AspJpeg.com - User Manual Chapter 7: Metadata Extraction EXIF IPTC ASP ASP.NET

User Manual

1.Introduction

2.Thumbnails

3.Using AspUpload

7.Metadata

10.PNG Output

Chapter 7: Metadata Extraction

7.1 Metadata Overview
7.2 EXIF Item List
7.3 IPTC Item List
7.4 TIFF Support
7.5 Metadata Preservation
7.6 Metadata Editing

7.1 Metadata Overview

Most digital cameras nowadays embed various pieces of information into the JPEG images they generate, including the current date/time, shooting conditions (e.g. whether a flash was used), camera settings (shutter, aperture, focal length), etc. Adobe Photoshop, the leading image editing package, is also capable of embedding various pieces of metadata into the JPEGs it produces, such as an author, title, copyright notice, etc.
The format for camera-embedded metadata is called EXIF, which stands for Exchangeable Image File Format. For more information on EXIF, visit www.exif.org. The format used by Photoshop is described by the International Press Telecommunications Council (www.iptc.org) and usually referred to as IPTC.
AspJpeg 1.4+ is capable of extracting EXIF and IPTC metadata from JPEG images via the OpenInfo and OpenInfoBinary methods. The difference between the two is the same as between Open and OpenBinary - the former opens a source image from disk, the latter from memory.
The OpenInfo and OpenInfoBinary methods both return an instance of the Info object which is a collection of InfoItem objects, each representing a separate EXIF or IPTC field. The InfoItem object supports the following properties: Name (String), Value (Variant, default property), Tag (Integer), and Description (String).
The following code sample extracts and displays all metadata fields from a JPEG image:
VB Script:

<%
' Directory with images
Path = Server.MapPath("../images/photo.jpg")
Set Jpeg = Server.CreateObject("Persits.Jpeg")
' Open image for metadata exraction
Set Info = Jpeg.OpenInfo( Path )
' Display collection of items
For Each Item in Info
  Response.Write "<TR><TD>"
  Response.Write Item.Name & "</TD><TD>"
  Response.Write Item.Description & "</TD><TD>"
  Response.Write Item.Value & "</TD>"
  Response.Write "</TD></TR>"
Next
%>

C#:

<script runat="server" LANGUAGE="C#">
void Page_Load(Object Source, EventArgs E)
{
IASPJpeg objJpeg;
objJpeg = new ASPJpeg();
// Compute path to source image
String strPath = Server.MapPath("../images/photo.jpg");
// Open source image for metadata exraction
IInfo objInfo = objJpeg.OpenInfo( strPath );
foreach( IInfoItem objItem in objInfo )
{
  TableRow objRow = new TableRow();
  TableCell objCell1 = new TableCell();
  objCell1.Controls.Add( new LiteralControl( objItem.Name ) );
  TableCell objCell2 = new TableCell();
  objCell2.Controls.Add(new LiteralControl(objItem.Description));
  TableCell objCell3 = new TableCell();
  objCell3.Controls.Add(
    new LiteralControl( objItem.Value.ToString() ) );
  objRow.Cells.Add( objCell1 );
  objRow.Cells.Add( objCell2 );
  objRow.Cells.Add( objCell3 );
  objTable.Rows.Add( objRow );
}
}
</script>

Click the links below to run this code sample:
http://localhost/aspjpeg/manual_07/07_metadata.asp
http://localhost/aspjpeg/manual_07/07_metadata.aspx
To obtain a value for a specific metadata field, such as a camera make, you should use Info's default Item property which accepts this field's name as an index. Since Item is the default property, the word "Item" can be omitted. In C#, square brackets must be used. The following code snippet obtains camera make information and assigns it to a string variable:
VB Script:

Dim s
s = Info("Make")

C#:

String s = Info["Make"].Value;

All valid EXIF and IPTC field names are listed below.

7.2 EXIF Item List

The following table lists all valid EXIF field names and descriptions. Most real-world images only contain a small subset of these items, if any. Note that in addition to general image and camera setting information, EXIF also provides for GPS-related data.

Name Description

NewSubfileType Subfile type

ImageWidth Image width

ImageLength Image height

BitsPerSample Number of bits per component

Compression Compression scheme

PhotometricInterpretation Pixel composition

ImageDescription Image title

Make Manufacturer of image input equipment

Model Model of image input equipment

StripOffsets Image data location

Orientation Orientation of image

SamplesPerPixel Number of components

RowsPerStrip Number of rows per strip

StripByteCounts Bytes per compressed strip

XResolution Image resolution in width direction

YResolution Image resolution in height direction

PlanarConfiguration Image data arrangement

ResolutionUnit Unit of X and Y resolution

TransferFunction Transfer function

Software Software used

DateTime File change date and time

Artist Person who created the image

HostComputer The computer and/or operating system in use

WhitePoint White point chromaticity

PrimaryChromaticities Chromaticities of primaries

JPEGInterchangeFormat Offset to JPEG SOI

JPEGInterchangeFormatLength Bytes of JPEG data

YCbCrCoefficients Color space transformation matrix coefficients

YCbCrSubSampling Subsampling ratio of Y to C

YCbCrPositioning Y and C positioning

ReferenceBlackWhite Pair of black and white reference values

Copyright Copyright holder

ExifTag Exif IFD Pointer

GPSTag GPSInfo IFD Pointer

ExposureTime Exposure time

FNumber F number

ExposureProgram Exposure program

SpectralSensitivity Spectral sensitivity

ISOSpeedRatings ISO speed ratings

OECF Optoelectric coefficient

ExifVersion Exif Version

DateTimeOriginal Date and time original image was generated

DateTimeDigitized Date and time image was made digital data

ComponentsConfiguration Meaning of each component

CompressedBitsPerPixel Image compression mode

ShutterSpeedValue Shutter speed

ApertureValue Aperture

BrightnessValue Brightness

ExposureBiasValue Exposure bias

MaxApertureValue Maximum lens aperture

SubjectDistance Subject distance

MeteringMode Metering mode

LightSource Light source

Flash Flash

FocalLength Lens focal length

SubjectArea Subject area

MakerNote Manufacturer notes

UserComment User comments

SubSecTime DateTime subseconds

SubSecTimeOriginal DateTimeOriginal subseconds

SubSecTimeDigitized DateTimeDigitized subseconds

FlashpixVersion Supported Flashpix version

ColorSpace Color space information

PixelXDimension Valid image width

PixelYDimension Valid image height

RelatedSoundFile Related audio file

InteroperabilityTag Interoperability IFD Pointer

FlashEnergy Flash energy

SpatialFrequencyResponse Spatial frequency response

FocalPlaneXResolution Focal plane X resolution

FocalPlaneYResolution Focal plane Y resolution

FocalPlaneResolutionUnit Focal plane resolution unit

SubjectLocation Subject location

ExposureIndex Exposure index

SensingMethod Sensing method

FileSource File source

SceneType Scene type

CFAPattern CFA pattern

CustomRendered Custom image processing

ExposureMode Exposure mode

WhiteBalance White balance

DigitalZoomRatio Digital zoom ratio

FocalLengthIn35mmFilm Focal length in 35 mm film

SceneCaptureType Scene capture type

GainControl Gain control

Contrast Contrast

Saturation Saturation

Sharpness Sharpness

DeviceSettingDescription Device settings description

SubjectDistanceRange Subject distance range

ImageUniqueID Unique image ID

RelatedImageFileFormat File format of image file

RelatedImageWidth Image width

RelatedImageLength Image height

GPSVersionID GPS tag version

GPSLatitudeRef North or South Latitude

GPSLatitude Latitude

GPSLongitudeRef East or West Longitude

GPSLongitude Longitude

GPSAltitudeRef Altitude reference

GPSAltitude Altitude

GPSTimeStamp GPS time (atomic clock)

GPSSatellites GPS satellites used for measurement

GPSStatus GPS receiver status

GPSMeasureMode GPS measurement mode

GPSDOP Measurement precision

GPSSpeedRef Speed unit

GPSSpeed Speed of GPS receiver

GPSTrackRef Reference for direction of movement

GPSTrack Direction of movement

GPSImgDirectionRef Reference for direction of image

GPSImgDirection Direction of image

GPSMapDatum Geodetic survey data used

GPSDestLatitudeRef Reference for latitude of destination

GPSDestLatitude Latitude of destination

GPSDestLongitudeRef Reference for longitude of destination

GPSDestLongitude Longitude of destination

GPSDestBearingRef Reference for bearing of destination

GPSDestBearing Bearing of destination

GPSDestDistanceRef Reference for distance to destination

GPSDestDistance Distance to destination

GPSProcessingMethod Name of GPS processing method

GPSAreaInformation Name of GPS area

GPSDateStamp GPS date

GPSDifferential GPS differential correction

WinTitle Windows Explorer Title

WinAuthor Windows Explorer Author

WinSubject Windows Explorer Subject

WinComments Windows Explorer Comments

WinKeywords Windows Explorer Keywords

In addition to the pre-defined fields, some images may contain custom fields not described in the EXIF specifications. Custom field names have the form Tag#nnn where nnn is a decimal number, e.g. Tag#37388.

7.3 IPTC Item List

The following IPTC fields are supported by AspJpeg:

IptcByline
IptcBylineTitle
IptcCredits
IptcSource
IptcObjectName
IptcDateCreated
IptcCity
IptcState
IptcCountry
IptcOriginalTransmissionReference
IptcCopyrightNotice
IptcCaption
IptcCaptionWriter
IptcHeadline
IptcSpecialInstructions
IptcCategory
IptcSupplementalCategories
IptcUrgency
IptcKeywords
IptcTimeCreated
IptcDigitalCreationDate
IptcDigitalCreationTime
IptcOriginatingProgram
IptcProgramVersion
IptcUno
IptcEditStatus
IptcFixtureIdentifier
IptcReleaseDate
IptcReleaseTime
IptcObjectCycle
IptcImageNotes
IptcTextSaved
IptcCustom1
IptcCustom2
IptcCustom3
IptcCustom4
IptcCustom5
IptcCustom6
IptcCustom7
IptcCustom8
IptcCustom9
IptcCustom10
IptcCustom11
IptcCustom12
IptcCustom13
IptcCustom14
IptcCustom15
IptcCustom16
IptcCustom17
IptcCustom18
IptcCustom19
IptcCustom20
IptcImageURL
IptcCopyrighted

Note: The IptcKeywords field may appear multiple times in the Info collection. IptcCopyrighted was introduced in version 2.0 of AspJpeg and is a True/False value.

7.4 TIFF Support

If the image being opened is a TIFF, the Info collection will contain a special field by the name of "TiffPages" which contains the number of pages (images) in that TIFF file. This value can be used to iterate through all images in a multi-page TIFF. To open an image with an index other than 1, the property TiffIndex should be used, as follows:

<%
path = "c:\path\multipage.tif"
Set Info = jpeg.OpenInfo(path)
nPages = Info("TiffPages")
For i = 1 To nPages
  jpeg.TiffIndex = i
  jpeg.Open path
  jpeg.Save "c:\path\" & i & ".jpg"
Next
%>

7.5 Metadata Preservation

Starting with Version 1.9, AspJpeg is capable of preserving the Exif and IPTC metadata of the original image when a thumbnail is created. To enable this functionality, you need to set the property PreserveMetadata to True before opening the image, as follows:

<%
Set Jpeg = Server.CreateObject("Persits.Jpeg")
Jpeg.PreserveMetadata = True
Jpeg.Open Path
...
%>

This property must be set before calling Open or OpenBinary, an exception will be thrown otherwise.
Note that preserving metadata in a thumbnail can considerably increase its file size.

7.6 Metadata Editing

As of Version 2.0, AspJpeg can be used to add or replace IPTC values in an image via the method AddMetadataItem. This method accepts two arguments: an IPTC tag from the table shown in Section 7.3 above, and a string value to be inserted under that tag. This method can be called multiple times, if necessary. To use AddMetadataItem, the property PreserveMetadata described in the previous section must be set to True before the image is opened:

<%
Set Jpeg = Server.CreateObject("Persits.Jpeg")
Jpeg.PreserveMetadata = True
Jpeg.Open Path
Jpeg.AddMetadataItem "IptcCaption", "New York City Skyline"
Jpeg.AddMetadataItem "IptcCaptionWriter", "John Smith"
...
Jpeg.Save ...
%>

All IPTC fields are strings with one exception: the IptcCopyrighted tag introduced in version 2.0 is an on/off flag. Use the string "True" to set this flag or "False" to clear it.
As of Version 2.0.0.1, you can add multiple IptcKeywords entries to an image by using the special tag "IptcKeywordsAdd". Unlike "IptcKeywords", it adds a new keyword entry instead of replacing an existing one. For example, the code snippet
Jpeg.AddMetadataItem "IptcKeywordsAdd", "Art"
Jpeg.AddMetadataItem "IptcKeywordsAdd", "Portrait"

adds two keyword entries to an image, "Art" and "Portrait", whereas the snippet
Jpeg.AddMetadataItem "IptcKeywords", "Art"
Jpeg.AddMetadataItem "IptcKeywords", "Portrait"

only adds a single entry, "Portrait" ("Art" is overwritten.)
As of Version 2.1, there is another special tag, "IptcKeywordsRemove", which removes all keyword entries. In previous versions, it was not possible to completely remove all existing keywords from an image, only overwrite (or clear) the last keyword entry while leaving all others intact. The IptcKeywordsRemove tag is to be used as follows:
Jpeg.AddMetadataItem "IptcKeywordsRemove", ""

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