ol.style.Image classes should defer rendering to the layer renderer

[ahocevar]

Currently ol.style.Image subclasses render an image to an AtlasManager. Instead, layer renderers should decide what to do with the information stored in an Image style. For example in the case of ol.style.Circle, the Canvas renderer could more efficiently just render a circle with the provided radius, fill and stroke, instead of the style having to add an image to the AtlasManager for every radius-fill-stroke combination.

[ahocevar]

Also see #5748.

[tschaub]

Agreed. It would be nice if the AtlasManager became a WebGL specific thing.

[ahocevar]

@wglas85, if you want to work on this, here is a brief outline of what I have in mind:

1. Create a new ol.style.Shape class

A ol.style.Shape would be configured with a ol.Size and a ol.geom.Geometry (can also be a ol.geom.GeometryCollection. The geometry defines the shape in pixel space, bounded by a rectangle with extent [0, 0, size[0], size[1]].

2. Make ol.style.RegularShape inherit from ol.style.Shape

All rendering code can be removed, and the shape will be described as ol.geom.Geometry instead.

A related refactoring would be to merge ol.style.Icon and ol.style.Image into a single class.

3. Update point renderers

This change mostly involves ol.renderer.vector.renderPointGeometry_, ol.render.canvas.Immediate.prototype.drawPoint and ol.render.canvas.Immediate.prototype.drawPoint (and WebGL counterparts, if any). Now that ol.style.Style#getImage() can return an ol.stlye.Icon or an ol.style.Shape, we do not always get an already rendered image any more. These functions are now responsible for creating and caching images (and making an AtlasManger handle them for WebGL only) from the geometry and size defined in an ol.style.Shape. And they can use ol.render.toContext for that.

With these changes in place, ol.style.* classes do not perform any computations any more, and are just containers for properties. And AtlasManager will no longer be an implicit dependency of ol.style.RegularShape, reducing the size of builds that do not need a WebGL renderer.

[wglas85]

@ahocevar could you please tell me how to temporarily activate the immediate canvas renderer, so I can start by implementing this bare-bone render without having to tackle all the replay complexities?

[ahocevar]

You won't need to touch any code inside functions of the replay or immediate APIs. This change would be mostly about removing all rendering code from ol.style.RegularShape, and replacing it with ol.render.toContext calls in the functions I listed above (which are both higher level than the immediate and replay APIs.

[wglas85]

@ahocevar I have performed first steps on this issue, there need some topics to be discussed, I'm going to arrange a meetup...

[wglas85]

@ahocevar I made some more progress, we still have to decide on some topics, most notable the software layer at which we render the shapes to pixels, when needed and which scaling to apply...

[stale]

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