Hi Frank,
Ok, I agree with you that the MapServer doc says that the getBytes()
function "Returns the image contents as a binary buffer". And I see in
tiff_read.py that tiff_vsimem() function is supposed to "# Test reading a
tiff from a memory buffer (#2931)", but there must be a difference between
the binary buffer that MS returns, and the memory buffer that tiff_vsimem is
reading as input.
I implemented your recommendation in the code below, and I am getting an
error when I try to open the buffer as it is done in the tiff_read code,
"TypeError: fileio() argument 1 must be encoded string without NULL bytes,
not str"
#! /usr/bin/python
from mapscript import mapObj
from osgeo import gdal
def getImageData():
mapobject = mapObj('basemap.map')
mapimage = mapobject.draw()
image_data = mapimage.getBytes()
return image_data
def cropImage(image_data):
open(image_data, mode='rb').read()
def main():
image_data = getImageData()
cropImage(image_data)
if __name__ == "__main__":
main()
I'm trying to figure out how to manipulate the getBytes() buffer in such a
way that "open" will work on it, but so far am not having any luck.
Roger
--
On Sat, Jan 16, 2010 at 5:57 PM, Frank Warmerdam <warmer...@pobox.com>wrote:
> Roger André wrote:
>
>> Hi Frank,
>>
>> Ok, that makes sense. So for this to work, I will have to know (somehow)
>> what type of image mapserver is sending, and then instantiate a MEM dataset
>> with the correct parameters. Problem with that is that I can't think of a
>> good way to read what those parameters are out of the binary string. It's
>> no problem when I know exactly what data is coming in, as is the case for my
>> very specific need, but it means that swapping in GDAL for PIL as a general
>> solution might not be possible.
>>
>
> Roger,
>
> I was confused about what you were using before, and now I realize you
> are calling imageObj.getBytes() in mapscript (as you did explain!).
>
> Reading the docs for this function:
> /*
>
> -------------------------------------------------------------------------
> getBytes returns a gdBuffer structure (defined in mapscript.i) which
> must
> be typemapped to an object appropriate to the target language. This
> typemap must also gdFree the data member of the gdBuffer. See the type-
> maps in java/javamodule.i and python/pymodule.i for examples.
>
> contributed by Jerry Pisk, jerry.p...@gmail.com
>
> -------------------------------------------------------------------------
> */
>
> it seems it may be difficult to do alot with the result. Hmm, in fact
> looking at the code:
>
> gdBuffer getBytes()
> {
> gdBuffer buffer;
>
> buffer.owns_data = MS_TRUE;
>
> buffer.data = msSaveImageBuffer(self, &buffer.size, self->format);
>
> if( buffer.data == NULL || buffer.size == 0 )
> {
> buffer.data = NULL;
> msSetError(MS_MISCERR, "Failed to get image buffer",
> "getBytes");
> return buffer;
> }
>
> return buffer;
> }
>
> it isn't doing what is suggested by the comments. It is actually encoding
> it into whatever the format selected was (perhaps jpeg or png) and
> returning
> the raw bytes of that.
>
> My approach before depended on the image being a raw image (uncompressed,
> without any format specific headers). What you need is a way of opening
> a supported file format, but from a memory buffer.
>
> This sequence from autotest/gcore/tiff_read.py gives a clue how this might
> be done in the GDAL Python bindings:
>
> content = open('data/byte.tif', mode='rb').read()
>
> # Create in-memory file
> gdal.FileFromMemBuffer('/vsimem/tiffinmem', content)
>
> ds = gdal.Open('/vsimem/tiffinmem', gdal.GA_Update)
>
> You won't need to worry about copying palettes or anything.
>
>
> One last question. How does a utility like gdal_translate figure out what
>> the input data type is?
>>
>
> gdal.Open() reads a small chunk from the beginning of the file, and
> then, in sequence, passes this to the Open method of each driver along
> with the filename and some other info. Each Open() has to decide if the
> referenced file is in it's format by whatever mechanism is appropriate.
> Usually this is done by looking for "magic" bytes for that format at
> the beginning of the file.
>
>
> Best regards,
> --
>
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up | Frank Warmerdam,
> warmer...@pobox.com
> light and sound - activate the windows |
> http://pobox.com/~warmerdam<http://pobox.com/%7Ewarmerdam>
> and watch the world go round - Rush | Geospatial Programmer for Rent
>
>
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