ryanb@microsoft.UUCP (Danger Mouse) (08/10/90)
Part 2 of 3
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iv) Color Resolution - Number of bits per primary color available
to the original image, minus 1. This value represents the size of
the entire palette from which the colors in the graphic were
selected, not the number of colors actually used in the graphic.
For example, if the value in this field is 3, then the palette of
the original image had 4 bits per primary color available to create
the image. This value should be set to indicate the richness of
the original palette, even if not every color from the whole
palette is available on the source machine.
v) Sort Flag - Indicates whether the Global Color Table is sorted.
If the flag is set, the Global Color Table is sorted, in order of
decreasing importance. Typically, the order would be decreasing
frequency, with most frequent color first. This assists a decoder,
with fewer available colors, in choosing the best subset of colors;
the decoder may use an initial segment of the table to render the
graphic.
Values : 0 - Not ordered.
1 - Ordered by decreasing importance, most
important color first.
vi) Size of Global Color Table - If the Global Color Table Flag is
set to 1, the value in this field is used to calculate the number
of bytes contained in the Global Color Table. To determine that
actual size of the color table, raise 2 to [the value of the field
+ 1]. Even if there is no Global Color Table specified, set this
field according to the above formula so that decoders can choose
the best graphics mode to display the stream in. (This field is
made up of the 3 least significant bits of the byte.)
vii) Background Color Index - Index into the Global Color Table for
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the Background Color. The Background Color is the color used for
those pixels on the screen that are not covered by an image. If the
Global Color Table Flag is set to (zero), this field should be zero
and should be ignored.
viii) Pixel Aspect Ratio - Factor used to compute an approximation
of the aspect ratio of the pixel in the original image. If the
value of the field is not 0, this approximation of the aspect ratio
is computed based on the formula:
Aspect Ratio = (Pixel Aspect Ratio + 15) / 64
The Pixel Aspect Ratio is defined to be the quotient of the pixel's
width over its height. The value range in this field allows
specification of the widest pixel of 4:1 to the tallest pixel of
1:4 in increments of 1/64th.
Values : 0 - No aspect ratio information is given.
1..255 - Value used in the computation.
d. Extensions and Scope. The scope of this block is the entire Data
Stream. This block cannot be modified by any extension.
e. Recommendations. None.
19. Global Color Table.
a. Description. This block contains a color table, which is a sequence of
bytes representing red-green-blue color triplets. The Global Color Table
is used by images without a Local Color Table and by Plain Text
Extensions. Its presence is marked by the Global Color Table Flag being
set to 1 in the Logical Screen Descriptor; if present, it immediately
follows the Logical Screen Descriptor and contains a number of bytes
equal to
3 x 2^(Size of Global Color Table+1).
This block is OPTIONAL; at most one Global Color Table may be present
per Data Stream.
b. Required Version. 87a
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c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+===============+
0 | | Red 0 Byte
+- -+
1 | | Green 0 Byte
+- -+
2 | | Blue 0 Byte
+- -+
3 | | Red 1 Byte
+- -+
| | Green 1 Byte
+- -+
up | |
+- . . . . -+ ...
to | |
+- -+
| | Green 255 Byte
+- -+
767 | | Blue 255 Byte
+===============+
d. Extensions and Scope. The scope of this block is the entire Data
Stream. This block cannot be modified by any extension.
e. Recommendation. None.
20. Image Descriptor.
a. Description. Each image in the Data Stream is composed of an Image
Descriptor, an optional Local Color Table, and the image data. Each
image must fit within the boundaries of the Logical Screen, as defined
in the Logical Screen Descriptor.
The Image Descriptor contains the parameters necessary to process a table
based image. The coordinates given in this block refer to coordinates
within the Logical Screen, and are given in pixels. This block is a
Graphic-Rendering Block, optionally preceded by one or more Control
blocks such as the Graphic Control Extension, and may be optionally
followed by a Local Color Table; the Image Descriptor is always followed
by the image data.
This block is REQUIRED for an image. Exactly one Image Descriptor must
be present per image in the Data Stream. An unlimited number of images
may be present per Data Stream.
b. Required Version. 87a.
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c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Image Separator Byte
+---------------+
1 | | Image Left Position Unsigned
+- -+
2 | |
+---------------+
3 | | Image Top Position Unsigned
+- -+
4 | |
+---------------+
5 | | Image Width Unsigned
+- -+
6 | |
+---------------+
7 | | Image Height Unsigned
+- -+
8 | |
+---------------+
9 | | | | | | <Packed Fields> See below
+---------------+
<Packed Fields> = Local Color Table Flag 1 Bit
Interlace Flag 1 Bit
Sort Flag 1 Bit
Reserved 2 Bits
Size of Local Color Table 3 Bits
i) Image Separator - Identifies the beginning of an Image
Descriptor. This field contains the fixed value 0x2C.
ii) Image Left Position - Column number, in pixels, of the left edge
of the image, with respect to the left edge of the Logical Screen.
Leftmost column of the Logical Screen is 0.
iii) Image Top Position - Row number, in pixels, of the top edge of
the image with respect to the top edge of the Logical Screen. Top
row of the Logical Screen is 0.
iv) Image Width - Width of the image in pixels.
v) Image Height - Height of the image in pixels.
vi) Local Color Table Flag - Indicates the presence of a Local Color
Table immediately following this Image Descriptor. (This field is
the most significant bit of the byte.)
Values : 0 - Local Color Table is not present. Use
Global Color Table if available.
1 - Local Color Table present, and to follow
immediately after this Image Descriptor.
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vii) Interlace Flag - Indicates if the image is interlaced. An image
is interlaced in a four-pass interlace pattern; see Appendix E for
details.
Values : 0 - Image is not interlaced.
1 - Image is interlaced.
viii) Sort Flag - Indicates whether the Local Color Table is
sorted. If the flag is set, the Local Color Table is sorted, in
order of decreasing importance. Typically, the order would be
decreasing frequency, with most frequent color first. This assists
a decoder, with fewer available colors, in choosing the best subset
of colors; the decoder may use an initial segment of the table to
render the graphic.
Values : 0 - Not ordered.
1 - Ordered by decreasing importance, most
important color first.
ix) Size of Local Color Table - If the Local Color Table Flag is
set to 1, the value in this field is used to calculate the number
of bytes contained in the Local Color Table. To determine that
actual size of the color table, raise 2 to the value of the field
+ 1. This value should be 0 if there is no Local Color Table
specified. (This field is made up of the 3 least significant bits
of the byte.)
d. Extensions and Scope. The scope of this block is the Table-based Image
Data Block that follows it. This block may be modified by the Graphic
Control Extension.
e. Recommendation. None.
21. Local Color Table.
a. Description. This block contains a color table, which is a sequence of
bytes representing red-green-blue color triplets. The Local Color Table
is used by the image that immediately follows. Its presence is marked by
the Local Color Table Flag being set to 1 in the Image Descriptor; if
present, the Local Color Table immediately follows the Image Descriptor
and contains a number of bytes equal to
3x2^(Size of Local Color Table+1).
If present, this color table temporarily becomes the active color table
and the following image should be processed using it. This block is
OPTIONAL; at most one Local Color Table may be present per Image
Descriptor and its scope is the single image associated with the Image
Descriptor that precedes it.
b. Required Version. 87a.
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c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+===============+
0 | | Red 0 Byte
+- -+
1 | | Green 0 Byte
+- -+
2 | | Blue 0 Byte
+- -+
3 | | Red 1 Byte
+- -+
| | Green 1 Byte
+- -+
up | |
+- . . . . -+ ...
to | |
+- -+
| | Green 255 Byte
+- -+
767 | | Blue 255 Byte
+===============+
d. Extensions and Scope. The scope of this block is the Table-based Image
Data Block that immediately follows it. This block cannot be modified by
any extension.
e. Recommendations. None.
22. Table Based Image Data.
a. Description. The image data for a table based image consists of a
sequence of sub-blocks, of size at most 255 bytes each, containing an
index into the active color table, for each pixel in the image. Pixel
indices are in order of left to right and from top to bottom. Each index
must be within the range of the size of the active color table, starting
at 0. The sequence of indices is encoded using the LZW Algorithm with
variable-length code, as described in Appendix F
b. Required Version. 87a.
c. Syntax. The image data format is as follows:
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
| | LZW Minimum Code Size Byte
+---------------+
+===============+
| |
/ / Image Data Data Sub-blocks
| |
+===============+
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i) LZW Minimum Code Size. This byte determines the initial number
of bits used for LZW codes in the image data, as described in
Appendix F.
d. Extensions and Scope. This block has no scope, it contains raster
data. Extensions intended to modify a Table-based image must appear
before the corresponding Image Descriptor.
e. Recommendations. None.
23. Graphic Control Extension.
a. Description. The Graphic Control Extension contains parameters used
when processing a graphic rendering block. The scope of this extension is
the first graphic rendering block to follow. The extension contains only
one data sub-block.
This block is OPTIONAL; at most one Graphic Control Extension may precede
a graphic rendering block. This is the only limit to the number of
Graphic Control Extensions that may be contained in a Data Stream.
b. Required Version. 89a.
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Extension Introducer Byte
+---------------+
1 | | Graphic Control Label Byte
+---------------+
+---------------+
0 | | Block Size Byte
+---------------+
1 | | | | | <Packed Fields> See below
+---------------+
2 | | Delay Time Unsigned
+- -+
3 | |
+---------------+
4 | | Transparent Color Index Byte
+---------------+
+---------------+
0 | | Block Terminator Byte
+---------------+
<Packed Fields> = Reserved 3 Bits
Disposal Method 3 Bits
User Input Flag 1 Bit
Transparent Color Flag 1 Bit
i) Extension Introducer - Identifies the beginning of an extension
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block. This field contains the fixed value 0x21.
ii) Graphic Control Label - Identifies the current block as a
Graphic Control Extension. This field contains the fixed value
0xF9.
iii) Block Size - Number of bytes in the block, after the Block
Size field and up to but not including the Block Terminator. This
field contains the fixed value 4.
iv) Disposal Method - Indicates the way in which the graphic is to
be treated after being displayed.
Values : 0 - No disposal specified. The decoder is
not required to take any action.
1 - Do not dispose. The graphic is to be left
in place.
2 - Restore to background color. The area used by the
graphic must be restored to the background color.
3 - Restore to previous. The decoder is required to
restore the area overwritten by the graphic with
what was there prior to rendering the graphic.
4-7 - To be defined.
v) User Input Flag - Indicates whether or not user input is
expected before continuing. If the flag is set, processing will
continue when user input is entered. The nature of the User input
is determined by the application (Carriage Return, Mouse Button
Click, etc.).
Values : 0 - User input is not expected.
1 - User input is expected.
When a Delay Time is used and the User Input Flag is set,
processing will continue when user input is received or when the
delay time expires, whichever occurs first.
vi) Transparency Flag - Indicates whether a transparency index is
given in the Transparent Index field. (This field is the least
significant bit of the byte.)
Values : 0 - Transparent Index is not given.
1 - Transparent Index is given.
vii) Delay Time - If not 0, this field specifies the number of
hundredths (1/100) of a second to wait before continuing with the
processing of the Data Stream. The clock starts ticking immediately
after the graphic is rendered. This field may be used in
conjunction with the User Input Flag field.
viii) Transparency Index - The Transparency Index is such that when
encountered, the corresponding pixel of the display device is not
modified and processing goes on to the next pixel. The index is
present if and only if the Transparency Flag is set to 1.
ix) Block Terminator - This zero-length data block marks the end of
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the Graphic Control Extension.
d. Extensions and Scope. The scope of this Extension is the graphic
rendering block that follows it; it is possible for other extensions to
be present between this block and its target. This block can modify the
Image Descriptor Block and the Plain Text Extension.
e. Recommendations.
i) Disposal Method - The mode Restore To Previous is intended to be
used in small sections of the graphic; the use of this mode imposes
severe demands on the decoder to store the section of the graphic
that needs to be saved. For this reason, this mode should be used
sparingly. This mode is not intended to save an entire graphic or
large areas of a graphic; when this is the case, the encoder should
make every attempt to make the sections of the graphic to be
restored be separate graphics in the data stream. In the case where
a decoder is not capable of saving an area of a graphic marked as
Restore To Previous, it is recommended that a decoder restore to
the background color.
ii) User Input Flag - When the flag is set, indicating that user
input is expected, the decoder may sound the bell (0x07) to alert
the user that input is being expected. In the absence of a
specified Delay Time, the decoder should wait for user input
indefinitely. It is recommended that the encoder not set the User
Input Flag without a Delay Time specified.
24. Comment Extension.
a. Description. The Comment Extension contains textual information which
is not part of the actual graphics in the GIF Data Stream. It is suitable
for including comments about the graphics, credits, descriptions or any
other type of non-control and non-graphic data. The Comment Extension
may be ignored by the decoder, or it may be saved for later processing;
under no circumstances should a Comment Extension disrupt or interfere
with the processing of the Data Stream.
This block is OPTIONAL; any number of them may appear in the Data Stream.
b. Required Version. 89a.
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c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Extension Introducer Byte
+---------------+
1 | | Comment Label Byte
+---------------+
+===============+
| |
N | | Comment Data Data Sub-blocks
| |
+===============+
+---------------+
0 | | Block Terminator Byte
+---------------+
i) Extension Introducer - Identifies the beginning of an extension
block. This field contains the fixed value 0x21.
ii) Comment Label - Identifies the block as a Comment Extension.
This field contains the fixed value 0xFE.
iii) Comment Data - Sequence of sub-blocks, each of size at most
255 bytes and at least 1 byte, with the size in a byte preceding
the data. The end of the sequence is marked by the Block
Terminator.
iv) Block Terminator - This zero-length data block marks the end of
the Comment Extension.
d. Extensions and Scope. This block does not have scope. This block
cannot be modified by any extension.
e. Recommendations.
i) Data - This block is intended for humans. It should contain
text using the 7-bit ASCII character set. This block should
not be used to store control information for custom processing.
ii) Position - This block may appear at any point in the Data
Stream at which a block can begin; however, it is recommended that
Comment Extensions do not interfere with Control or Data blocks;
they should be located at the beginning or at the end of the Data
Stream to the extent possible.
25. Plain Text Extension.
a. Description. The Plain Text Extension contains textual data and the
parameters necessary to render that data as a graphic, in a simple form.
The textual data will be encoded with the 7-bit printable ASCII
characters. Text data are rendered using a grid of character cells
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defined by the parameters in the block fields. Each character is rendered
in an individual cell. The textual data in this block is to be rendered
as mono-spaced characters, one character per cell, with a best fitting
font and size. For further information, see the section on
Recommendations below. The data characters are taken sequentially from
the data portion of the block and rendered within a cell, starting with
the upper left cell in the grid and proceeding from left to right and
from top to bottom. Text data is rendered until the end of data is
reached or the character grid is filled. The Character Grid contains an
integral number of cells; in the case that the cell dimensions do not
allow for an integral number, fractional cells must be discarded; an
encoder must be careful to specify the grid dimensions accurately so that
this does not happen. This block requires a Global Color Table to be
available; the colors used by this block reference the Global Color Table
in the Stream if there is one, or the Global Color Table from a previous
Stream, if one was saved. This block is a graphic rendering block,
therefore it may be modified by a Graphic Control Extension. This block
is OPTIONAL; any number of them may appear in the Data Stream.
b. Required Version. 89a.
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c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Extension Introducer Byte
+---------------+
1 | | Plain Text Label Byte
+---------------+
+---------------+
0 | | Block Size Byte
+---------------+
1 | | Text Grid Left Position Unsigned
+- -+
2 | |
+---------------+
3 | | Text Grid Top Position Unsigned
+- -+
4 | |
+---------------+
5 | | Text Grid Width Unsigned
+- -+
6 | |
+---------------+
7 | | Text Grid Height Unsigned
+- -+
8 | |
+---------------+
9 | | Character Cell Width Byte
+---------------+
10 | | Character Cell Height Byte
+---------------+
11 | | Text Foreground Color Index Byte
+---------------+
12 | | Text Background Color Index Byte
+---------------+
+===============+
| |
N | | Plain Text Data Data Sub-blocks
| |
+===============+
+---------------+
0 | | Block Terminator Byte
+---------------+
i) Extension Introducer - Identifies the beginning of an extension
block. This field contains the fixed value 0x21.
ii) Plain Text Label - Identifies the current block as a Plain Text
Extension. This field contains the fixed value 0x01.
iii) Block Size - Number of bytes in the extension, after the Block
Size field and up to but not including the beginning of the data
portion. This field contains the fixed value 12.
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iv) Text Grid Left Position - Column number, in pixels, of the left
edge of the text grid, with respect to the left edge of the Logical
Screen.
v) Text Grid Top Position - Row number, in pixels, of the top edge
of the text grid, with respect to the top edge of the Logical
Screen.
vi) Image Grid Width - Width of the text grid in pixels.
vii) Image Grid Height - Height of the text grid in pixels.
viii) Character Cell Width - Width, in pixels, of each cell in the
grid.
ix) Character Cell Height - Height, in pixels, of each cell in the
grid.
x) Text Foreground Color Index - Index into the Global Color Table
to be used to render the text foreground.
xi) Text Background Color Index - Index into the Global Color Table
to be used to render the text background.
xii) Plain Text Data - Sequence of sub-blocks, each of size at most
255 bytes and at least 1 byte, with the size in a byte preceding
the data. The end of the sequence is marked by the Block
Terminator.
xiii) Block Terminator - This zero-length data block marks the end
of the Plain Text Data Blocks.
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