Image Maps: A need for a common, understandable Coordinate Reference System

[Moonbase59]

Did some testing with 3.19.0, 3.19.1 and the new image overlays.

What I like:

• Keeping markers etc. in the same layer, i.e. only really overlaying some parts, so that markers etc. stay intact. (As opposed to several "real" layers (like in caves) where you’d want to use separate markers.)
• Possibility to either specify or not specify coordinates (for full-image vs. partial-image PNG overlays).
• Real great: Possibility to switch on/off one or more layers. So one can show "where the action is" or maybe routes Frodo travelled …

Possible improvements:

• It’d be nice if the

  • currently selected layer, and the
  • currently selected overlays

  could be saved in the map instance. Thus, re-opening the map note would re-enable the last selected layer and/or the last selected overlays. If people use logo/watermark/thing/monster overlays, this might be beneficial.

What confuses me:

• Yet "another" coordinate system (top left/bottom right). Other mapping stuff usually works in Southwest/Northeast bounding boxes. Then again, it’s the same system (0,0 = top left) programs like Photoshop or GIMP use, so measuring for partial overlays is probably easier.
• What "coordinates" are these?
  • Pixels?
  • Percents?
  • Pseudo map coordinates relying on scale and/or bounds?
  • Do they use [x,y] or [y,x] notation?
• If specifying something smaller/larger than the overlay image, will it shrink/enlarge the overlay image?

I can imagine several use cases for overlay images, which all require an understandable coordinate system:

• Overlaying "highlighted" parts of the terrain (i.e. show a dimmed map with a highlighted "action" or "play" area). This might be easiest using pixel coordinates (since the original map is also in pixels, and the overlays probably created using an image editing program).
• Overlaying "things" or monsters on a map (battery vending machine in the Colossal Caves, image of a dragon’s lair, etc.). This would possibly need "gaming map coordinates".
• Overlaying a logo, watermark, or map legend. This could use either pixel, percent, or "gaming" coordinates.

Conclusion

Before we get lost in a maze of twisty little coordinates, all different, there is a need for a common, understandable coordinate reference system, even for image maps. Which should work well with coordinates shown on the map, have a common reference point, coordinate notation (lat,lon or y,x), and sequence for bounding boxes.

My thoughts:

• Pixels can/should only be used for input, if ever. They are only relevant in calculating the gaming coordinates in the first place (from the image dimensions), and possibly for the exact placement of transparent overlay images.
• Percentages might come in handy for placement of logos, watermarks and map legends, but are otherwise not too useful. Then again, these might be parsed easily (using an appended % sign).
• Gaming map coordinates (as calculated using bounds and scale) seem to be the most useful for all-round use, i.e. markers, distance calculation on the map, giving coordinates to other players. They …
  • can be used inside the map (markers, distances, show & copy locations)
  • can be used outside the map (i.e., in supporting notes, YAML, communicating with other players)
  • should be used for establishing the initial map view (I think; instead of %lat/lon)
  • could be used for placement of thing/monster images on the map
  • could be used to place overlay images, like routes or terrain areas (albeit not as easy as pixels, but one could directly pick up points from the map!)
  • can be used with GeoJSON (in reversed lon,lat notation but bboxes also use SW/NE)
  • should always use the same sequence (i.e., [lat,lon] or [y,x]; except reversed in GeoJSON)
  • should always use a defined sequence when specifying bounding boxes (bounds, areas, overlays). I strongly prefer "Southwest, Northeast" notation here, since it’s usual in other maps (real-world, too)
  • must use the same reference (i.e., "where is 0,0?"). I somehow prefer 0,0 to be in the lower left ("Southwest") for mapping. It’s more natural for users (imagine a cartesian coordinate system), and—if using bounds—we already have 0,0 in the lower left corner.
• The only drawbacks I can see if using "game map coordinates" in each and every place are …
  • One has to do some initial bounds calculation. But in order to play a good game, players will surely use the same map for reference, and prefer "real" (gaming) coordinates.
  • Calculating the placement of terrain/route overlays is more difficult if just cutting/pasting parts of a map using an image editor. But I think since we gain the advantage of picking point coordinates directly from the map and being able to copy-paste these, i.e. for placement of things, monsters, logos, watermarks, map legends, it might be an overall gain. Also, the memory penalty of using a fullsize transparent PNG with just a small area filled in isn’t too much. (In case of highlighting parts of the terrain.)

Taken from #125 (comment) and put here, for better visibility, and so it doesn’t get forgotten.