viewport
TODO's
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TODO Add gridX and gridY camera listeners, with support for loading from an infinite grid of cells.
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TODO Add camera-movement listener, to let nodes watch for camera movement, and thus let nodes detect when they are approaching critical screen boundaries:
enteringScreen leavingScreen
Node distance is radially calculated (using the viewport's diagonal) from the camera's center, adjusted by some constant.
hysteresis factor gives the +/- from some preset large distance (probably some hundreds of bud radiuses). Ignoring hysteresis, then when the camera moves, the node's relative position may be changed. This distance is recalculated, and if it is above some threshold plus hysteresis constant, and the node's state was 'near', then the node's leavingScreen is called, and the node's state is set to 'far'.
Likewise, if the distance is lower than the same threshold minus hysteresis constant, and the node's state was 'far', then the node's enteringScreen is called, and the node's state is set to 'near'.
This distance is checked when the node is painted and also when the camera is moved.
- TODO Figure out how changing the grid size might change things.
Grid updates based only on camera movement. Updates are reported in terms of cells made visible in either direction. The number of potentially visible grid cells is determined for each axis using the camera's axis size adjusted by some constant.