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R8218/private/nitfReadJPEG.m
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nitfReadJPEG.m
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function X = nitfReadJPEG(details, region)
%nitfReadJPEG Extract JPEG-compressed image from NITF file.
% X = nitfReadJPEG(DETAILS, REGION) returns the image X from the NITF
% file described in the DETAILS structure. The REGION argument is
% currently not used.
%
% See MIL-STD-188-198 for details of this scheme.
% Copyright 2010-2013 The MathWorks, Inc.
% Currently there's no pixel subsetting for NITF I/O.
if (~isempty(region))
error(message('images:nitfread:jpegSubsetting'))
end
% Get the JPEG data from the file.
compData = getJpegStream(details);
% Get the number of blocks, dimensions and markers.
nColsBlocks = double(swapbytes(typecast(compData(15:16), 'uint16')));
nRowsBlocks = double(swapbytes(typecast(compData(17:18), 'uint16')));
h = details.tileHeight;
w = details.tileWidth;
[startIdx, stopIdx, maskedBlocks] = getBlockLocations(compData, details, nColsBlocks, nRowsBlocks);
% Create the full-sized image.
X = createOutputArray(details);
% Read all of the blocks.
name = [tempname '.jpg'];
cleaner = onCleanup(@() delete(name));
idx = 1;
r = 1;
for j = 1:nRowsBlocks
c = 1;
for i = 1:nColsBlocks
% If the block is masked, update the position and go to the next.
if (maskedBlocks(idx))
c = c + w;
idx = idx + 1;
continue;
end
start = startIdx(idx);
stop = stopIdx(idx);
frameData = compData(start:stop);
% Read the portion of the image and store it.
data = getImageDataFromFrame(name, frameData);
X(r:(r+h-1), c:(c+w-1), :) = data;
% Update the position.
c = c + w;
idx = idx + 1;
end
r = r + h;
end
function compData = getJpegStream(details)
% Open the NITF file.
fid = fopen(details.filename, 'rb');
if (fid == 0)
error(message('images:nitfread:jpegFileOpen', details.filename))
else
c = onCleanup(@() fclose(fid));
end
% Move to the location in the file where the JPEG stream begins.
details.offsetStart = convertOffsetType(details.offsetStart);
status = fseek(fid, details.offsetStart, 'bof');
if (status ~= 0)
error(message('images:nitfread:jpegFseek'));
end
% Read the JPEG data, which comprises the rest of the NITF file.
compData = fread(fid, inf, 'uint8=>uint8');
% Trim leading 0xFF pad values before SOI.
while ((numel(compData > 2)) && (compData(2) == 255))
compData(1) = [];
end
% Validate the JPEG stream. It should start with an SOI marker (0xFF 0xD8)
% and then an APP6 marker (0xFF 0xE6).
if (~isequal(compData(1:2), [255; 216]))
error(message('images:nitfread:badEncapsulation'))
end
if (~isequal(compData(3:4), [255; 230]))
error(message('images:nitfread:missingAppMarker'))
end
function [startIdx, stopIdx, skipIdx] = getBlockLocations(compData, details, nColsBlocks, nRowsBlocks)
% Find the Image markers
SOIidx = findpattern(compData', sscanf('ff d8', '%x')');
EOIidx = findpattern(compData', sscanf('ff d9', '%x')');
SOFidx = sort([findpattern(compData', sscanf('ff c0', '%x')'), ...
findpattern(compData', sscanf('ff c1', '%x')'), ...
findpattern(compData', sscanf('ff c2', '%x')'), ...
findpattern(compData', sscanf('ff c3', '%x')')]);
if ((numel(SOIidx) ~= numel(EOIidx)) || ...
(numel(SOIidx) ~= numel(SOFidx)))
error(message('images:nitfread:jpegSoiEoiCount'))
end
% Does the NITF file use block masking, where some tiles are "virtual"?
if (isfield(details, 'blockOffsets'))
blockMasking = true;
skipIdx = (details.blockOffsets == intmax('uint32'));
else
blockMasking = false;
skipIdx = false(nRowsBlocks * nColsBlocks, 1);
end
if ((numel(SOIidx) ~= (nColsBlocks * nRowsBlocks)) && ~blockMasking)
warning(message('images:nitfread:wrongSOFIndexCount'));
end
% Compute where each tile's JPEG data starts and ends.
if (~blockMasking)
% If there's no masking, the data starts and stops with each SOI/EOI pair.
startIdx = SOIidx;
stopIdx = EOIidx + 1;
else
% If there is masking, the data start positions are given by the
% blockOffset values. NaN indicates a masked out frame.
startIdx = details.blockOffsets + 1;
startIdx(skipIdx) = NaN;
% Each tile ends directly before the next one starts. (Masked frames
% are tricky, so just use the end of the stream.) Shift the start
% values one over and subtract one to determine where they end. The
% last stop value is the last compressed value location.
stopIdx = [(startIdx(2:end) - 1) numel(compData)];
stopIdx(isnan(stopIdx)) = numel(compData);
stopIdx(skipIdx) = NaN;
end
function X = createOutputArray(details)
imgSize = [details.imageHeight, details.imageWidth, details.samplesPerPixel];
if (details.bitsPerSample <= 8)
X = zeros(imgSize, 'uint8');
elseif (details.bitsPerSample <= 16)
X = zeros(imgSize, 'uint16');
else
error(message('images:nitfread:jpegBitDepth', details.bitsPerSample));
end
function img = getImageDataFromFrame(name, frameData)
fid = fopen(name, 'wb');
if (fid == 0)
error(message('images:nitfread:tmpJpegFileOpen', name))
else
c = onCleanup(@() fclose(fid));
end
fwrite(fid, frameData, 'uint8');
delete(c);
img = imread(name, 'jpeg');
function idx = findpattern(array, pattern)
%FINDPATTERN Find a pattern in an array.
% Despite its name, "strfind" is very good at finding numeric patterns in
% numeric vectors.
idx = strfind(array, pattern);
function out = convertOffsetType(in)
if (in > intmax('uint32'))
error(message('images:nitfread:offsetTooBigForJPEG'))
end
out = double(in);
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