Changes between Version 1 and Version 2 of rfc15_nodatabitmask
- Timestamp:
- Aug 9, 2007, 1:09:47 PM (17 years ago)
Legend:
- Unmodified
- Added
- Removed
- Modified
-
rfc15_nodatabitmask
v1 v2 1 = RFC 14: N oData Bitmasks =1 = RFC 14: Null Masks = 2 2 3 3 Author: Frank Warmerdam[[BR]] … … 9 9 Some file formats support a concept of a bitmask to identify pixels that are not valid data. This can be particularly valuable with byte image formats where a nodata pixel value can not be used because all pixel values have a valid meaning. This RFC tries to formalize a way of recognising and accessing such nodata bitmasks through the GDAL API. 10 10 11 == 11 The basic approach is to treat such masks as raster bands, but not regular raster bands on the datasource. Instead they are freestanding raster bands in a manner similar to the overview raster band objects. The nodata masks are represented as GDT_Byte bands with a value of zero indicating nodata and 255 indicating valid data. Intermediate values do not normally occur but in they future they might be interpreted as a confidence value or alpha transparency values. 12 13 == API == 14 15 GDALRasterBand is extended with the following methods: 12 16 13 17 {{{ 14 virtual CPLErr BitmaskIO( GDALRWFlag, int, int, int, int, 15 GByte *, int, int, GDALDataType, 16 int, int *, int, int, int ); 18 GDALRasterBand *GetNullMask(); 19 GDALNullMaskType GetNullMaskType(); 17 20 }}} 18 21 22 Note that the GetNullMask() should always return a GDALRasterBand mask, even if it is only an all 255 mask with thetype 23 24 The GDALNullMaskType enumeration has the following values: 25 26 * GDALNMTAllValid: There are no invalid pixels, all mask values will be 255. 27 * GDALNMTPerBand: The mask band is meaningful and is unique to this band (other bands may have masks with different areas of validity) 28 * GDALNMTPerDataset: The mask band is meaningful and is shared between all bands on this dataset. 29 30 Note that the GetNullMask() should always return a GDALRasterBand mask, even if it is only an all 255 mask with the type set to GDALNMTAllValid. 31 32 == Default Implementation == 33 34 The GDALRasterBand class will include a default implementation of GetNullMask() that returns one of two default implementations. The first default implement is a GDALNMTAllValid band that returns all 255. The second default implementation would return a mask computed by reading the imagery data and comparing it to the nodata value. The nodata based one would be used when the base band has a nodata value. 35 36 == Drivers Updated == 37 38 These drivers will be updated: 39 40 * JPEG Driver: support the "zlib compressed mask appended to the file" approach used by a few data providers. 41 * GRASS Driver: updated to support handling null values as masks. 42 43 Possibly updated: 44 * HDF4 Driver: This driver might possibly be updated to return real mask if we can figure out a way. 45 * SDE Driver: This driver might be updated if Howard has sufficient time and enthusiasm. 19 46 20 47 == Implementation Plan == … … 24 51 == SWIG Implications == 25 52 53 The GetNullMask() and GetNullMaskType() methods (and corrsponding defines) will need to be added. The bitmask should work like a normal raster band for swig purposes so minimal special work should be required. 26 54 27 55 == Testing == 56 57 == Issues == 58 59 I'm tempted to change GetNullMaskType() into GetNullMaskFlags() and have the result be an or-ed set of flags. It would allow us to differentiate mask bands auto generated from nodata, and add special interpretations in the future, like is-alpha or is-confidence. 60 61 * I haven't filled in how creation of masks works. 62 * I haven't talked about bitmask-via-spill file possibilities for the PAM subsystem.
