Changes between Version 67 and Version 68 of WKTRaster/SpecificationWorking03

Timestamp:

Apr 23, 2011, 8:22:27 AM (13 years ago)

Author:

Bborie Park

Comment:

Fleshed out ST_AsPNG

WKTRaster/SpecificationWorking03

@@ -357 +357 @@
 ----
 '''ST_AsPNG(raster, band) -> PNG as "bytea"'''
+
+Bborie: Like the JPEG raster format, the PNG format has limitations:
+
+  1. PNG only allows 1 (greyscale) or 3 (RGB) bands of data
+
+  2. PNG only supports 8BUI and 16BUI pixeltypes. Any other pixeltype will be written as 8BUI, though the results are probably useless
+
+  3. PNG cannot embed spatial reference information within the file but can have an associated world file
+
+Like JPEG, the limitations can be resolved:
+
+  1. Use ST_Band to specify which band(s) should be passed to the ST_AsPNG function. If a raster whose number of specified bands does not equal 1 or 3 is provided, a warning is raised and the first or the first three bands are used.
+
+  2. Throw an exception if any of the specified bands is not 8BUI or 16BUI. The user should use ST_Reclass to convert any non-8BUI or 16BUI bands to 8BUI or 16BUI.
+
+  3. Nothing can be done within this function. ST_Georeference() can be used to the contents of the associated world file
+
+A proposed set of variations of the ST_AsPNG function:
+
+1. ST_AsPNG(rast raster, options text[])
+
+    rast: the raster with one or three bands in 8BUI or 16BUI pixel type to generate a PNG image from
+
+    options: array of creation options to pass to the GDAL JPEG driver
+
+{{{
+ST_AsPNG(rast, ARRAY['ZLEVEL=9'])
+}}}
+
+2. ST_AsPNG(rast raster)
+
+    Like !#1 above but use the driver's default creation options
+
+3. ST_AsPNG(rast raster, nbands int[], options text[])
+
+    nbands: an integer array specifying the band indices of the raster to include in the PNG file
+
+{{{
+ST_AsPNG(rast, ARRAY[3,1,2], ARRAY['ZLEVEL=9'])
+}}}
+
+4. ST_AsPNG(rast raster, nbands int[])
+
+    Like !#3, but use the default creation options
+
+{{{
+ST_AsPNG(rast, ARRAY[3])
+}}}
+
+5. ST_AsPNG(rast raster, nbands int[], compression int)
+
+    compression: number between 1 and 9 indicating the amount of time to spend on compression. 1 is fastest with least compression. 9 is slowest with best compression
+
+{{{
+ST_AsPNG(rast, ARRAY[2,1,3], 3)
+}}}
+
+6. ST_AsPNG(rast raster, nband int, options text[])
+
+    nband: index of the band to include
+
+{{{
+ST_AsPNG(rast, 2, ARRAY['ZLEVEL=5'])
+}}}
+
+7. ST_AsPNG(rast raster, nband int, compression int)
+
+{{{
+ST_AsPNG(rast, 1, 8)
+}}}
+
+8. ST_AsPNG(rast raster, nband int)
+
+{{{
+ST_AsPNG(rast, 1)
+}}}
 
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