Graphics Mode Specification

Contents

• Introduction and Overview
• Technical details
  • Mode specifiers
  • Mode numbers
  • Sprite mode words
  • Mode selectors
  • Mode strings

Introduction and Overview

Graphics modes can be specified in a number of ways, which have been added to with each iteration of the Operating System. Originally only mode numbers were allowed, but hardware improved and more flexibility was required, so the mode specification was extended.

Graphics modes may be supplied to a number of interfaces, most of which will eventually come down to a call to OS_ReadModeVariable. Some of the interfaces that you may find using mode specifications are:

Interface	Usage
OS_ReadModeVariable	Read values for a given mode.
OS_CheckModeValid	Return whether the mode specified can be selected.
OS_ScreenMode	Operations on the graphics mode
ColourTrans_*ForMode	Colour operations for a given mode
OS_SpriteOp	Sprite creation operations may be supplied modes
Sprite Header	Defines the type of data within a sprite
Wimp_SetMode	Selects the mode used by the Window Manager

Technical details

Mode specifiers

Mode specification is always through a single 32bit word value known as a mode specifier. This allows it to be supplied in many of the places that a mode number was used in earlier interfaces. This mode specifier can represent a number of ways of describing a mode. The following mode specifier formats are defined:

• Mode number
• Sprite mode word
• Sprite pointer
• Mode selector

These can be distinguished by the following algorithm:

• If the mode specifier is < 256:
  • This is a mode number, and shadow bank selection.
  • The mode number is in the low 7 bits, and shadow bank selection is given in bit 7.
  • If the mode number is not recognised Service_ModeExtension is issued to determine the mode's parameters.
  • Modes up to 7 are supported from the BBC onwards.
  • Shadow modes are supported from the Master onwards (although they are less reliable from RISC OS 3.6 onwards)
• If the mode specifier has bit 0 set, this is a sprite mode word:
  • Sprite mode words are given in the sprite header, but may also be supplied to many of the mode functions (except for display selection).
  • They only contain the DPI (and thus eigenfactors), and type of data within the sprite. No resolution information is available.
  • Sprite mode words are supported from RISC OS 3.5 onwards.
• If the mode specifier has bit 0 and 1 clear, this is a pointer to data, whose meaning is differentiated by the value of the first word.
  • If the first word has bit 0 clear, the data is a sprite (the mode specifier is a sprite pointer):
    • Sprite pointers allow information about the width and height to be included in the information, and allow the use of palette data as well. These types of mode specification are usually only used with ColourTrans_*operations.
    • Sprite pointers are supported from RISC OS 3 onwards.
  • If the first word has bit 0 set, the data is a mode selector:
    • Mode selectors expose the base specifications for the mode and modifications to mode variables.
    • Mode selectors allow for extended formats, but only a single format is currently defined.
    • Mode selectors are supported from RISC OS 3.5 onwards.
• If the value has bit 0 clear and bit 1 set, this is an invalid mode specification.

Mode numbers

Mode numbers may be extended through the Service_ModeExtension interface. This allows new numbered modes to be defined, either completely or based on other modes.

Sprite mode words

Sprite mode words allow some of the parameters of the mode to be determined, but because they do not include resolution information they cannot be selected. Sprite mode words are only supported from RISC OS 3.5 onwards.

The sprite mode word format has undergone a few revisions. The current definition of the sprite mode word is:

Bit(s)	Meaning
0	Set (indicator that this is a new format sprite, together with set bits in bits 27-31)
1-13	Horizontal dots per inch, should be 180, 90, 45, 23/22, 11
14-26	Vertical dots per inch, should be 180, 90, 45, 23/22, 11
27-30	Sprite type : Value Meaning 0 Old format mode word (mode is a standard number) 1 1 bpp 2 2 bpp 3 4 bpp 4 8 bpp 5 15 bpp in 16bit values 6 24 bpp in 32bit values 7 CMYK 8 24 bpp compact format (allocated but not used) 9 JPEG data (allocated but not used) 10-15 Reserved
31	Set:	Alpha channel data present. May not be set for type 0 sprites
	Clear:	Binary mask data present

For sprite types 1-4, the palette is only supported from RISC OS 3.6 onwards.

Although the DPI value should be the values defined above, values outside these may be supported. Certain interfaces, such as PlotSpriteTransformed, may use this information to render the sprites to the correct size for the display. Other interfaces, such as OS_ReadModeVariable and PlotSpriteScaled, may quantise these DPI values to the closest eigenfactor.

CMYK format sprites are supported from Select 2 onwards. JPEG data has been supported by third party extensions.

Compatibility

RISC OS < 3.5  

Does not support sprite mode words.

RISC OS >= 3.5  

Sprite types 0 to 6 supported, but does not support palettes on types 1-4.

RISC OS >= 3.6  

Supports palettes on sprite types 1-4.

RISCOS Ltd  RISC OS >= Select 2  

Sprite types 0-6 and 7 (CMHK) supported.

RISCOS Ltd  RISC OS >= Select 3  

Supports alpha channel data in addition to the types supported by Select 2.

RISC OS Pyromaniac  RISC OS >= 7.19  

Sprite types 0-6 supported.

Gerph  JPEGSprites   

Adds support for sprite type 9 (JPEG) to those supported by RISC OS 3.5.

Document information

Maintainer(s):	Gerph <gerph@gerph.org>
History:	Revision Date Author Changes 1 22 Nov 2020 Gerph Initial version Created from PRM and Select technical documentation. 2 19 May 2023 Gerph Compatibility with Pyromaniac Added details about the compatibility of the interfaces with RISC OS Pyromaniac and RISC OS Select.
Disclaimer:	© Gerph, 2020-2023.