Library design: Layer coodinates/origin

Question

I am working on a vector map library where you can add plain image and vector layers to a map. The map itself consists of an infinite coordinate system and each layer is a coordinate system of it's own.

However I'm uncertain how layers should be added to the map coordinate system.

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The current implementation places the layer's center around given coordinates and scales the layer to a given size:

map.add(new layer(image="url", cx, cy, width, height)) -> Figure1

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Another option would be to place the layer's origin (bottom left corner) to the given coordinates:

map.add(new layer(image="url", ox, oy, width, height)) -> Figure2

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The third option would be to let the user provide 2 coordinates i.e. bottom left and top right so that width and height can be omitted:

map.add(new layer(image="url", {x, y}, {x, y})) -> Figure3

This Polymaps example uses the third approach

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Question:

What would you recommend: Are there any conventions, how layers (image or vector) are usually positioned on a map?

Answer 1

Objects in vector layers have their coordinates specified in some coordinate system, so you do not need to set their position explicitly when adding them to a map. (If the layer and the map use different coordinate systems or projections, it is necessary to convert the coordinates, but this would usually be more complex than a simple affine transform.)

Bitmap layers need to be georeferenced to specify the mapping from pixel to map coordinates. Typically, this is done by choosing some control points and specifying the map coordinates of those points. This is similar to your third option, but the control points are usually not at the corners of the bitmap, and it is common to use three or more to be able to reduce errors.