The image_signature of an <Image> instance specifies the structure of an individual texel, including which of the bits_of_xxx, min_value_of_xxx, and max_value_of_xxx fields are applicable. Essentially, the image_signature indicates how many components are in an individual texel, what they mean, and which bits_of_xxx / min_value_of_xxx / max_value_of_xxx fields are therefore applicable; the bits_of_xxx and min / max fields themselves specify the size and ranges of the individual components of a texel. The individual image signatures are discussed in detail in the rest of this section.

If image_signature is set to SE_IMG_SIG_LUMINANCE, then each texel in the <Image> consists of a luminance value, representing the luminance (also called intensity) of the texel. A luminance of 0 indicates that the texel is black, while a luminance of 1 indicates that the texel is white; all intermediate values are shades of grey. An <Image> with this signature may be called a luminance <Image>, intensity <Image>, or a greyscale <Image>. The effect is that of a "black and white" television.

A LUMINANCE <Image> can be composited with a compatible ALPHA image to produce a LUMINANCE_AND_ALPHA <Image>. An example of compositing LUMINANCE <Images> is the case of large areas of terrain <Polygons>, where the same textures for dirt are repeated over and over. To avoid creating a "quilt" effect on the terrain, a luminance <Image> can be composited with the terrain (using different offsets at different locations to make the effect appear random) to "dirty" the textures and give the terrain a more realistic appearance.

FURTHER CONSTRAINTS:

1. bits_of_luminance must equal the size per texel (in bits); all other bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx fields) must be zero.

Used to indicate that each texel in the <Image> consists of 1 colour component, namely the first for its colour model (G for RGB, M for CMY, or S for HSV). The first colour component of the colour model must be the only value in the texel.

Note that a COLOUR_COORDINATE_1 <Image> can be composited with compatible COLOUR_COORDINATE_2 and COLOUR_COORDINATE_3 <Images> of the same dimensions to produce a 123_COLOUR <Image>. (These can also be composited with a compatible alpha map to produce a 123COLOUR_ALPHA map.)

FURTHER CONSTRAINTS:

1. bits_of_colour_coordinate_1 must equal the size per texel (in bits); all other bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx) must be zero.
2. When an object has K <Image Mapping Functions> providing texture mapping information, at most one of the K <Image Mapping Functions> can be associated with a COLOUR_COORDINATE_1 <Image>. (Multiple COLOUR_COORDINATE_1 <Images> produce an undefined result.) EXAMPLES:
   1. An RGB texel must have red as the texel value.
   2. An CMY texel must have cyan as the texel value.
   3. An HSV texel must have hue as the texel value.

See SE_IMG_SIG_COLOUR_COORDINATE_2's documentation.

See SE_IMG_SIG_COLOUR_COORDINATE_3's documentation.

If image_signature is set to SE_IMG_SIG_ALPHA, then each texel in the <Image> consists of an alpha value, representing the coverage of the texel. An alpha of 0 indicates that the texel is transparent, while an alpha of 1 indicates that the texel constitutes part of the important content of the <Image>. An <Image> with this signature may be called an alpha channel or an alpha map. For further details, see [FOLEY_VAN_DAM] Section 17.6, "Image Compositing".

Note that an alpha map can be composited with:

• a compatible 123_COLOUR <Image> to produce a 123COLOUR_ALPHA map
• compatible COLOUR_COORDINATE_1, COLOUR_COORDINATE_2, and COLOUR_COORDINATE_3 <Images> to produce a 123COLOUR_ALPHA map.
• a compatible LUMINANCE <Image> to produce a LUMINANCE_AND_ALPHA map.

1. bits_of_alpha must equal the size per texel (in bits); all other bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx) fields must be zero.
2. When an object has K <Image Mapping Functions> providing its texture mapping information, at most one of the K <Image Mapping Functions> can legally provide an alpha map. (Multiple alpha maps produce an undefined result.)

Used to indicate that the <Image> represents a 2-D height bump map (or just height map), specifying information used to modify the surface normals of a smooth surface. When used with a ray-tracing technique, bump mapping introduces variations in intensity across the surface, so that it simulates a rough, wrinkled, or dimpled surface (e.g., the surface of the ocean).

A UV type of bump map can be derived from a height bump map by computing the texel gradients in both the X and Y directions.

Rather than manipulating the colour of a flat surface, bump mapping modifies the physical texture of the surface. For a description of bump mapping, see [WATT].

FURTHER CONSTRAINTS:

1. bits_of_bump_map_height must equal the size per texel (in bits); all other bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx) fields must be zero.
2. When an object has K <Image Mapping Functions> providing its texture mapping information, at most one of the K <Image Mapping Functions> can legally provide a bump map. (Multiple bump maps produce an undefined result.)

Used to indicate that each texel in the <Image> consists of 1 value, an EDCS Classification Code (ECC).

CONSTRAINTS:

1. The size per texel must be the size of EDCS_Classification_Code.
2. Since no bits_of, min_value, or max_value fields are needed for this signature, all bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx fields) must be zero.

Used to indicate that the <Image> is (functionally) a composite of a luminance <Image> and an alpha <Image> (see SE_IMG_SIG_ALPHA, SE_IMG_SIG_LUMINANCE). Each texel consists of an intensity value followed by an alpha value. No other ordering is possible with this signature.

A LUMINANCE_AND_ALPHA <Image> can be down-sampled to produce an ALPHA <Image> and a LUMINANCE <Image>.

FURTHER CONSTRAINTS:

1. bits_of_luminance + bits_of_alpha must equal the size per texel (in bits); all other bits_of_xxx fields must be zero.
2. When an object has K <Image Mapping Functions> providing its texture mapping information, at most one of of the K <Image Mapping Functions> can legally provide a luminance & alpha map. (Multiple alpha maps produce an undefined result.)

Used to indicate that each texel in the <Image> consists of 3 colour components (RGB, CMY, or HSV). The first colour component of the colour model must be the first value in the texel, the second colour component of the colour model must be the second value in the texel, and the third colour component of the colour model must be the third value in the texel. No other ordering is possible with this signature. Note that a 123COLOUR <Image> can be down-sampled to produce a COLOUR_COORDINATE_1, COLOUR_COORDINATE_2, or COLOUR_COORDINATE_3 <Image>.

FURTHER CONSTRAINTS:

1. bits_of_colour_coordinate_1 + bits_of_colour_coordinate_2 + bits_of_colour_coordinate_3 must equal the size per texel (in bits); all other bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx) must be zero.
2. When an object has K <Image Mapping Functions> providing texture mapping information, at most one of the K <Image Mapping Functions> can be associated with a 123 colour map <Image>. (Multiple 123 colour maps produce an undefined result.)

1. An RGB texel must have red as the first value in the texel, green next and finally blue.
2. A CMY texel must have cyan as the first value in the texel, magenta next, and finally yellow.
3. An HSV texel must have hue as the first value in the texel, saturation next and finally brightness value.

Used to indicate that each texel in the <Image> consists of 3 colour components (RGB, CMY, or HSV) and an alpha value. The first colour component of the colour model must be the first value in the texel, the second colour component of the colour model must be the second value in the texel, the third colour component of the colour model must be the third value in the texel, and the alpha value must be the last value in the texel. No other ordering is possible with this signature.

Note that a 123COLOUR_ALPHA <Image> can be down-sampled to produce a COLOUR_COORDINATE_1, COLOUR_COORDINATE_2, COLOUR_COORDINATE_3, and / or ALPHA <Image>.

FURTHER CONSTRAINTS:

1. bits_of_first_colour+bits_of_second_colour+bits_of_third_colour+ bits_of_alpha must equal the size per texel (in bits); all other bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx) must be zero.
2. When an object has K <Image Mapping Functions> providing texture mapping information, at most one of the K <Image Mapping Functions> can be associated with a 123 colour & alpha map <Image> (Multiple 123 colour and alpha maps produce an undefined result.)

EXAMPLES:

1. An RGBA texel must have red as the first value in the texel, green next, then blue, and finally alpha.
2. A CMYA texel must have cyan as the first value in the texel, magenta next, then yellow, and finally alpha.
3. An HSVA texel must have hue as the first value in the texel, saturation next, then brightness value, and finally alpha.

Used to indicate that each texel in the <Image> consists of 1 value, an index into the <Property Tables> referenced by this <Image>. These <Property Tables> describe the material. Normally, a <Property Table Reference> is used to find the corresponding <Property Table>, find which <Axis> is referred to, and which hash value measurement along that <Axis> is being referenced. The bits_of_material1 field is used in place of the <Property Table Reference>' index_on_axis fields.

See the examples for the Image class.

CONSTRAINTS:

1. The <Image> must have at least one <Property Table Reference>. (The data producer may have as many <Property Table References> as desired, as long as there is at least one, and bits_of_material# will handle them.)
2. All the <Property Tables> being referred to by the <Property Table Reference> must be of the same size, since the material reference corresponds to all of them.
3. bits_of_material1 must equal the size per texel (in bits); all other bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx) must be zero.

Used to indicate that each texel in the <Image> represents a linear combination of 2 materials in the <Property Tables> referenced by this <Image>. That is, each texel consists of 3 values: 2 indexes into the <Property Tables> referenced by this <Image>, and the percentage (an integer, 0 - 100%) of material 2. These <Property Tables> describe the materials. Normally, a <Property Table Reference> is used to find the corresponding <Property Table>, find which <Axis> is referred to, and which hash value measurement along that <Axis> is being referenced. The bits_of_material1 and bits_of_material2 fields are used in place of the <Property Table References>' index_on_axis fields.

See the examples for the <Image> class.

CONSTRAINTS:

1. The <Image> must have at least one <Property Table Reference>. (The data producer may have as many <Property Table References> as desired, as long as there is at least one, and bits_of_material# will handle them.)
2. All the <Property Tables> being referred to by the <Property Table References> must be of the same size, since the material reference corresponds to all of them.
3. bits_of_material1+bits_of_material2+bits_of_material2_percentage must equal the size per texel (in bits); all other bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx) must be zero.

Used to indicate that each texel in the <Image> represents a linear combination of 3 materials in the <Property Tables> referenced by this <Image>. That is, each texel consists of 4 values: 3 indexes into the <Property Tables> referenced by this <Image>, and the percentages (integers, 0 - 100%) of materials 2 and 3 These <Property Tables> describe the materials. Normally, a <Property Table Reference> is used to find the corresponding <Property Table>, find which <Axis> is referred to, and which hash value measurement along that <Axis> is being referenced. The bits_of_material1, bits_of_material2, and bits_of_material3 fields are used in place of the <Property Table References>' index_on_axis fields.

See the examples for the <Image> class.

CONSTRAINTS:

1. The <Image> must have at least one <Property Table Reference>. (The data producer may have as many <Property Table References> as desired, as long as there is at least one, and bits_of_material# will handle them.)
2. All the <Property Tables> being referred to by the <Property Table References> must be of the same size, since the material reference corresponds to all of them.
3. bits_of_material1+bits_of_material2+bits_of_material2_percentage +bits_of_material3+bits_of_material3_percentage must equal the size per texel (in bits); all other bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx) must be zero.

Used to indicate that the <Image> consists of references to other <Images> (i.e., each texel within the <Image> is the index of another <Image>). This mechanism allows an <Image> to define high-resolution insets.

Each texel is to be replaced by the entire <Image> identified by the <Image> whose index is specified by that texel. This allows data providers to create a gigantic <Image> formed by many smaller <Image>. The index is an index into the <Image Library> containing this <Image>.

FURTHER CONSTRAINTS:

1. bits_of_image_index must equal the size per texel (a positive value), while all other bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx fields) must be zero.
2. Each texel within the <Image> must resolve to a valid index within the <Image Library> containing this image, but neither directly nor via other SE_IMG_SIG_IMAGE_INDEX <Images> to the index of this <Image>. (This would cause infinite recursion when trying to resolve the image to its component parts).
3. All referenced <Images> must have the same values as the main <Image> for colour_model, data_is_little_endian, data_is_3D, component_data_type, scan_direction, and scan_direction_z.
4. All referenced <Images> must have either the same image signature X, or SE_IMG_SIG_IMAGE_INDEX which resolves to referenced images with signature X, so that the main <Image> can be resolved to a single image signature.

Used to indicate that the <Image>represents a bump map in UV space, specifying information used to modify the surface normals of a smooth surface. See SE_IMG_SIG_BUMP_MAP_HEIGHT for further information on bump maps.

FURTHER CONSTRAINTS:

1. (bits_of_bump_map_u + bits_of_bump_map_v) must equal the size per texel (in bits); all other bits_of_xxx fields (and their corresponding min_value_of_xxx, max_value_of_xxx) fields must be zero.
2. When an object has K <Image Mapping Functions> providing its texture mapping information, at most one of the K <Image Mapping Functions> can legally provide a bump map. (Multiple bump maps produce an undefined result.)