| 69 | | However, returning only the bits per pixel is not sufficient to quickly and efficiently process the raster stream. The user should also be made aware of the bits used per pixel. In certain circumstances, the raster image does not completely use all available bits and pads the extra bits in the raster stream with NULL values. Without knowing the bits used per pixel, the client has to spend processing cycles determining how many bits are used thus affecting the performance of their applications. |
| | 69 | However, returning only the bits per pixel is not sufficient to quickly and efficiently scale the raster for display in cases where dynamic contrast stretching is appropriate. In cases where stetching is needed (for instance 16bit greyscale images or 4bit images represented as 8bit) it is often the case that the full dynamic range of the data type is not being used. In order to scale the raster to 0-255 for display it is necessary to know what actual range of values is used. For some data types (floating point elevations for instance) there is no obvious range implicit in the data type. |
| | 70 | |
| | 71 | Client application can work out the range by making a first pass reading the whole dataset to scan for a min/max, and then pulling out subareas they are actually interested in and scaling those. But this is very expensive, and discards any opportunity to use pre-existing metadata from the datasource to establish the value min/max range. |
