diff --git a/src/tag.rs b/src/tag.rs
index 3acdbf5..c2ea1b7 100644
--- a/src/tag.rs
+++ b/src/tag.rs
@@ -26,6 +26,7 @@
 
 use std::fmt;
 
+use value;
 use value::Value;
 
 /// A tag of a TIFF field.
@@ -109,7 +110,7 @@ macro_rules! generate_well_known_tag_constants {
         $( |$ctx:path| $(
             // Copy the doc attribute to the actual definition.
             $( #[$attr:meta] )*
-            ($name:ident, $num:expr, $defval:expr, $desc:expr)
+            ($name:ident, $num:expr, $defval:expr, $dispval:ident, $desc:expr)
         ),+, )+
     ) => (
         /// A module that contains Exif tag constants.
@@ -138,18 +139,25 @@ macro_rules! generate_well_known_tag_constants {
         // Use a separate module to avoid name conflicts between
         // const Tag and static TagInfo.
         mod tag_info {
+            use std::fmt;
+            use value::Value;
             use value::DefaultValue;
 
             pub struct TagInfo {
                 pub name: &'static str,
                 pub desc: &'static str,
                 pub default: DefaultValue,
+                pub dispval: fn(&mut fmt::Write, &Value) -> fmt::Result,
             }
 
             $($(
                 #[allow(non_upper_case_globals)]
                 pub static $name: TagInfo = TagInfo {
-                    name: stringify!($name), desc: $desc, default: $defval };
+                    name: stringify!($name),
+                    desc: $desc,
+                    default: $defval,
+                    dispval: super::$dispval,
+                };
             )+)+
         }
 
@@ -172,323 +180,400 @@ generate_well_known_tag_constants!(
 
     /// A pointer to the Exif IFD.  This is used for the internal structure
     /// of Exif data and will not be returned to the user.
-    (ExifIFDPointer, 0x8769, DefaultValue::None,
+    (ExifIFDPointer, 0x8769, DefaultValue::None, d_tbd,
      "Exif IFD pointer"),
     /// A pointer to the GPS IFD.  This is used for the internal structure
     /// of Exif data and will not be returned to the user.
-    (GPSInfoIFDPointer, 0x8825, DefaultValue::None,
+    (GPSInfoIFDPointer, 0x8825, DefaultValue::None, d_tbd,
      "GPS Info IFD pointer"),
 
     |Context::Exif|
 
     /// A pointer to the interoperability IFD.  This is used for the internal
     /// structure of Exif data and will not be returned to the user.
-    (InteropIFDPointer, 0xa005, DefaultValue::None,
+    (InteropIFDPointer, 0xa005, DefaultValue::None, d_tbd,
      "Interoperability IFD pointer"),
 
     // TIFF primary and thumbnail attributes [EXIF23 4.6.4 Table 4,
     // 4.6.8 Table 17, and 4.6.8 Table 21].
     |Context::Tiff|
 
-    (ImageWidth, 0x100, DefaultValue::None,
+    (ImageWidth, 0x100, DefaultValue::None, d_tbd,
      "Image width"),
-    (ImageLength, 0x101, DefaultValue::None,
+    (ImageLength, 0x101, DefaultValue::None, d_tbd,
      "Image height"),
-    (BitsPerSample, 0x102, DefaultValue::Short(&[8, 8, 8]),
+    (BitsPerSample, 0x102, DefaultValue::Short(&[8, 8, 8]), d_tbd,
      "Number of bits per component"),
-    (Compression, 0x103, DefaultValue::None,
+    (Compression, 0x103, DefaultValue::None, d_tbd,
      "Compression scheme"),
-    (PhotometricInterpretation, 0x106, DefaultValue::None,
+    (PhotometricInterpretation, 0x106, DefaultValue::None, d_tbd,
      "Pixel composition"),
-    (ImageDescription, 0x10e, DefaultValue::None,
+    (ImageDescription, 0x10e, DefaultValue::None, d_tbd,
      "Image title"),
-    (Make, 0x10f, DefaultValue::None,
+    (Make, 0x10f, DefaultValue::None, d_tbd,
      "Manufacturer of image input equipment"),
-    (Model, 0x110, DefaultValue::None,
+    (Model, 0x110, DefaultValue::None, d_tbd,
      "Model of image input equipment"),
-    (StripOffsets, 0x111, DefaultValue::None,
+    (StripOffsets, 0x111, DefaultValue::None, d_tbd,
      "Image data location"),
-    (Orientation, 0x112, DefaultValue::Short(&[1]),
+    (Orientation, 0x112, DefaultValue::Short(&[1]), d_tbd,
      "Orientation of image"),
-    (SamplesPerPixel, 0x115, DefaultValue::Short(&[3]),
+    (SamplesPerPixel, 0x115, DefaultValue::Short(&[3]), d_tbd,
      "Number of components"),
-    (RowsPerStrip, 0x116, DefaultValue::None,
+    (RowsPerStrip, 0x116, DefaultValue::None, d_tbd,
      "Number of rows per strip"),
-    (StripByteCounts, 0x117, DefaultValue::None,
+    (StripByteCounts, 0x117, DefaultValue::None, d_tbd,
      "Bytes per compressed strip"),
-    (XResolution, 0x11a, DefaultValue::Rational(&[(72, 1)]),
+    (XResolution, 0x11a, DefaultValue::Rational(&[(72, 1)]), d_tbd,
      "Image resolution in width direction"),
-    (YResolution, 0x11b, DefaultValue::Rational(&[(72, 1)]),
+    (YResolution, 0x11b, DefaultValue::Rational(&[(72, 1)]), d_tbd,
      "Image resolution in height direction"),
-    (PlanarConfiguration, 0x11c, DefaultValue::Short(&[1]),
+    (PlanarConfiguration, 0x11c, DefaultValue::Short(&[1]), d_tbd,
      "Image data arrangement"),
-    (ResolutionUnit, 0x128, DefaultValue::Short(&[2]),
+    (ResolutionUnit, 0x128, DefaultValue::Short(&[2]), d_tbd,
      "Unit of X and Y resolution"),
-    (TransferFunction, 0x12d, DefaultValue::None,
+    (TransferFunction, 0x12d, DefaultValue::None, d_tbd,
      "Transfer function"),
-    (Software, 0x131, DefaultValue::None,
+    (Software, 0x131, DefaultValue::None, d_tbd,
      "Software used"),
-    (DateTime, 0x132, DefaultValue::None,
+    (DateTime, 0x132, DefaultValue::None, d_tbd,
      "File change date and time"),
-    (Artist, 0x13b, DefaultValue::None,
+    (Artist, 0x13b, DefaultValue::None, d_tbd,
      "Person who created the image"),
-    (WhitePoint, 0x13e, DefaultValue::None,
+    (WhitePoint, 0x13e, DefaultValue::None, d_tbd,
      "White point chromaticity"),
-    (PrimaryChromaticities, 0x13f, DefaultValue::None,
+    (PrimaryChromaticities, 0x13f, DefaultValue::None, d_tbd,
      "Chromaticities of primaries"),
-    (JPEGInterchangeFormat, 0x201, DefaultValue::None,
+    (JPEGInterchangeFormat, 0x201, DefaultValue::None, d_tbd,
      "Offset to JPEG SOI"),
-    (JPEGInterchangeFormatLength, 0x202, DefaultValue::None,
+    (JPEGInterchangeFormatLength, 0x202, DefaultValue::None, d_tbd,
      "Bytes of JPEG data"),
-    (YCbCrCoefficients, 0x211, DefaultValue::Unspecified,
+    (YCbCrCoefficients, 0x211, DefaultValue::Unspecified, d_tbd,
      "Color space transformation matrix coefficients"),
-    (YCbCrSubSampling, 0x212, DefaultValue::None,
+    (YCbCrSubSampling, 0x212, DefaultValue::None, d_tbd,
      "Subsampling ratio of Y to C"),
-    (YCbCrPositioning, 0x213, DefaultValue::Short(&[1]),
+    (YCbCrPositioning, 0x213, DefaultValue::Short(&[1]), d_tbd,
      "Y and C positioning"),
-    (ReferenceBlackWhite, 0x214, DefaultValue::ContextDependent,
+    (ReferenceBlackWhite, 0x214, DefaultValue::ContextDependent, d_tbd,
      "Pair of black and white reference values"),
-    (Copyright, 0x8298, DefaultValue::None,
+    (Copyright, 0x8298, DefaultValue::None, d_tbd,
      "Copyright holder"),
 
     // Exif IFD attributes [EXIF23 4.6.5 Table 7 and 4.6.8 Table 18].
     |Context::Exif|
 
-    (ExposureTime, 0x829a, DefaultValue::None,
+    (ExposureTime, 0x829a, DefaultValue::None, d_tbd,
      "Exposure time"),
-    (FNumber, 0x829d, DefaultValue::None,
+    (FNumber, 0x829d, DefaultValue::None, d_tbd,
      "F number"),
-    (ExposureProgram, 0x8822, DefaultValue::None,
+    (ExposureProgram, 0x8822, DefaultValue::None, d_tbd,
      "Exposure program"),
-    (SpectralSensitivity, 0x8824, DefaultValue::None,
+    (SpectralSensitivity, 0x8824, DefaultValue::None, d_tbd,
      "Spectral sensitivity"),
-    (PhotographicSensitivity, 0x8827, DefaultValue::None,
+    (PhotographicSensitivity, 0x8827, DefaultValue::None, d_tbd,
      "Photographic sensitivity"),
-    (OECF, 0x8828, DefaultValue::None,
+    (OECF, 0x8828, DefaultValue::None, d_tbd,
      "Optoelectric conversion factor"),
-    (SensitivityType, 0x8830, DefaultValue::None,
+    (SensitivityType, 0x8830, DefaultValue::None, d_tbd,
      "Sensitivity type"),
-    (StandardOutputSensitivity, 0x8831, DefaultValue::None,
+    (StandardOutputSensitivity, 0x8831, DefaultValue::None, d_tbd,
      "Standard output sensitivity"),
-    (RecommendedExposureIndex, 0x8832, DefaultValue::None,
+    (RecommendedExposureIndex, 0x8832, DefaultValue::None, d_tbd,
      "Recommended exposure index"),
-    (ISOSpeed, 0x8833, DefaultValue::None,
+    (ISOSpeed, 0x8833, DefaultValue::None, d_tbd,
      "ISO speed"),
-    (ISOSpeedLatitudeyyy, 0x8834, DefaultValue::None,
+    (ISOSpeedLatitudeyyy, 0x8834, DefaultValue::None, d_tbd,
      "ISO speed latitude yyy"),
-    (ISOSpeedLatitudezzz, 0x8835, DefaultValue::None,
+    (ISOSpeedLatitudezzz, 0x8835, DefaultValue::None, d_tbd,
      "ISO speed latitude zzz"),
     // The absence of this field means non-conformance to Exif, so the default
     // value specified in the standard (e.g., "0231") should not apply.
-    (ExifVersion, 0x9000, DefaultValue::None,
+    (ExifVersion, 0x9000, DefaultValue::None, d_tbd,
      "Exif version"),
-    (DateTimeOriginal, 0x9003, DefaultValue::None,
+    (DateTimeOriginal, 0x9003, DefaultValue::None, d_tbd,
      "Date and time of original data generation"),
-    (DateTimeDigitized, 0x9004, DefaultValue::None,
+    (DateTimeDigitized, 0x9004, DefaultValue::None, d_tbd,
      "Date and time of digital data generation"),
-    (OffsetTime, 0x9010, DefaultValue::None,
+    (OffsetTime, 0x9010, DefaultValue::None, d_tbd,
      "Offset data of DateTime"),
-    (OffsetTimeOriginal, 0x9011, DefaultValue::None,
+    (OffsetTimeOriginal, 0x9011, DefaultValue::None, d_tbd,
      "Offset data of DateTimeOriginal"),
-    (OffsetTimeDigitized, 0x9012, DefaultValue::None,
+    (OffsetTimeDigitized, 0x9012, DefaultValue::None, d_tbd,
      "Offset data of DateTimeDigitized"),
-    (ComponentsConfiguration, 0x9101, DefaultValue::ContextDependent,
+    (ComponentsConfiguration, 0x9101, DefaultValue::ContextDependent, d_tbd,
      "Meaning of each component"),
-    (CompressedBitsPerPixel, 0x9102, DefaultValue::None,
+    (CompressedBitsPerPixel, 0x9102, DefaultValue::None, d_tbd,
      "Image compression mode"),
-    (ShutterSpeedValue, 0x9201, DefaultValue::None,
+    (ShutterSpeedValue, 0x9201, DefaultValue::None, d_tbd,
      "Shutter speed"),
-    (ApertureValue, 0x9202, DefaultValue::None,
+    (ApertureValue, 0x9202, DefaultValue::None, d_tbd,
      "Aperture"),
-    (BrightnessValue, 0x9203, DefaultValue::None,
+    (BrightnessValue, 0x9203, DefaultValue::None, d_tbd,
      "Brightness"),
-    (ExposureBiasValue, 0x9204, DefaultValue::None,
+    (ExposureBiasValue, 0x9204, DefaultValue::None, d_tbd,
      "Exposure bias"),
-    (MaxApertureValue, 0x9205, DefaultValue::None,
+    (MaxApertureValue, 0x9205, DefaultValue::None, d_tbd,
      "Maximum lens aperture"),
-    (SubjectDistance, 0x9206, DefaultValue::None,
+    (SubjectDistance, 0x9206, DefaultValue::None, d_tbd,
      "Subject distance"),
-    (MeteringMode, 0x9207, DefaultValue::Short(&[0]),
+    (MeteringMode, 0x9207, DefaultValue::Short(&[0]), d_tbd,
      "Metering mode"),
-    (LightSource, 0x9208, DefaultValue::Short(&[0]),
+    (LightSource, 0x9208, DefaultValue::Short(&[0]), d_tbd,
      "Light source"),
-    (Flash, 0x9209, DefaultValue::Unspecified,
+    (Flash, 0x9209, DefaultValue::Unspecified, d_tbd,
      "Flash"),
-    (FocalLength, 0x920a, DefaultValue::None,
+    (FocalLength, 0x920a, DefaultValue::None, d_tbd,
      "Lens focal length"),
-    (SubjectArea, 0x9214, DefaultValue::None,
+    (SubjectArea, 0x9214, DefaultValue::None, d_tbd,
      "Subject area"),
-    (MakerNote, 0x927c, DefaultValue::None,
+    (MakerNote, 0x927c, DefaultValue::None, d_tbd,
      "Manufacturer notes"),
-    (UserComment, 0x9286, DefaultValue::None,
+    (UserComment, 0x9286, DefaultValue::None, d_tbd,
      "User comments"),
-    (SubSecTime, 0x9290, DefaultValue::None,
+    (SubSecTime, 0x9290, DefaultValue::None, d_tbd,
      "DateTime subseconds"),
-    (SubSecTimeOriginal, 0x9291, DefaultValue::None,
+    (SubSecTimeOriginal, 0x9291, DefaultValue::None, d_tbd,
      "DateTimeOriginal subseconds"),
-    (SubSecTimeDigitized, 0x9292, DefaultValue::None,
+    (SubSecTimeDigitized, 0x9292, DefaultValue::None, d_tbd,
      "DateTimeDigitized subseconds"),
-    (Temperature, 0x9400, DefaultValue::None,
+    (Temperature, 0x9400, DefaultValue::None, d_tbd,
      "Temperature"),
-    (Humidity, 0x9401, DefaultValue::None,
+    (Humidity, 0x9401, DefaultValue::None, d_tbd,
      "Humidity"),
-    (Pressure, 0x9402, DefaultValue::None,
+    (Pressure, 0x9402, DefaultValue::None, d_tbd,
      "Pressure"),
-    (WaterDepth, 0x9403, DefaultValue::None,
+    (WaterDepth, 0x9403, DefaultValue::None, d_tbd,
      "Water depth"),
-    (Acceleration, 0x9404, DefaultValue::None,
+    (Acceleration, 0x9404, DefaultValue::None, d_tbd,
      "Acceleration"),
-    (CameraElevationAngle, 0x9405, DefaultValue::None,
+    (CameraElevationAngle, 0x9405, DefaultValue::None, d_tbd,
      "Camera elevation angle"),
-    (FlashpixVersion, 0xa000, DefaultValue::Undefined(b"0100"),
+    (FlashpixVersion, 0xa000, DefaultValue::Undefined(b"0100"), d_tbd,
      "Supported Flashpix version"),
-    (ColorSpace, 0xa001, DefaultValue::Unspecified,
+    (ColorSpace, 0xa001, DefaultValue::Unspecified, d_tbd,
      "Color space information"),
-    (PixelXDimension, 0xa002, DefaultValue::None,
+    (PixelXDimension, 0xa002, DefaultValue::None, d_tbd,
      "Valid image width"),
-    (PixelYDimension, 0xa003, DefaultValue::Unspecified,
+    (PixelYDimension, 0xa003, DefaultValue::Unspecified, d_tbd,
      "Valid image height"),
-    (RelatedSoundFile, 0xa004, DefaultValue::None,
+    (RelatedSoundFile, 0xa004, DefaultValue::None, d_tbd,
      "Related audio file"),
-    (FlashEnergy, 0xa20b, DefaultValue::None,
+    (FlashEnergy, 0xa20b, DefaultValue::None, d_tbd,
      "Flash energy"),
-    (SpatialFrequencyResponse, 0xa20c, DefaultValue::None,
+    (SpatialFrequencyResponse, 0xa20c, DefaultValue::None, d_tbd,
      "Spatial frequency response"),
-    (FocalPlaneXResolution, 0xa20e, DefaultValue::None,
+    (FocalPlaneXResolution, 0xa20e, DefaultValue::None, d_tbd,
      "Focal plane X resolution"),
-    (FocalPlaneYResolution, 0xa20f, DefaultValue::None,
+    (FocalPlaneYResolution, 0xa20f, DefaultValue::None, d_tbd,
      "Focal plane Y resolution"),
-    (FocalPlaneResolutionUnit, 0xa210, DefaultValue::Short(&[2]),
+    (FocalPlaneResolutionUnit, 0xa210, DefaultValue::Short(&[2]), d_tbd,
      "Focal plane resolution unit"),
-    (SubjectLocation, 0xa214, DefaultValue::None,
+    (SubjectLocation, 0xa214, DefaultValue::None, d_tbd,
      "Subject location"),
-    (ExposureIndex, 0xa215, DefaultValue::None,
+    (ExposureIndex, 0xa215, DefaultValue::None, d_tbd,
      "Exposure index"),
-    (SensingMethod, 0xa217, DefaultValue::None,
+    (SensingMethod, 0xa217, DefaultValue::None, d_tbd,
      "Sensing method"),
-    (FileSource, 0xa300, DefaultValue::Undefined(&[3]),
+    (FileSource, 0xa300, DefaultValue::Undefined(&[3]), d_tbd,
      "File source"),
-    (SceneType, 0xa301, DefaultValue::Undefined(&[1]),
+    (SceneType, 0xa301, DefaultValue::Undefined(&[1]), d_tbd,
      "Scene type"),
-    (CFAPattern, 0xa302, DefaultValue::None,
+    (CFAPattern, 0xa302, DefaultValue::None, d_tbd,
      "CFA pattern"),
-    (CustomRendered, 0xa401, DefaultValue::Short(&[0]),
+    (CustomRendered, 0xa401, DefaultValue::Short(&[0]), d_tbd,
      "Custom image processing"),
-    (ExposureMode, 0xa402, DefaultValue::None,
+    (ExposureMode, 0xa402, DefaultValue::None, d_tbd,
      "Exposure mode"),
-    (WhiteBalance, 0xa403, DefaultValue::None,
+    (WhiteBalance, 0xa403, DefaultValue::None, d_tbd,
      "White balance"),
-    (DigitalZoomRatio, 0xa404, DefaultValue::None,
+    (DigitalZoomRatio, 0xa404, DefaultValue::None, d_tbd,
      "Digital zoom ratio"),
-    (FocalLengthIn35mmFilm, 0xa405, DefaultValue::None,
+    (FocalLengthIn35mmFilm, 0xa405, DefaultValue::None, d_tbd,
      "Focal length in 35 mm film"),
-    (SceneCaptureType, 0xa406, DefaultValue::Short(&[0]),
+    (SceneCaptureType, 0xa406, DefaultValue::Short(&[0]), d_tbd,
      "Scene capture type"),
-    (GainControl, 0xa407, DefaultValue::None,
+    (GainControl, 0xa407, DefaultValue::None, d_tbd,
      "Gain control"),
-    (Contrast, 0xa408, DefaultValue::Short(&[0]),
+    (Contrast, 0xa408, DefaultValue::Short(&[0]), d_tbd,
      "Contrast"),
-    (Saturation, 0xa409, DefaultValue::Short(&[0]),
+    (Saturation, 0xa409, DefaultValue::Short(&[0]), d_tbd,
      "Saturation"),
-    (Sharpness, 0xa40a, DefaultValue::Short(&[0]),
+    (Sharpness, 0xa40a, DefaultValue::Short(&[0]), d_tbd,
      "Sharpness"),
-    (DeviceSettingDescription, 0xa40b, DefaultValue::None,
+    (DeviceSettingDescription, 0xa40b, DefaultValue::None, d_tbd,
      "Device settings description"),
-    (SubjectDistanceRange, 0xa40c, DefaultValue::None,
+    (SubjectDistanceRange, 0xa40c, DefaultValue::None, d_tbd,
      "Subject distance range"),
-    (ImageUniqueID, 0xa420, DefaultValue::None,
+    (ImageUniqueID, 0xa420, DefaultValue::None, d_tbd,
      "Unique image ID"),
-    (CameraOwnerName, 0xa430, DefaultValue::None,
+    (CameraOwnerName, 0xa430, DefaultValue::None, d_tbd,
      "Camera owner name"),
-    (BodySerialNumber, 0xa431, DefaultValue::None,
+    (BodySerialNumber, 0xa431, DefaultValue::None, d_tbd,
      "Body serial number"),
-    (LensSpecification, 0xa432, DefaultValue::None,
+    (LensSpecification, 0xa432, DefaultValue::None, d_tbd,
      "Lens specification"),
-    (LensMake, 0xa433, DefaultValue::None,
+    (LensMake, 0xa433, DefaultValue::None, d_tbd,
      "Lens make"),
-    (LensModel, 0xa434, DefaultValue::None,
+    (LensModel, 0xa434, DefaultValue::None, d_tbd,
      "Lens model"),
-    (LensSerialNumber, 0xa435, DefaultValue::None,
+    (LensSerialNumber, 0xa435, DefaultValue::None, d_tbd,
      "Lens serial number"),
-    (Gamma, 0xa500, DefaultValue::None,
+    (Gamma, 0xa500, DefaultValue::None, d_tbd,
      "Gamma"),
 
     // GPS attributes [EXIF23 4.6.6 Table 15 and 4.6.8 Table 19].
     |Context::Gps|
 
     // Depends on the Exif version.
-    (GPSVersionID, 0x0, DefaultValue::ContextDependent,
+    (GPSVersionID, 0x0, DefaultValue::ContextDependent, d_tbd,
      "GPS tag version"),
-    (GPSLatitudeRef, 0x1, DefaultValue::None,
+    (GPSLatitudeRef, 0x1, DefaultValue::None, d_tbd,
      "North or south latitude"),
-    (GPSLatitude, 0x2, DefaultValue::None,
+    (GPSLatitude, 0x2, DefaultValue::None, d_tbd,
      "Latitude"),
-    (GPSLongitudeRef, 0x3, DefaultValue::None,
+    (GPSLongitudeRef, 0x3, DefaultValue::None, d_tbd,
      "East or West Longitude"),
-    (GPSLongitude, 0x4, DefaultValue::None,
+    (GPSLongitude, 0x4, DefaultValue::None, d_tbd,
      "Longitude"),
-    (GPSAltitudeRef, 0x5, DefaultValue::Byte(&[0]),
+    (GPSAltitudeRef, 0x5, DefaultValue::Byte(&[0]), d_tbd,
      "Altitude reference"),
-    (GPSAltitude, 0x6, DefaultValue::None,
+    (GPSAltitude, 0x6, DefaultValue::None, d_tbd,
      "Altitude"),
-    (GPSTimeStamp, 0x7, DefaultValue::None,
+    (GPSTimeStamp, 0x7, DefaultValue::None, d_tbd,
      "GPS time (atomic clock)"),
-    (GPSSatellites, 0x8, DefaultValue::None,
+    (GPSSatellites, 0x8, DefaultValue::None, d_tbd,
      "GPS satellites used for measurement"),
-    (GPSStatus, 0x9, DefaultValue::None,
+    (GPSStatus, 0x9, DefaultValue::None, d_tbd,
      "GPS receiver status"),
-    (GPSMeasureMode, 0xa, DefaultValue::None,
+    (GPSMeasureMode, 0xa, DefaultValue::None, d_tbd,
      "GPS measurement mode"),
-    (GPSDOP, 0xb, DefaultValue::None,
+    (GPSDOP, 0xb, DefaultValue::None, d_tbd,
      "Measurement precision"),
-    (GPSSpeedRef, 0xc, DefaultValue::Ascii(&[b"K"]),
+    (GPSSpeedRef, 0xc, DefaultValue::Ascii(&[b"K"]), d_tbd,
      "Speed unit"),
-    (GPSSpeed, 0xd, DefaultValue::None,
+    (GPSSpeed, 0xd, DefaultValue::None, d_tbd,
      "Speed of GPS receiver"),
-    (GPSTrackRef, 0xe, DefaultValue::Ascii(&[b"T"]),
+    (GPSTrackRef, 0xe, DefaultValue::Ascii(&[b"T"]), d_tbd,
      "Reference for direction of movement"),
-    (GPSTrack, 0xf, DefaultValue::None,
+    (GPSTrack, 0xf, DefaultValue::None, d_tbd,
      "Direction of movement"),
-    (GPSImgDirectionRef, 0x10, DefaultValue::Ascii(&[b"T"]),
+    (GPSImgDirectionRef, 0x10, DefaultValue::Ascii(&[b"T"]), d_tbd,
      "Reference for direction of image"),
-    (GPSImgDirection, 0x11, DefaultValue::None,
+    (GPSImgDirection, 0x11, DefaultValue::None, d_tbd,
      "Direction of image"),
-    (GPSMapDatum, 0x12, DefaultValue::None,
+    (GPSMapDatum, 0x12, DefaultValue::None, d_tbd,
      "Geodetic survey data used"),
-    (GPSDestLatitudeRef, 0x13, DefaultValue::None,
+    (GPSDestLatitudeRef, 0x13, DefaultValue::None, d_tbd,
      "Reference for latitude of destination"),
-    (GPSDestLatitude, 0x14, DefaultValue::None,
+    (GPSDestLatitude, 0x14, DefaultValue::None, d_tbd,
      "Latitude of destination"),
-    (GPSDestLongitudeRef, 0x15, DefaultValue::None,
+    (GPSDestLongitudeRef, 0x15, DefaultValue::None, d_tbd,
      "Reference for longitude of destination"),
-    (GPSDestLongitude, 0x16, DefaultValue::None,
+    (GPSDestLongitude, 0x16, DefaultValue::None, d_tbd,
      "Longitude of destination"),
-    (GPSDestBearingRef, 0x17, DefaultValue::Ascii(&[b"T"]),
+    (GPSDestBearingRef, 0x17, DefaultValue::Ascii(&[b"T"]), d_tbd,
      "Reference for bearing of destination"),
-    (GPSDestBearing, 0x18, DefaultValue::None,
+    (GPSDestBearing, 0x18, DefaultValue::None, d_tbd,
      "Bearing of destination"),
-    (GPSDestDistanceRef, 0x19, DefaultValue::Ascii(&[b"K"]),
+    (GPSDestDistanceRef, 0x19, DefaultValue::Ascii(&[b"K"]), d_tbd,
      "Reference for distance to destination"),
-    (GPSDestDistance, 0x1a, DefaultValue::None,
+    (GPSDestDistance, 0x1a, DefaultValue::None, d_tbd,
      "Distance to destination"),
-    (GPSProcessingMethod, 0x1b, DefaultValue::None,
+    (GPSProcessingMethod, 0x1b, DefaultValue::None, d_tbd,
      "Name of GPS processing method"),
-    (GPSAreaInformation, 0x1c, DefaultValue::None,
+    (GPSAreaInformation, 0x1c, DefaultValue::None, d_tbd,
      "Name of GPS area"),
-    (GPSDateStamp, 0x1d, DefaultValue::None,
+    (GPSDateStamp, 0x1d, DefaultValue::None, d_tbd,
      "GPS date"),
-    (GPSDifferential, 0x1e, DefaultValue::None,
+    (GPSDifferential, 0x1e, DefaultValue::None, d_tbd,
      "GPS differential correction"),
-    (GPSHPositioningError, 0x1f, DefaultValue::None,
+    (GPSHPositioningError, 0x1f, DefaultValue::None, d_tbd,
      "Horizontal positioning error"),
 
     // Interoperability attributes [EXIF23 4.6.7 Table 16 and 4.6.8 Table 20].
     |Context::Interop|
 
-    (InteroperabilityIndex, 0x1, DefaultValue::None,
+    (InteroperabilityIndex, 0x1, DefaultValue::None, d_tbd,
      "Interoperability identification"),
 );
 
+// For Value::display_as().
+pub fn display_value_as<'a>(value: &'a Value, tag: Tag) -> value::Display<'a> {
+    match get_tag_info(tag) {
+        Some(ti) => value::Display { fmt: ti.dispval, value: value },
+        None => value::Display { fmt: d_default, value: value },
+    }
+}
+
+fn d_tbd(_w: &mut fmt::Write, _value: &Value) -> fmt::Result {
+    unimplemented!();
+}
+
+fn d_default(w: &mut fmt::Write, value: &Value) -> fmt::Result {
+    match *value {
+        Value::Byte(ref v) => d_sub_comma(w, v),
+        Value::Ascii(ref v) => {
+            let mut first = true;
+            for x in v {
+                if !first {
+                    try!(w.write_char('\n'));
+                }
+                first = false;
+                try!(d_escape_ascii(w, x));
+            }
+            Ok(())
+        },
+        Value::Short(ref v) => d_sub_comma(w, v),
+        Value::Long(ref v) => d_sub_comma(w, v),
+        Value::Rational(ref v) => d_sub_comma(w, v),
+        Value::SByte(ref v) => d_sub_comma(w, v),
+        Value::Undefined(ref s) => d_sub_hex(w, s),
+        Value::SShort(ref v) => d_sub_comma(w, v),
+        Value::SLong(ref v) => d_sub_comma(w, v),
+        Value::SRational(ref v) => d_sub_comma(w, v),
+        Value::Float(ref v) => d_sub_comma(w, v),
+        Value::Double(ref v) => d_sub_comma(w, v),
+        Value::Unknown(t, c, o) =>
+            write!(w, "unknown value (type={}, count={}, offset={:#x})",
+                   t, c, o),
+    }
+}
+
+fn d_sub_comma<T>(w: &mut fmt::Write, slice: &[T])
+                  -> fmt::Result where T: fmt::Display {
+    let mut first = true;
+    for x in slice {
+        try!(match first {
+            true => write!(w, "{}", x),
+            false => write!(w, ", {}", x),
+        });
+        first = false;
+    }
+    Ok(())
+}
+
+fn d_sub_hex(w: &mut fmt::Write, bytes: &[u8]) -> fmt::Result {
+    try!(w.write_str("0x"));
+    for x in bytes {
+        try!(write!(w, "{:02x}", x));
+    }
+    Ok(())
+}
+
+fn d_escape_ascii(w: &mut fmt::Write, bytes: &[u8]) -> fmt::Result {
+    for &c in bytes {
+        match c {
+            b'\\' => {
+                try!(w.write_char('\\'));
+                try!(w.write_char(c as char));
+            },
+            0x20...0x7e => try!(w.write_char(c as char)),
+            _ => try!(write!(w, "\\x{:02x}", c)),
+        }
+    }
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use tag;
diff --git a/src/value.rs b/src/value.rs
index 7779271..44ed689 100644
--- a/src/value.rs
+++ b/src/value.rs
@@ -68,6 +68,29 @@ pub enum Value<'a> {
     Unknown(u16, u32, u32),
 }
 
+impl<'a> Value<'a> {
+    /// Returns an object that implements `std::fmt::Display` for
+    /// printing a value in a tag-specific format.
+    /// The tag of the value is specified as the argument.
+    #[inline]
+    pub fn display_as(&self, tag: ::tag_priv::Tag) -> Display {
+        ::tag_priv::display_value_as(self, tag)
+    }
+}
+
+/// Helper struct for printing a value in a tag-specific format.
+pub struct Display<'a> {
+    pub fmt: fn(&mut fmt::Write, &Value) -> fmt::Result,
+    pub value: &'a Value<'a>,
+}
+
+impl<'a> fmt::Display for Display<'a> {
+    #[inline]
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        (self.fmt)(f, self.value)
+    }
+}
+
 // Static default values.
 pub enum DefaultValue {
     None,