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<html>
<head>
<title>Image Specification</title>
</head>
<body bgcolor="#FFFFFF">
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<table border=0 width=98%>
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<hr>
<center>
<h1>HDF5 Image and Palette Specification</h1>
<h3><i>(Major revision: designated Version 1.1 of the Image and Palette Specification)</i></h3>
</center>
The HDF5 specification defines the standard objects and storage for the
standard HDF5 objects. (For information about the HDF5 library, model and
specification, see the HDF documentation.) This document is an additional
specification do define a standard profile for how to store image data
in HDF5. Image data in HDF5 is stored as HDF5 datasets with standard attributes
to define the properties of the image.
<p>This specification defines:
<ul>
<li>
Standard attributes for an Image dataset</li>
<li>
Standard storage and attributes for Palettes</li>
<li>
Standard for associating Palettes with Images.</li>
</ul>
<h2>
1. HDF5 Image Specification</h2>
<h3>
1.1 Overview</h3>
Image data is stored as an HDF5 dataset with values of HDF5 class Integer
or Float. A common example would be a two dimensional dataset, with
elements of type Integer. However, this specification does not limit
the dimensions or number type that may be used for an Image.
<p>The dataset for an image is distinguished from other datasets by giving
it an attribute "CLASS=IMAGE". In addition, the Image dataset has
an attribute "PALETTE" that points to one or more palettes.
<p>A Palette is ha HDF5 dataset. A Pallet dataset has an attribute
"an" and other attributes indicating the type and size of the palette.
A Palette is an independent object, which can be shared among several Image
datasets.
<h3>
1.2 Image Attributes</h3>
<dl>
<dt>
Attribute name="<b>CLASS</b>"</dt>
<dd>
This attribute is of type H5T_C_S1, with size 6.</dd>
<dd>
For all Images, the value of this attribute is "IMAGE".</dd>
</dl>
<dd>
This attribute identifies this data set as intended to be interpreted as
an image that conforms to the specifications on this page.</dd>
<dl>
<dt>
Attribute name="<b>IMAGE_SUBCLASS</b>"</dt>
<dd>
If present, the value of this attribute indicates the type of Palette that
should be used with the Image. This attribute is of type H5T_C_S1,
with size according to the string. The values are:</dd>
<dl>
<dt>
"IMAGE_GRAYSCALE"</dt>
<dd>
A grayscale image, values 0-255 indicating brightness.</dd>
<dt>
"IMAGE_BITMAP"</dt>
<dd>
A "bit map" image (???)</dd>
<dt>
"IMAGE_TRUECOLOR"</dt>
<dd>
A truecolor image</dd>
<dt>
"IMAGE_INDEXED"</dt>
<dd>
An indexed image</dd>
</dl>
</dl>
<dt>
Attribute name="<b>IMAGE_WHITE_IS_ZERO</b>"</dt>
<dl>
<dd>
This attribute is of type H5T_NATIVE_UCHAR. 0 = false, 1 = true .
This is used for images with IMAGE_SUBCLASS="IMAGE_GRAYSCALE" or "IMAGE_BITMAP".</dd>
</dl>
<dl>
<dt>
Attribute name="<b>IMAGE_MINMAXRANGE</b>"</dt>
<dd>
If present, this attribute is an array of two numbers, of the same HDF5
datatype as the data. The first element is the minimum value of the
data, and the second is the maximum. This is used for images with
IMAGE_SUBCLASS="IMAGE_GRAYSCALE", "IMAGE_BITMAP" or "IMAGE_INDEXED".</dd>
</dl>
<dt>
Attribute name="<b>IMAGE_BACKGROUNDINDEX</b>"</dt>
<dl>
<dd>
If set, this attribute indicates the index value that should be interpreted
as the "background color". This attribute is HDF5 type H5T_NATIVE_UINT.</dd>
</dl>
<dt>
Attribute name="<b>IMAGE_TRANSPARENCY</b>"</dt>
<dl>
<dd>
If set, this attribute indicates the index value that should be interpreted
as the "transparent color". This attribute is HDF5 type H5T_NATIVE_UINT.
This attribute may not be used for IMAGE_SUBCLASS="IMAGE_TRUE_COLOR".</dd>
</dl>
<dt>
Attribute name="<b>IMAGE_ASPECTRATIO</b>"</dt>
<dl>
<dd>
If set, this attribute indicates the aspect ratio. This attribute
is HDF5 type H5T_NATIVE_UINT.</dd>
</dl>
<dt>
Attribute name="<b>IMAGE_COLORMODEL</b>"</dt>
<dl>
<dd>
If set, this attribute indicates the color model of Palette that should
be used with the Image. This attribute is of type H5T_C_S1, with
size 4,5, or 6. The value is one of the color models described in
the Palette specification below. This attribute may be used only
for IMAGE_SUBCLASS="IMAGE_TRUECOLOR" or "IMAGE_INDEXED".</dd>
</dl>
<dt>
Attribute name="<b>IMAGE_GAMMACORRECTION</b>"</dt>
<dl>
<dd>
If set, this attribute gives the Gamma correction. The type is H5T_NATIVE_FLOAT.
This attribute may be used only for IMAGE_SUBCLASS="IMAGE_TRUECOLOR" or
"IMAGE_INDEXED".</dd>
</dl>
<dt>
Attribute name="<b>PALETTE</b>"</dt>
<dl>
<dd>
A dataset within an HDF5 file may optionally specify an array of palettes
to be viewed with. The dataset will have an attribute field called "<b>PALETTE</b>"
which contains an array of object reference pointers which refer to palettes
in the file. The first palette in this array will be the default palette
that the data may be viewed with.</dd>
</dl>
Attribute name="<b>IMAGE_VERSION</b>"
<dl>
<dd>
This attribute is of type H5T_C_S1, with size corresponding to the length
of the version string. This attribute identifies the version number
of this specification to which it conforms. The initial version number
is "1.0".</dd>
<br>
<p>
<br>
<br>
<center><table BORDER=2 BGCOLOR="#FFFFFF" >
<caption>Attributes of an Image Dataset</caption>
<tr>
<td><b>Attribute Name</b></td>
<td></td>
<td><b>Type</b></td>
<td><b>Value</b></td>
</tr>
<tr>
<td>CLASS</td>
<td>R</td>
<td>String</td>
<td>"IMAGE"</td>
</tr>
<tr>
<td>PALETTE</td>
<td>O</td>
<td>Array of Object References</td>
<td><references to Palette datasets><sup>1</sup></td>
</tr>
<tr>
<td>IMAGE_SUBCLASS</td>
<td>O<sup>2</sup></td>
<td>String</td>
<td>
<dt>
"IMAGE_GRAYSCALE",</dt>
<dt>
"IMAGE_BITMAP",</dt>
<dt>
"IMAGE_TRUECOLOR",</dt>
<dt>
"IMAGE_INDEXED"</dt>
</td>
</tr>
<tr>
<td>PALETTE</td>
<td>O</td>
<td>Array of Object Reference</td>
<td><references to Palette datasets></td>
</tr>
<tr>
<td>IMAGE_WHITE_IS_ZERO</td>
<td>O<sup>3,4</sup></td>
<td>Unsigned Integer</td>
<td>0 = false, 1 = true</td>
</tr>
<tr>
<td>IMAGE_MINMAXRANGE</td>
<td>O<sup>3,5</sup></td>
<td>Unsigned Integer[2]</td>
<td>The (<minimum>, <maximum>) value of the data.</td>
</tr>
<tr>
<td>IMAGE_BACKGROUNDINDEX</td>
<td>O<sup>3</sup></td>
<td>Unsigned Integer</td>
<td>The index of the background color.</td>
</tr>
<tr>
<td>IMAGE_TRANSPARENCY</td>
<td>O<sup>3,5</sup></td>
<td>Unsigned Integer</td>
<td>The index of the transparent color.</td>
</tr>
<tr>
<td>IMAGE_ASPECTRATIO</td>
<td>O<sup>3,4</sup></td>
<td>Unsigned Integer</td>
<td>The aspect ratio.</td>
</tr>
<tr>
<td>IMAGE_COLORMODEL</td>
<td>O<sup>3,6</sup></td>
<td>String</td>
<td>The color model, as defined below in the Palette specification for
attribute <b>PAL_COLORMODEL</b>.</td>
</tr>
<tr>
<td>IMAGE_GAMMACORRECTION</td>
<td>O<sup>3,6</sup></td>
<td>Float</td>
<td>The gamma correction.</td>
</tr>
<tr>
<td>IMAGE_VERSION</td>
<td>R</td>
<td>String</td>
<td><specification version number></td>
</tr>
</table></center>
<dl><font size=-1>1. The first element of the array is the default
Palette.</font>
<br><font size=-1>2. This attribute is <b>required</b> for images
that are converted from one of the standard types listed.</font>
<br><font size=-1>3. This attribute is <b>required</b> if set for images
converted from the applicable standard type.</font>
<br><font size=-1>4. This applies to gray scale images: IMAGE_SUBCLASS="IMAGE_GRAYSCALE"
or "IMAGE_BITMAP".</font>
<br><font size=-1>5. This applies to gray scale images: IMAGE_SUBCLASS="IMAGE_GRAYSCALE",
"IMAGE_BITMAP", or "IMAGE_INDEXED".</font>
<br><font size=-1>5. This applies to gray scale images: IMAGE_SUBCLASS="IMAGE_TRUECOLOR",
or "IMAGE_INDEXED".</font></dl>
</dl>
<h3>
1.3 Storage Properties for Images</h3>
Image datasets may be stored with any chunking or compression properties
supported by HDF5.
<h2>
2. HDF5 Palette Specification</h2>
<h3>
2.1 Overview</h3>
A palette is the means by which color is applied to an image and is also
referred to as a color lookup table. It is a table in which every row contains
the numerical representation of a particular color. In the example of an
8-bit standard RGB color model palette, this numerical representation of
a color is presented as a triplet specifying the intensity of red, green,
and blue components that make up each color.
<center>
<p><img SRC="Palettes.fm.anc.gif" ></center>
<p>In this example, the color component numeric type is an 8-bit unsigned
integer. While this is most common and recommended for general use, other
component color numeric datatypes, such as a 16-bit unsigned integer ,
may be used. This type is specified as the type attribute of the palette
dataset. (see H5Tget_type(), H5Tset_type())
<p>The minimum and maximum values of the component color numeric are specified
as attribute of the palette dataset. See below (attribute PAL_MINMAXNUMERIC).
If these attributes do not exist, it is assumed that the range of values
will fill the space of the color numeric type. i.e. with an 8-bit unsigned
integer, the valid range would be 0 to 255 for each color component.
<p>The HDF5 palette specification additionally allows for color models
beyond RGB. YUV, HSV, CMY, CMYK, YCbCr color models are supported, and
may be specified as a color model attribute of the palette dataset. <i>(see
"Palette Attributes" for details)</i>.
<p>In HDF 4 and earlier, palettes were limited to 256 colors. The HDF5
palette specification allows for palettes of varying length. The length
is specified as the number of rows of the palette dataset.
<p>In a standard palette, the color entries are indexed directly. HDF5
supports the notion of a range index table. Such a table defines an ascending
ordered of ranges that map dataset values to the palette. If a range index
table exists for the palette, the PAL_TYPE attribute will be set to "RANGEINDEX",
and the PAL_RANGEINDEX attribute will contain an object reference to a
range index table array. If not, the PAL_TYPE attribute either does not
exist, or will be set to "STANDARD".
<p>The range index table array consists of a one dimensional array with
the same length as the palette dataset - 1. Ideally, the range index would
be of the same type as the dataset it refers to, however this is not a
requirement.
<p><b>Example 2: A range index array of type floating point</b>
<center>
<p><img SRC="PaletteExample1.gif" ></center>
<p>The range index array attribute defines the "<i>to</i>" of the range.
Notice that the range index array attribute is one less entry in size than
the palette. The first entry of 0.1259, specifies that all values below
and up to 0.1259 inclusive, will map to the first palette entry. The second
entry signifies that all values greater than 0.1259 up to 0.3278 inclusive,
will map to the second palette entry, etc. All value greater than the last
range index array attribute (100000) map to the last entry in the palette.
<h3>
2.2. Palette Attributes</h3>
A palette exists in an HDF file as an independent data set with accompanying
attributes.
<p>These attributes are defined as follows:
<dl>
<dt>
Attribute name="<b>CLASS</b>"</dt>
<dd>
This attribute is of type H5T_C_S1, with size 8.</dd>
<dd>
For all palettes, the value of this attribute is "PALETTE". This attribute
identifies this palette data set as a palette that conforms to the specifications
on this page.</dd>
<dt>
Attribute name="<b>PAL_COLORMODEL</b>"</dt>
<dd>
This attribute is of type H5T_C_S1, with size 4, 5, or 6.</dd>
<dd>
Possible values for this are "RGB", "YUV", "CMY", "CMYK", "YCbCr", "HSV".</dd>
<dd>
This defines the color model that the entries in the palette data set represent.</dd>
<dl>
<dt>
"RGB"</dt>
<dd>
Each color index contains a triplet where the the first value defines the
red component, second defines the green component, and the third the blue
component.</dd>
<dt>
"CMY"</dt>
<dd>
Each color index contains a triplet where the the first value defines the
cyan component, second defines the magenta component, and the third the
yellow component.</dd>
<dt>
"CMYK"</dt>
<dd>
Each color index contains a quadruplet where the the first value defines
the cyan component, second defines the magenta component, the third the
yellow component, and the forth the black component.</dd>
<dt>
"YCbCr"</dt>
<dd>
Class Y encoding model. Each color index contains a triplet where the the
first value defines the luminance, second defines the Cb Chromonance, and
the third the Cr Chromonance.</dd>
<dt>
"YUV"</dt>
<dd>
Composite encoding color model. Each color index contains a triplet where
the the first value defines the luminance component, second defines the
chromonance component, and the third the value component.</dd>
<dt>
"HSV"</dt>
<dd>
Each color index contains a triplet where the the first value defines the
hue component, second defines the saturation component, and the third the
value component. The hue component defines the hue spectrum with a low
value representing magenta/red progressing to a high value which would
represent blue/magenta, passing through yellow, green, cyan. A low value
for the saturation component means less color saturation than a high value.
A low value for <i>value</i> will be darker than a high value.</dd>
</dl>
<dt>
Attribute name="<b>PAL_TYPE</b>"</dt>
<dd>
This attribute is of type H5T_C_S1, with size 10 or 11.</dd>
<br>The current supported values for this attribute are : "STANDARD8" or
"RANGEINDEX"
<p>A PAL_TYPE of "STANDARD8" defines a palette dataset such that the first
entry defines index 0, the second entry defines index 1, etc. up until
the length of the palette - 1. This assumes an image dataset with direct
indexes into the palette.
<p>If the PAL_TYPE is set to "RANGEINDEX", there will be an additional
attribute with a name of "<b>PAL_RANGEINDEX</b>", (See example 2
for more details)
<dt>
Attribute name="<b>PAL_RANGEINDEX</b>"</dt>
<dd>
The <b>PAL_RANGEINDEX</b> attribute contains an HDF object reference pointer
which specifies a range index array in the file to be used for color lookups
for the palette. (Only for PAL_TYPE="RANGEINDEX"</dd>
<dt>
Attribute name="<b>PAL_MINMAXNUMERIC</b>"</dt>
<dl>
<dt>
If present, this attribute is an array of two numbers, of the same HDF5
datatype as the palette elements or color numerics.</dt>
<br>They specify the minimum and maximum values of the color numeric components.
For example, if the palette was an RGB of type Float, the color numeric
range for Red, Green, and Blue could be set to be between 0.0 and 1.0.
The intensity of the color guns would then be scaled accordingly to be
between this minimum and maximum attribute.</dl>
Attribute name="<b>PAL_VERSION</b>"
<dl>This attribute is of type H5T_C_S1, with size corresponding to the
length of the version string. This attribute identifies the version
number of this specification to which it conforms. The initial version
number is "1.0".</dl>
<center><table BORDER=2 BGCOLOR="#FFFFFF" >
<caption><b>Attributes of a Palette Dataset</b></caption>
<tr>
<td><b>Attribute Name</b></td>
<td></td>
<td><b>Type</b></td>
<td><b>Value</b></td>
</tr>
<tr>
<td>CLASS</td>
<td>R</td>
<td>String</td>
<td>"PALETTE"</td>
</tr>
<tr>
<td>PAL_COLORMODEL</td>
<td>R</td>
<td>String</td>
<td>Color Model: "RGB", YUV", "CMY", "CMYK", "YCbCr", or "HSV"</td>
</tr>
<tr>
<td>PAL_TYPE</td>
<td>R</td>
<td>String</td>
<td>"STANDARD8" or "RANGEINDEX"</td>
</tr>
<tr>
<td>RANGE_INDEX</td>
<td>R<sup>1</sup></td>
<td>Object Reference </td>
<td><Object Reference to Dataset of range index values></td>
</tr>
<tr>
<td>PAL_MINMAXNUMERIC</td>
<td>O</td>
<td>Array[2] of <same datatype as palette></td>
<td>The first value is the <Minimum value for color values>, the second
value is <Maximum value for color values><sup>2</sup></td>
</tr>
<tr>
<td>PAL_VERSION</td>
<td>R</td>
<td>String</td>
<td><specification version number></td>
</tr>
</table></center>
<dl><font size=-1>1. The RANGE_INDEX attribute is required if the
PAL_TYPE is "RANGEINDEX". Otherwise, the RANGE_INDEX attribute should
be omitted.</font>
<br><font size=-1>2. The minimum and maximum are optional.
If not set, the range is assumed to the maximum range of the number type.
If one of these attributes is set, then both should be set. The value
of the minumum must be less than or equal to the value of the maximum.</font></dl>
</dl>
<h3>
2.3. Storage Layout for Palettes</h3>
The values of the Palette are stored as a dataset. The datatype can
be any HDF 5 atomic numeric type. The dataset will have dimensions
(<tt>nentries</tt> by <tt>ncomponents</tt>), where '<tt>nentries</tt>'
is the number of colors (usually 256) and '<tt>ncomponents'</tt> is the
number of values per color (3 for <b>RGB</b>, 4 for <b>CMYK</b>, etc.)
<br>
<h2>
3. Consistency and Correlation of Image and Palette Attributes</h2>
The objects in this specification are an extension to the base HDF5 specification
and library. They are accessible with the standard HDF5 library,
but the semantics of the objects are not enforced by the base library.
For example, it is perfectly possible to add an attribute called <b>IMAGE</b>
to <i>any</i> dataset, or to include an object reference to <i>any</i>
HDF5 dataset as a "palette" in a <b>PALETTE</b> attribute. this would
be a perfectly valid HDF5 file, but not conformant to this specification.
The rules defined in this specification must be implemented with appropriate
software, and applications must use conforming software to assure correctness.
<p>The Image and Palette specifications include several mutually redundant
standard attributes, such as the <b>IMAGE_COLORMODEL</b> and the <b>PAL_COLORMODEL</b>.
These attributes are informative not normative, in that it is acceptable
to attach a Palette to an Image dataset even if their attributes do not
match. Software is not required to enforce consistency, and files
may contain mismatched associations of Images and Palettes. In all
cases, it is up to applications to determine what kinds of images and color
models can be supported.
<p>For example, an Image that was created from a file with an "RGB" may
have a "YUV" Palette in its <b>PALETTE</b> attribute array. This
would be a legal HDF5 file and also a conforms to this specification, although
it may or may not be correct or useable for a given application.
<p>
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Describes HDF5 Release 1.4, February 2001
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