gecko-dev/image/decoders/EXIF.cpp

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/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#include "EXIF.h"
#include "mozilla/Endian.h"
namespace mozilla {
namespace image {
// Section references in this file refer to the EXIF v2.3 standard, also known
// as CIPA DC-008-Translation-2010.
// See Section 4.6.4, Table 4.
// Typesafe enums are intentionally not used here since we're comparing to raw
// integers produced by parsing.
enum EXIFTag
{
OrientationTag = 0x112,
};
// See Section 4.6.2.
enum EXIFType
{
ByteType = 1,
ASCIIType = 2,
ShortType = 3,
LongType = 4,
RationalType = 5,
UndefinedType = 7,
SignedLongType = 9,
SignedRational = 10,
};
static const char* EXIFHeader = "Exif\0\0";
static const uint32_t EXIFHeaderLength = 6;
/////////////////////////////////////////////////////////////
// Parse EXIF data, typically found in a JPEG's APP1 segment.
/////////////////////////////////////////////////////////////
EXIFData
EXIFParser::ParseEXIF(const uint8_t* aData, const uint32_t aLength)
{
if (!Initialize(aData, aLength))
return EXIFData();
if (!ParseEXIFHeader())
return EXIFData();
uint32_t offsetIFD;
if (!ParseTIFFHeader(offsetIFD))
return EXIFData();
JumpTo(offsetIFD);
Orientation orientation;
if (!ParseIFD0(orientation))
return EXIFData();
// We only care about orientation at this point, so we don't bother with the
// other IFDs. If we got this far we're done.
return EXIFData(orientation);
}
/////////////////////////////////////////////////////////
// Parse the EXIF header. (Section 4.7.2, Figure 30)
/////////////////////////////////////////////////////////
bool
EXIFParser::ParseEXIFHeader()
{
return MatchString(EXIFHeader, EXIFHeaderLength);
}
/////////////////////////////////////////////////////////
// Parse the TIFF header. (Section 4.5.2, Table 1)
/////////////////////////////////////////////////////////
bool
EXIFParser::ParseTIFFHeader(uint32_t& aIFD0OffsetOut)
{
// Determine byte order.
if (MatchString("MM\0*", 4))
mByteOrder = ByteOrder::BigEndian;
else if (MatchString("II*\0", 4))
mByteOrder = ByteOrder::LittleEndian;
else
return false;
// Determine offset of the 0th IFD. (It shouldn't be greater than 64k, which
// is the maximum size of the entry APP1 segment.)
uint32_t ifd0Offset;
if (!ReadUInt32(ifd0Offset) || ifd0Offset > 64 * 1024)
return false;
// The IFD offset is relative to the beginning of the TIFF header, which
// begins after the EXIF header, so we need to increase the offset
// appropriately.
aIFD0OffsetOut = ifd0Offset + EXIFHeaderLength;
return true;
}
/////////////////////////////////////////////////////////
// Parse the entries in IFD0. (Section 4.6.2)
/////////////////////////////////////////////////////////
bool
EXIFParser::ParseIFD0(Orientation& aOrientationOut)
{
uint16_t entryCount;
if (!ReadUInt16(entryCount))
return false;
for (uint16_t entry = 0 ; entry < entryCount ; ++entry) {
// Read the fields of the entry.
uint16_t tag;
if (!ReadUInt16(tag))
return false;
// Right now, we only care about orientation, so we immediately skip to the
// next entry if we find anything else.
if (tag != OrientationTag) {
Advance(10);
continue;
}
uint16_t type;
if (!ReadUInt16(type))
return false;
uint32_t count;
if (!ReadUInt32(count))
return false;
// We should have an orientation value here; go ahead and parse it.
Orientation orientation;
if (!ParseOrientation(type, count, aOrientationOut))
return false;
// Since the orientation is all we care about, we're done.
return true;
}
// We didn't find an orientation field in the IFD. That's OK; we assume the
// default orientation in that case.
aOrientationOut = Orientation();
return true;
}
bool
EXIFParser::ParseOrientation(uint16_t aType, uint32_t aCount, Orientation& aOut)
{
// Sanity check the type and count.
if (aType != ShortType || aCount != 1)
return false;
uint16_t value;
if (!ReadUInt16(value))
return false;
switch (value) {
case 1: aOut = Orientation(Angle::D0, Flip::Unflipped); break;
case 2: aOut = Orientation(Angle::D0, Flip::Horizontal); break;
case 3: aOut = Orientation(Angle::D180, Flip::Unflipped); break;
case 4: aOut = Orientation(Angle::D180, Flip::Horizontal); break;
case 5: aOut = Orientation(Angle::D90, Flip::Horizontal); break;
case 6: aOut = Orientation(Angle::D90, Flip::Unflipped); break;
case 7: aOut = Orientation(Angle::D270, Flip::Horizontal); break;
case 8: aOut = Orientation(Angle::D270, Flip::Unflipped); break;
default: return false;
}
// This is a 32-bit field, but the orientation value only occupies the first
// 16 bits. We need to advance another 16 bits to consume the entire field.
Advance(2);
return true;
}
bool
EXIFParser::Initialize(const uint8_t* aData, const uint32_t aLength)
{
if (aData == nullptr)
return false;
// An APP1 segment larger than 64k violates the JPEG standard.
if (aLength > 64 * 1024)
return false;
mStart = mCurrent = aData;
mLength = mRemainingLength = aLength;
mByteOrder = ByteOrder::Unknown;
return true;
}
void
EXIFParser::Advance(const uint32_t aDistance)
{
if (mRemainingLength >= aDistance) {
mCurrent += aDistance;
mRemainingLength -= aDistance;
} else {
mCurrent = mStart;
mRemainingLength = 0;
}
}
void
EXIFParser::JumpTo(const uint32_t aOffset)
{
if (mLength >= aOffset) {
mCurrent = mStart + aOffset;
mRemainingLength = mLength - aOffset;
} else {
mCurrent = mStart;
mRemainingLength = 0;
}
}
bool
EXIFParser::MatchString(const char* aString, const uint32_t aLength)
{
if (mRemainingLength < aLength)
return false;
for (uint32_t i = 0 ; i < aLength ; ++i) {
if (mCurrent[i] != aString[i])
return false;
}
Advance(aLength);
return true;
}
bool
EXIFParser::MatchUInt16(const uint16_t aValue)
{
if (mRemainingLength < 2)
return false;
bool matched;
switch (mByteOrder) {
case ByteOrder::LittleEndian:
matched = LittleEndian::readUint16(mCurrent) == aValue;
break;
case ByteOrder::BigEndian:
matched = BigEndian::readUint16(mCurrent) == aValue;
break;
default:
NS_NOTREACHED("Should know the byte order by now");
matched = false;
}
if (matched)
Advance(2);
return matched;
}
bool
EXIFParser::ReadUInt16(uint16_t& aValue)
{
if (mRemainingLength < 2)
return false;
bool matched = true;
switch (mByteOrder) {
case ByteOrder::LittleEndian:
aValue = LittleEndian::readUint16(mCurrent);
break;
case ByteOrder::BigEndian:
aValue = BigEndian::readUint16(mCurrent);
break;
default:
NS_NOTREACHED("Should know the byte order by now");
matched = false;
}
if (matched)
Advance(2);
return matched;
}
bool
EXIFParser::ReadUInt32(uint32_t& aValue)
{
if (mRemainingLength < 4)
return false;
bool matched = true;
switch (mByteOrder) {
case ByteOrder::LittleEndian:
aValue = LittleEndian::readUint32(mCurrent);
break;
case ByteOrder::BigEndian:
aValue = BigEndian::readUint32(mCurrent);
break;
default:
NS_NOTREACHED("Should know the byte order by now");
matched = false;
}
if (matched)
Advance(4);
return matched;
}
} // namespace image
} // namespace mozilla