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TODO list for glaciers
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HIGHEST PRIORITY:

HIGH PRIORITY:
- [727] glacier growth from zero length still takes a long time, and then speeds up
- [766] add snow to valley floor above ELA
- [850] flags can be put in the cross-section of the ice
- [851] flags can't be dragged from valley floor to ice surface while glacier is moving

MEDIUM PRIORITY:

LOW PRIORITY:
- [725] finalize Help items
- [735] improve smoothness of DistanceAxisNode (x-axis for coordinates feature)
- [558] edges of thermometer & ice thickness tool icons are clipped
- [734] dragging out and immediately into the toolbox doesn't delete tool
- [736] ticks & labels on sliders should be white, code to set their color is not working (Mac OS 10.4 only?)
- [735] Spanish translation
- YAW_X_OFFSET should be computed as a function of YAW_ROTATION and PITCH_Y_OFFSET
- remove registration points from background image in any versions published to PhET website
- make sure everything works properly when clock is paused

DEFERRED FOR INTERVIEWS:
- [728] change color of glacier cross-section
- [731] change label on steady state button
- behavior of glacial budget meter (snap-to-ice) is hard to discover
- do we want to show values or relative ranges (temperature=cold-to-hot,snowfall=none-to-lots) for Intro climate controls?

DEFERRED FOR FUTURE VERSION:
- [724] make borehole drill fire every N years
- [729] "steady state" feature needs to fill glacier with debris and surface ripples
- "steady state" feature needs to advance clock to the time at which the glacier would reach its steady state
- "steady state" feature needs to fill in missing data in time-dependent graphs
- climate experiments interface (start with a time estimate)
- Experiments module
- always collect data for time-based graphs, keep "latest" & "historical" data points, compress historical, limit historical size
- add zoom in/out button for time-based graphs; show scrolling "latest" data or complete "historical"

REFACTOR:
- eliminate code duplication in modules
- GlaciersModel is too big
- PlayArea is too big
- generalize all charts, there's too much duplicated code
- PlayArea.verticallyAlignZoomedViewport makes a change to viewport and results in itself being called again (it works, but not very elegant)
- delete trash can code & image file if we're sure we'll never need it

PERFORMANCE OPTIMIZATIONS:
- Valley.getX uses divide-and-conquer
- Climate.updateELA uses divide-and-conquer
- Glacier.findSurfaceAtELA does divide-and-conquer
- Glacier.getIceSpeed(x,z) doesn't reuse speed at x values
- Valley.getElevationLeftOfHeadwall does lookup and interpolation

MIGRATE TO PHETCOMMON:
- DoubleSpinner
- SliderOnlyLayoutStrategy
- HorizontalLayoutStrategy

CLEANUP:
- setPickable(false) on non-interactive nodes
- rescale image files to minimize file size
- disable all debug output
- optimize memory management by reusing mutable objects
- remove unused images

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