LECTURE D'UNE IMAGE GIF
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Date d'inscription
vendredi 14 mars 2003
Statut
Membre
Dernière intervention
31 juillet 2004
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17 déc. 2003 à 14:15
BombStrike Messages postés 23 Date d'inscription lundi 1 juillet 2002 Statut Membre Dernière intervention 27 juin 2004 - 27 juin 2004 à 19:08
BombStrike Messages postés 23 Date d'inscription lundi 1 juillet 2002 Statut Membre Dernière intervention 27 juin 2004 - 27 juin 2004 à 19:08
Cette discussion concerne un article du site. Pour la consulter dans son contexte d'origine, cliquez sur le lien ci-dessous.
https://codes-sources.commentcamarche.net/source/18649-lecture-d-une-image-gif
https://codes-sources.commentcamarche.net/source/18649-lecture-d-une-image-gif
27 juin 2004 à 19:08
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 | | | | | See below
+---------------+
2 | | Delay Time Unsigned
+- -+
3 | |
+---------------+
4 | | Transparent Color Index Byte
+---------------+
+---------------+
0 | | Block Terminator Byte
+---------------+
= 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.
19 déc. 2003 à 14:02
En regardant de plus près, j'ai vu que j'ai fait une erreur. L'octet indiquant la fin du fichier GIF est le 0x3B.
De plus, Pour indiquer la fin d'une image et le début de la suivante on trouve le code 0x00.
Encore une chose, la temporisation de l'animation se trouve dans un bloc optionnelle (0x21) avec le premier code 0xF9 suivit de la longueur du bloc (4). Mais je ne connais pas la signification de la suite.
Si quelqu'un connaît...
19 déc. 2003 à 10:15
- Entête du fichier GIF (palette globale, écran global, ...)
- Bloc descripteur d'image de la première image
- Palette locale si elle existe
- Données compressées de la première image
- Bloc descripteur de la deuxième image
- Palette locale si elle existe
- Données compressées de la deuxième image
- Ainsi de suite jusqu'a la fin du fichier.
Je crois que ça répond à ta question. Et puisque tu poses cette question, je vais améliorer mon programme pour pouvoir lire toutes les images de ces types de fichier.
17 déc. 2003 à 14:15
nn