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import struct
from django.core.exceptions import ValidationError
from .const import (
BANDTYPE_FLAG_HASNODATA,
BANDTYPE_PIXTYPE_MASK,
GDAL_TO_POSTGIS,
GDAL_TO_STRUCT,
POSTGIS_HEADER_STRUCTURE,
POSTGIS_TO_GDAL,
STRUCT_SIZE,
)
def pack(structure, data):
"""
Pack data into hex string with little endian format.
"""
return struct.pack("<" + structure, *data)
def unpack(structure, data):
"""
Unpack little endian hexlified binary string into a list.
"""
return struct.unpack("<" + structure, bytes.fromhex(data))
def chunk(data, index):
"""
Split a string into two parts at the input index.
"""
return data[:index], data[index:]
def from_pgraster(data):
"""
Convert a PostGIS HEX String into a dictionary.
"""
if data is None:
return
# Split raster header from data
header, data = chunk(data, 122)
header = unpack(POSTGIS_HEADER_STRUCTURE, header)
# Parse band data
bands = []
pixeltypes = []
while data:
# Get pixel type for this band
pixeltype_with_flags, data = chunk(data, 2)
pixeltype_with_flags = unpack("B", pixeltype_with_flags)[0]
pixeltype = pixeltype_with_flags & BANDTYPE_PIXTYPE_MASK
# Convert datatype from PostGIS to GDAL & get pack type and size
pixeltype = POSTGIS_TO_GDAL[pixeltype]
pack_type = GDAL_TO_STRUCT[pixeltype]
pack_size = 2 * STRUCT_SIZE[pack_type]
# Parse band nodata value. The nodata value is part of the
# PGRaster string even if the nodata flag is True, so it always
# has to be chunked off the data string.
nodata, data = chunk(data, pack_size)
nodata = unpack(pack_type, nodata)[0]
# Chunk and unpack band data (pack size times nr of pixels)
band, data = chunk(data, pack_size * header[10] * header[11])
band_result = {"data": bytes.fromhex(band)}
# Set the nodata value if the nodata flag is set.
if pixeltype_with_flags & BANDTYPE_FLAG_HASNODATA:
band_result["nodata_value"] = nodata
# Append band data to band list
bands.append(band_result)
# Store pixeltype of this band in pixeltypes array
pixeltypes.append(pixeltype)
# Check that all bands have the same pixeltype.
# This is required by GDAL. PostGIS rasters could have different pixeltypes
# for bands of the same raster.
if len(set(pixeltypes)) != 1:
raise ValidationError("Band pixeltypes are not all equal.")
return {
"srid": int(header[9]),
"width": header[10],
"height": header[11],
"datatype": pixeltypes[0],
"origin": (header[5], header[6]),
"scale": (header[3], header[4]),
"skew": (header[7], header[8]),
"bands": bands,
}
def to_pgraster(rast):
"""
Convert a GDALRaster into PostGIS Raster format.
"""
# Prepare the raster header data as a tuple. The first two numbers are
# the endianness and the PostGIS Raster Version, both are fixed by
# PostGIS at the moment.
rasterheader = (
1,
0,
len(rast.bands),
rast.scale.x,
rast.scale.y,
rast.origin.x,
rast.origin.y,
rast.skew.x,
rast.skew.y,
rast.srs.srid,
rast.width,
rast.height,
)
# Pack raster header.
result = pack(POSTGIS_HEADER_STRUCTURE, rasterheader)
for band in rast.bands:
# The PostGIS raster band header has exactly two elements, a 8BUI byte
# and the nodata value.
#
# The 8BUI stores both the PostGIS pixel data type and a nodata flag.
# It is composed as the datatype with BANDTYPE_FLAG_HASNODATA (1 << 6)
# for existing nodata values:
# 8BUI_VALUE = PG_PIXEL_TYPE (0-11) | BANDTYPE_FLAG_HASNODATA
#
# For example, if the byte value is 71, then the datatype is
# 71 & ~BANDTYPE_FLAG_HASNODATA = 7 (32BSI)
# and the nodata value is True.
structure = "B" + GDAL_TO_STRUCT[band.datatype()]
# Get band pixel type in PostGIS notation
pixeltype = GDAL_TO_POSTGIS[band.datatype()]
# Set the nodata flag
if band.nodata_value is not None:
pixeltype |= BANDTYPE_FLAG_HASNODATA
# Pack band header
bandheader = pack(structure, (pixeltype, band.nodata_value or 0))
# Add packed header and band data to result
result += bandheader + band.data(as_memoryview=True)
return result
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