Accessors

The bitmaps from the individual sub-blocks are logically arranged as tiles on a plane. In order to get a bitmap containing the tile-composite (or a part of it) the use of an accessor is required. In this example, we have three tiles on a plane and request to get a certain section (aka region-of-interest ROI):

The resulting bitmap in this case will look like this:

When creating the output bitmap, the pixels are converted to the destination pixel type (if necessary). This conversion is done in the following way:

Pixel Type Conversion

source pixel type	destination pixel type	operation
Gray8	BGR24	R8 = G8 = B8 ← Gray8
Gray8	BGR48	R8 = G8 = B8 ← Gray8
Gray16	BGR24	R8 = G8 = B8 ← Gray16/256
Gray16	BGR48	R8 = G8 = B8 ← Gray16
BGR24	Gray8	Gray8 ← \(\frac{R8+G8+B8}{3}\)
BGR48	Gray8	Gray8 ← \((\frac{R16}{256}+\frac{G16}{256}+\frac{B16}{256})/3\)
Gray8	Gray16	Gray8 ← Gray16/256
RGB24	RGB48	R16 ← R8, G16 ← G8, B16 ← B8
RGB48	RGB24	R8 ← \(\frac{R16}{256}\), G16 ← \(\frac{G16}{256}\), B16 ← \(\frac{B16}{256}\)

We have three types of accessors available:

Accessor Types

accessor	purpose
SingleChannelTileAccessor	get a multi-tile composite only from pyramid-layer 0
SingleChannelPyramidLayerTileAccessor	get a multi-tile composite from an explicitly specified pyramid-layer
SingleChannelScalingTileAccessor	get a multi-tile composite at an arbitrary zoom-level

The SingleChannelTileAccessor will only operate on tiles on pyramid-level 0 - i.e. only on sub-blocks for which physical_size = logical_size. The size (in pixels) of the resulting multi-tile-composite is the same as the ROI specified.

The SingleChannelPyramidLayerTileAccessor allows access to a specific pyramid-level. But it will only consider sub-blocks from this pyramid-level when composing the tile-composite.

The SingleChannelScalingTileAccessor allows specifying an arbitrary zoom-level, it will choose the appropriate pyramid-level and scale the bitmaps (if necessary) to fit the desired zoom-level.