Zermeno, Robert J CIV NAVAIR, 472100D wrote:
Can anyone let me know how to properly read a 24bpp JPEG2000 image data?
This image has 1 Band overview. I am using the C API.
I have a few questions. Please respond even if you know something about any
of the questions I have.
1. Can someone reference me to where I can acquire what Band overviews are
in respect to JPEG2000?
Robert,
I don't understand this question.
2. What does GDALGetOverview() really return when referring to image data?
I understand it returns a GDALRasterBandH in which I can call
GDALReadBlock(), but it does not successfully (I believe its because my
image array is not big enough).
GDALGetOverview() returns a GDALRasterBandH which can be used to access
the reduced resolution overview of the main full res image. In the
case of JPEG2000 the overview does not exist on disk as a distinct object,
but rather is mixed into the compressed data stream for the image is a
whole. But the overview band objects fairly efficiently extract the
image overview data.
I do not understand your point about GDALReadBlock not working. I will
note that it is often better to use the RasterIO() interface as the band
or dataset level rather than fetching by blocks with JPEG2000.
3. In GDALReadBlock(), what does it mean when this method returns a
"natural" block from the raster band? So, since my image is say 24bpp 900 x
900, will return a line of 900 x 3 (for RGB per pixel)?
Many file formats have a "natural block size". For scanline oriented
formats the block size is one full scanline. For tiled images, it is one
tile. JPEG2000 does not really have a natural block size, so we arbitrarily
establish a block size (usually 2048x128 for the JP2KAK JPEG2000 driver)
that provides for reasonably efficient access.
4. When using GDALReadBlock() when the image has an overview, I still
retrieve image data before even calling GDALGetOverview(). What data am I
actually getting?
You are presumably getting blocks of imagery from the full resolution image.
My main issue is understanding how the image data in JPEG2000 is retrieved
by GDAL and what GDALGetOverview() and GDALReadBlock() really returns to the
programmer when your image contains an Overview?
Well, the overview data is extracted from the JPEG2000 datastream by setting
a window and resolution level with the lower level library (usually
Kakadu) and then reading the imagery back. The lower level library takes
care of the complex logic.
Best regards,
--
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I set the clouds in motion - turn up | Frank Warmerdam, [email protected]
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush | Geospatial Programmer for Rent
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