will

Implement at 15% size and no MAP_STROKE_WIDTH or similar.

[anbj]

God morgen Pål,

20% is too much, 15% is good, 10% is good.

[seisman]

MAP_STROKE_WIDTH or similar.

Sounds good

[PaulWessel]

To specifically answer @seisman questions and cases:

#!/bin/bash
gmt begin cases png
	gmt basemap -R0/6/0/2 -JX15c/5c -B
	echo 1 1 | gmt plot -Sx32p			# black cross with the default pen
	echo 2 1 | gmt plot -Sx32p -W1p,blue		# 1p-thick blue cross (for backward compatibility)
	echo 3 1 | gmt plot -Sx32p -Gred		# 32*0.15-thick red cross (treat cross as a symbol)
	echo 4 1 | gmt plot -Sx32p -Gred -W1p,black	# 1p-thick black cross (for backward compatibility)
	echo 5 1 | gmt plot -Sx32p -Gred -W1p		# what will happen? 1p black cross or 1p red cross? 
gmt end show

As for the two cases that fails to meet @seismanʻs expectation above:

1. Since Bush 1 administration we have decided that if a user is plotting a cross and do not set a pen (and setting -Gcolor has no affect since no filling), then we defaulted to the default pen to avoid user surprise at no plotting. Since not giving -W now scales the pen width I think the users will be happier with a size-appropriate pen than a skinny default pen. Those who have a script that never set -W can easily add one.
2. Stroke symbols always ignored -G, but the only reason anyone would use -G with a stroke symbol is of course they assume that will work. It never did, but does now.

So my proposal will

Case 1. Break an ancient and in retrospect bad default handling for no -W
Case 2. Retain backwards compatibility of -W is set (use 2)
Case 3. Get stroke color from -G if it is set
Case 4. No longer ignore -G for any symbol which always sets symbol color now
Case 5. Basically same as use 4

This PR will let all symbols assign color the same way (-G or CPT(z)). Note: I wish I had implemented this at the start bu such is life.

[PaulWessel]

Final notes:

• This scheme also fixes the problem of trying to place symbol codes in the data file. There were no way to use cross there and magically get variable pens or colors.
• Finally, assigning colors and width to line segments is completely separate and will stay as before.

[Esteban82]

Hei, @joa-quim @anbj @Esteban82 @maxrjones @seisman @remkos : Any opinion on this

Fine by me.