Bug #1206

In specific Canon makernote tags, certain values are meant to be ignored

Added by Sridhar Boovaraghavan about 1 year ago. Updated about 1 year ago.

Status:AssignedStart date:13 Aug 2016
Priority:NormalDue date:
Assignee:Sridhar Boovaraghavan% Done:


Target version:0.27


We already saw in Issue #1203 that certain canon makernote tags were returning values of incorrect type.

Further investigation (basically, a comparison with exiftool, see notes attached to Issue #1203) make it clear that some of the returned values are defaults that are meant to be ignored.

A selection of these are listed below. The table contains the tag name and the value which if it has, should be ignored.

There are other tag values that need to be ignored as well, but those are in the form of values needing to be >0 etc.

A good start would be to implement a filter type function that defines for each tag what an ignorable value means. If this function returns true, then we return ed.end() when the user calls findMetadatum.

Exif.CanonCs.RecordMode -1
Exif.CanonCs.Contrast 0x7fff
Exif.CanonCs.Saturation 0x7fff
Exif.CanonCs.Sharpness 0x7fff
Exif.CanonCs.CameraISO 0x7fff
Exif.CanonCs.AFPoint 0
Exif.CanonCs.FlashActivity -1
Exif.CanonCs.FocusContinuous -1
Exif.CanonCs.AESetting -1
Exif.CanonCs.ImageStabilization -1
Exif.CanonCs.SpotMeteringMode -1
Exif.CanonCs.PhotoEffect -1
Exif.CanonCs.ColorTone 0x7fff
Exif.CanonCs.SRAWQuality -1
Exif.CanonSi.FlashGuideNumber -1
Exif.CanonSi.AFPointsInFocus 0
Exif.CanonMi.FrameRate 65535
Exif.CanonMi.FrameCount 65535
Exif.CanonFi.FilterEffect -1
Exif.CanonFi.ToningEffect -1

Related issues

Related to Exiv2 - Bug #1203: Exif.CanonCs.FocusContinuous reported as Short Assigned 11 Aug 2016
Related to Exiv2 - Feature #1219: Adding Camera Temperature Assigned 31 Aug 2016

Associated revisions

Revision 4438
Added by Sridhar Boovaraghavan about 1 year ago

This is mainly a fix for #1206, but also interprets missing Canon Exif
Tags in exiv2 with the help of Phil Harvey's exiftool (see

Even with these changes (toward #1204 and #1205), exiv2 lags behind
exiftool in some areas of interpretation of Canon tags. Ideally, a
catch-up effort to bring the code in source: canonmn.cpp in line with
lib/Image/ExifTool/Canon.pm. v10.25 of exiftool was used as reference
for this change.

#1206 seeks to address the fact that when Canon does not have data for
certain tags, they use specific default values in those fields. These
default values need to be ignored and not displayed. This change
brings this feature to exiv2, something that already exiftool does.

With regards to implementation, the struct TagInfo in source: tags.hpp
is extended with four new fields.

The first field is a bool that if set to true (default false), denotes
that this field has ignorable default values.

The second field is the default value that needs to be ignored. This
can be of four types (String, Long, Float, Rational). These four types
were chosen as they had conversion functions in the Value class.

The third field is the comparison type (default equal_to). There are
six comparison types possible (equal_to, not_equal_to, less,
less_equal, greater, greater_equal). This is the comparison applied to
the value stored in the tag's field and the default value specified
above. For e.g. if the value in the tag Exif.CanonCs.RecordMode is -1,
then it needs to be ignored.

The fourth field is the data type (default Long). This could have been
guessed from the type of the second field, but that would necessitate
making this structure into a template calling for changes in multitude
of files.

Usage: In source: canonmn.cpp, several exif tags now have ignorable
default properties. I will list a few examples.

1. Exif.CanonCs.FocusMode: TagInfo(0x0007, "FocusMode", N_("Focus Mode"), N_("Focus mode setting"), canonCsId, makerTags, signedShort, 1, EXV_PRINT_TAG(canonCsFocusMode)),

There are no changes - i.e. this is an example of how the TagInfo
structure was being populated.

2. Exif.CanonCs.RecordMode: TagInfo(0x0009, "RecordMode", N_("Record Mode"), N_("Record mode setting"), canonCsId, makerTags, signedShort, 1, EXV_PRINT_TAG(canonCsRecordMode), true, s_1_),

Take a look at the two new arguments. The first one (true) specifies
that there is a default value that can be ignored. The second one s_1_
specifies the value (-1, in this case) to be ignored.

const UShortValue CanonMakerNote::s_1_(65535, unsignedShort); // Till bug is resolved

Note s_1_ is temporarily having the value 65535 till #1203 that causes
signedShorts to be interpreted as unsignedShorts is resolved.

3. Exif.CanonSi.TargetAperture: TagInfo(0x0004, "TargetAperture", N_("Target Aperture"), N_("Target Aperture"), canonSiId, makerTags, unsignedShort, 1, printSi0x0015, true, us0_, TagInfo::less_equal),

Note the third argument TagInfo::less_equal. This combined with the
second argument us0_ (the number 0) signifies that any values in this
tag that are less than or equal (<=) to 0 should be ignored.

4. TagInfo(0x0028, "ImageUniqueID", N_("Image Unique ID"), N_("Image Unique ID"), canonId, makerTags, asciiString, -1, printValue, true, s0x16_, TagInfo::equal_to, TagInfo::String),

The previous examples have all been of Long type. This shows a case
where the default value is a string.

const AsciiValue CanonMakerNote::s0x16_("0000000000000000");

Once these tag values have been defined, the actual mechanics of
ignoring these default values happens in Image::exifData().

Before the exifData is returned, we loop through the data, ask the
data whether it needs to be ignored (which in turn checks its
underlying tagInfo and compares it with the default value, if
specified) and if so, deletes that element.

A compile-time switch called EXV_DONT_IGNORE_UNDEFINED which when set
to a non-zero value will cause the behavior to revert back to the
original where all values are reported irregardless of the fact that
they need to be ignored.


#1 Updated by Sridhar Boovaraghavan about 1 year ago

  • Priority changed from Normal to High

#2 Updated by Robin Mills about 1 year ago

  • Category set to metadata
  • Status changed from New to Assigned
  • Assignee set to Sridhar Boovaraghavan
  • Priority changed from High to Normal
  • Target version set to 0.26

Are these keys to be always ignored, or only when they have those default values? Should we report those values if the user provides the --verbose option on the command-line?

#3 Updated by Sridhar Boovaraghavan about 1 year ago

No, these keys are not always meant to be ignored, but only when they have those specific values.

Regarding the verbose option, I don't know. Does exiv2 do anything different as in print the raw (uninterpreted) values in addition to the interpreted ones? If so, then we can consider printing those values when verbose is used.

#4 Updated by Robin Mills about 1 year ago

I don't thing verbose does much. And it's a feature of exiv2(.exe) and not the library. If you decide to remove those in canonmn.cpp, the --verbose flag will not be able to see those keys.

--vebose gives a little more output by reporting file paths and stuff.

954 rmills@rmillsmbp:~/gnu/exiv2/trunk $ exiv2 -pv --grep LensID/i ~/Stonehenge.jpg 
0x000c NikonLd3     LensIDNumber                Byte        1  146
955 rmills@rmillsmbp:~/gnu/exiv2/trunk $ exiv2 -pa --grep LensID/i ~/Stonehenge.jpg 
Exif.NikonLd3.LensIDNumber                   Byte        1  Sigma 18-250mm F3.5-6.3 DC OS Macro HSM
956 rmills@rmillsmbp:~/gnu/exiv2/trunk $ exiv2 --verbose -pa --grep LensID/i ~/Stonehenge.jpg 
File 1/1: /Users/rmills/Stonehenge.jpg
Exif.NikonLd3.LensIDNumber                   Byte        1  Sigma 18-250mm F3.5-6.3 DC OS Macro HSM
957 rmills@rmillsmbp:~/gnu/exiv2/trunk $ exiv2 --verbose -pv --grep LensID/i ~/Stonehenge.jpg 
File 1/1: /Users/rmills/Stonehenge.jpg
0x000c NikonLd3     LensIDNumber                Byte        1  146
958 rmills@rmillsmbp:~/gnu/exiv2/trunk $ 
--verbose also reports when the return value of exiv2(.exe) is not zero.

A very important use of --verbose is with --version. It prints a lot of information about the build AND the paths to dynamic libraries loaded by exiv2(.exe).

#5 Updated by Robin Mills about 1 year ago

  • Target version changed from 0.26 to 0.27

Development of this feature has been deferred for v0.27 and to be developed in branches/develop. r4449

#6 Updated by Andreas Huggel about 1 year ago

Based on the list above, all of these tags are in 'binary arrays', i.e., composite IFD tags which consist of a number of values that are decoded into different tags, e.g., a single IFD tag consisting of three short values 1, 2, 3 which are decoded into three different tags, tag1 with value short 1, tag2 with value short 2 and tag3 with value short 3.

My understanding is that Canon decided to use a certain value to identify tags which are not present in certain images (maybe because the camera used doesn't support the feature related to the particular tag). So, e.g., the second value of the above sample array could be set to -1 to indicate that tag2 is not present, as, because of the choice to use a binary array for the tags, that tag cannot simply be omitted from the binary structure.

Exiv2 should ignore such tags, as if they were really not present in the image.

A straightforward implementation will be to enhance the TIFF parser to not read such tags at all:

New settings should be added to Internal::ArrayCfg to make it possible to define the tags to be ignored. The actual configurations for the known arrays are in tiffimage.cpp. TiffReader::visitBinaryArray can then use the new configuration to skip these tags.

The write code may require minor changes as well.

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