Brush inputs/settings calibrated for viewzoom and viewrotation +new input (viewzoom)

[briend]

fixes #66
This branch and the associated mypaint gui branch (https://github.com/briend/mypaint/tree/view-zoom) add zoom-level and view-rotation as inputs to the brush library. The brush library then applies these inputs to the relevant brush inputs or settings such that the brush is not affected by zoom or rotation of the canvas. Two additional changes that were necessary to include in this pull-- ascension formula change and replacement of fmodf with custom mod function for negative values. Also, brush_radius is removed from the formula that affects speed because it is no longer balanced by the the zoom-effect.

inputs/settings affected: direction, ascension, speed, brush-dab-angle. Speeds are clamped at 4.0. The only brushes that should be affected are those that were expecting the overflow of speed (beyond 4.0) and the rare overflow of ascension, so it should be pretty backwards-compatible.

[briend]

I made viewzoom visible as an actual brush input instead of just behind-the-scenes calibrating speed. This makes it possible to have a brush that automatically changes its size as you zoom in (which is also helpful for the surfacemap pattern-generator that I'm working on). Here's an example. Say you have a brush with a base radius of 2.59 and you want it to remain the same size on-screen when you zoom in and out. You just drag the view-zoom setting to -2.59 and now the brush should reach size zero when you reach 6400% zoom, and grow as you zoom out as well. You don't even have to mess with the curves or points by default. If anyone can verify the math that'd be great :-)

Having this as a setting can be really cool if you didn't want a 1:1 ratio. Or maybe you just want 4 brush sizes and have it automatically pick from the 4 at different zoom levels-- you'd just add points and square them off instead of ramping it.

[briend]

Not sure why the check is failing. I will test it on Windows 10 soon

[0ion9]

That's a really cool way of doing it -- hadn't considered the possibilities of square graphs.

(though I'm kind of losing track of how many pull requests you have pending, heh ;)

So to be really explicit:

Say you have a brush with a base radius of 2.59 and you want it to remain the same size on-screen when you zoom in and out. You just drag the view-zoom setting to -2.59

.. A base-radius of N and a view-zoom of minus N -> brush is rendered with "real" base-radius N at 100% zoom; "real" base radius N/2 at 200% zoom; "real" base radius N*2 at 50% zoom?

And if view-zoom was set to plus N, "real" brush radius would be what (for each of: 100%, 200%, 50%)?

(the reference to size zero @ 6400% confused me, partially because size zero isn't actually infinitely small in MyPaint)

[briend]

.. A base-radius of N and a view-zoom of minus N -> brush is rendered with "real" base-radius N at 100% zoom; "real" base radius N/2 at 200% zoom; "real" base radius N*2 at 50% zoom?

I think that's right-- if you mean "real" as in "pixels" (or exp(N)).
base_radius = exp(mypaint_mapping_get_base_value(self->settings[MYPAINT_BRUSH_SETTING_RADIUS_LOGARITHMIC]))
(mypaint_mapping_get_base_value(self->settings[MYPAINT_BRUSH_SETTING_RADIUS_LOGARITHMIC])) - logf(base_radius * 1 / self->states[MYPAINT_BRUSH_STATE_VIEWZOOM])

And if view-zoom was set to plus N, "real" brush radius would be what (for each of: 100%, 200%, 50%)?

100% would brush radius of N - log(exp(N) * 1 / 1.0)
200% would be brush radius of N - log(exp(N) * 1 / 2.0)
50% would be brush radius of N - log(exp(N) * 1 / 0.5)

I never realized that brush size of zero is actually 1 pixel. (exp(0)=1). So now having negative brush sizes makes sense for sub-pixel rendering I suppose exp(-2) is .13 pixels. So my example assumes that you want your brush to be 1 pixel at maximum zoom. That seems to make sense, but you'll never get to .13 this way. Also, after playing with it more it feels more natural to reach 1 pixel a lot sooner than 6400%. But that would entirely depend on your preference for a particular brush. So I think having this control is a lot more flexible than a checkbox-- albeit it is way more complicated-looking.

here are charts for base radius of 2 and -1. Obviously if you start with a base value of -1 you won't be able to get to -5 when you zoom in to 6400%. Or maybe you can, hopefully it doesn't overlflow like that but I think it might.

[odysseywestra]

@briend seems like your pull request failed a test on appveyor. Could you check the logs and see if you address the problem?

[briend]

I'm still not quite sure what step_viewzoom and step_viewrotation should do. I think that part can be cleaned up/removed. If I understand it is supposed to interpolate the input data if changes, but I don't think the zoom or rotation will change during a draw event-- or at least they aren't likely to change. I've played around with "spinning the view" while drawing and it's not very practical.

[briend]

rebased with master