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Sprite Mode Word (français)

Documentation de Référence du programmeur?
» Sprite Mode Word

Le mode Sprite

Traduction en cours

Le mode Sprite est une valeur 32 bits particulière du mode graphique utilisée pour définir le format du sprite. Il y a trois formats principaux supportés selon la version de RISC OS.

Mode graphique

Les modes graphiques sont les seuls modes supportés par les versions antérieures à RISC OS 3.5

If the mode number is one of an extension mode (i.e. a mode defined by a third-party module) then some operations on the sprite will only perform correctly if that mode (or a very similar one) is present on the host system.

Notez que l’utilisation de modes graphiques compris entre 128 et 255 pour définir un mode Sprite n’est pas autorisé.

Le mode Sprite du RISC OS 3.5

Le RISC OS 3.5 étend les possibilités du mode graphique de la façon suivante:

wttt tyyy yyyy yyyy yyxx xxxx xxxx xxx1
Plage de bits Contents
w (bit 31) Wide mask flag
t (bits 27-30) Type de Sprite
y (bits 14-26) Densité de pixels vertical
x (bits 1-13) Densité de pixels horizontal

Le bit 0 doit valoir ‘1’ pour définir un mode Sprite.

Notez que de nombreuses API accepte uniquement des valeurs de mode Sprite dont la densité de pixels est exprimée en ratio: 180 DPI (valeur du radio 0), 90 DPI (valeur du radio 1), 45 DPI (valeur du radio 2) ou 22 ou 23 DPI (valeur de ratio 3).

Le mode sprite de RISC OS 5

A partir du RISC OS 5.21, un nouveau format a été ajouté permettant un plus grand nombre de type de sprite au détriment du nombre de combinaisons de densité de pixels.

w111 1ttt tttt 0000 ffff ffff yyxx 0001
Plage de bits Contents
w (bit 31) Wide mask flag
t (bits 20-26) Sprite type
f (bits 8-15) Contains bits 8-15 of the Mode Flags
y (bits 6-7) Y eigen value (0=180 DPI, 1=90 DPI, etc.)
x (bits 4-5) X eigen value

Les bits 27-30, 16-19 et 0-3 doivent contenir le motif &78000001.

Determining mode word format

  • Is the mode word an unsigned value less than 256?
    • Yes: It’s a mode number
  • Is the bottom bit zero?
    • Yes: It’s a pointer to a Mode Selector Block.
  • Are bits 27-30 all set to 1?
    • Yes: It’s a RISC OS 5 style sprite mode word
    • No: It’s a RISC OS 3.5 style sprite mode word

The above rules are essentially just an expanded form of the set of rules presented for decoding mode specifiers, as many APIs which accept mode words actually accept all types of mode specifier. However it’s important to remember that pointers to mode selector blocks are not valid sprite mode words, as there is no way the pointer or the block can be saved out to a sprite file. Therefore if you are writing a public API which only accepts sprite mode words, you may want to implement a check for mode selectors and throw an error if one is passed in by accident.

Some APIs may also accept pointers to sprite areas – these can be distinguished from pointers to mode selector blocks by the fact that bit 0 of the first word of a sprite area will always be zero, whereas bit 0 of the first word of a mode selector block will always be one.

Also note that even if it looks like a RISC OS 3.5 or RISC OS 5 sprite mode word, other factors may mean that it’s a completely invalid mode word (e.g. DPI values of zero in a RISC OS 3.5 mode word, or the fixed bits of a RISC OS 5 mode word containing an invalid pattern).

RISC OS 3.5 sprite types

Type Description NColour value ModeFlags value Log2BPP value
0 Invalid (used to identify mode numbers) - - -
1 1bpp palletised 1 0 0
2 2bpp palletised 3 0 1
3 4bpp palletised 15 0 2
4 8bpp palletised 63 0 3
5 16bpp 1:5:5:5 TBGR 65535 0 4
6 32bpp 8:8:8:8 TBGR -1 0 5
7 32bpp CMYK -1 &1000 5
8 24bpp 16777215 0 6
9 JPEG data - - -
10 16bpp 5:6:5 TBGR 65535 &80 4
11-14 Reserved - - -
15 Invalid (used to identify RISC OS 5 sprite mode words) - - -

Note that for sprite type 4, the values returned by the kernel are for a sprite with a 64 entry palette instead of a full 256 entry palette.

RISC OS 5 sprite types

Types 0-15 are as above.

Type Description NColour value Log2BPP value
16 16bpp 4:4:4:4 4095 4
17 4:2:0 YCbCr 420 7
18 4:2:2 YCbCr 422 7
19-127 Reserved - -

When reading the mode flags, bits 8-15 of the sprite mode word directly specify bits 8-15 of the mode flags. The other flags (e.g. bit 7) are as specified in the tables above.

Gestion des types de sprites inconnus

A partir de RISC OS 3.5, essayer d’utiliser la fonction OS_ReadModeVariable pour obtenir les détails d’un type de sprite non supporté va indiquer que le sprite est de type 6 (c.à.d 32bpp). En comparant les valeurs retournées aux valeurs fournies il est possible de savoir quels type de sprites le système d’exploitation est capable de gérer.

Mask types

See Format Of Sprite for details of how the sprite mode word affects the format of the sprite mask.

Voir aussi

  • Screen Modes
  • Mode Flags?
  • Mode Variable Numbers?
  • Mode Selector Block
  • Numéro de mode
  • Format Of Sprite?
Page d’origine

English

Revised on February 25, 2018 14:36:55 by Etienne SOBOLE (3572)? (90.63.247.106)
Edit | Back in time (2 revisions) | See changes | History | Views: Print | Source | Linked from: Sprite Mode Word, Mode Specifier (français)

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