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55 lines
2.5 KiB
55 lines
2.5 KiB
14 years ago
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This is an attempt to explain how autosizing paintdevices work
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The tileddatamanager which is the underlying object that organizes the pixel
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storage is autoextending. So it starts with zero tiles, and as writing is
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being done new tiles are added as needed.
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There is also a default tile that is only readable (in theory). Whenever
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access is only readaccess you get this tile for areas that don't have their
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own tile yet. The idea is that the default tile is filled with transparent
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pixels. (Or, with special layers like the background pattern or the selection,
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with something else relevant. bsar)
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So imagine a new image of size 640x640. The layer has no tiles, but when the
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layer is being read (for viewing) we probe all over the image (640x640) and
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read from the default tile wherever we probe.
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The extent of the layer (paintdevice) is 0x0 because nothing has been
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written yet.
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Then some drawing is done in one corner and a tile is created automatically.
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Now the extent is 64x64 (tilesize defined at compilation) because a single
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tile exists.
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If some drawing is done in the opposite corner another tile is created. The
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extent is now 640x640, but only two tiles exist (one in each opposite
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corner). When probing for viewing the default tile is what you get except
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for the two tiles.
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If we now apply a filer that lightens the colors (or something else) we
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could iterate over the extent. That way we are sure to get all the real
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tiles that have been created.
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As there is no way for us to know which pixel have true tiles and which have
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the default, we would write to all the pixels within the extent, which would
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automatically create ALL the tiles inside the extent.
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(XXX: Of course, the tile manager could decide that the written value
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is the same as the default value, and if that's true for all pixels in a tile,
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not create a tile at all) (bsar))
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In other cases like a gradient fill we would probably want to fill the
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current image area, so we we would just write to the paintdevice, and tiles
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would be automatically created for us.
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So in short: if you read - data will be available (either real tiles or a
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default) - and if you write to the paintdevice - tiles will be created as
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needed. So simply don't worry.
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But in some cases, when you want to modify already written pixels, as in
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some filters you want to know where true data have been written. Otherwise
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we would have to modify an infinite number of pixels to be sure everything is
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changed. This is where the extent comes in handy. If you have absolutely
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no leeway, use exactBounds which is slow, but sure.
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