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https://github.com/netsurf-browser/netsurf
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140 lines
5.1 KiB
Plaintext
140 lines
5.1 KiB
Plaintext
Tinct
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=====
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The ability to plot alpha-blended sprites is only provided by certain
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versions of RISC OS provided by RISC OS Ltd (ie RISC OS Select), and
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currently not by the latest versions provided by Castle (ie RISC OS 5)
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There is currently no separate module available from RISC OS Ltd, nor a
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version of Select that will function on an Iyonix. As such, users of machines
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that will not run RISC OS 4 (and thus Select) cannot display images with
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variable transparency (eg PNGs).
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This module provides the necessary functionality to display alpha-blended
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sprites both scaled and otherwise. It also provides functions for dithering
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and performing bi-linear filtering to improve their appearance.
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Technical information
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¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
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To ensure future compatibilty, this module does not patch the OS in any way
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and works in a totally legal way. It also does not write to itself in any
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way, so is suitable for running from ROM.
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Redirection to sprites is supported, although due to the overheads involved
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with caching the colour translation tables it is not recommended that this is
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done. There are some exceptions to this, however, as output to a 32bpp mode
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does not cause the tables to be re-calculated, and redirecting to a sprite
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that has the same characteristics as the current mode and palette has a
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minimal overhead.
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Tinct comes in two different variants - one that is slightly faster and
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only suitable for memory architectures that support LDRH and STRH (denoted by
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a '_h' filename suffix), and one that is suitable for all machines (denoted
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by a '_b' filename suffix).
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Format of a sprite with 8-bit alpha channel
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¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
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The sprite format used by Tinct differs from those used by RISC OS Select,
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and no facilities to manipulate or create the required format is provided.
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All sprites used by Tinct must be 32bpp, and cannot have a mask.
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The basic format of the sprite is shown below, with the restrictions to the
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standard sprite format marked with an asterisk (*):
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[+0] Offset to next sprite
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[+4] Sprite name, up to 12 characters with trailing zeroes
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[+16] Width in words - 1
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[+20] Height in scan lines - 1
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[+24] * First bit used (must be 0)
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[+28] * Last bit used (must be 31)
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[+32] Offset to sprite image
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[+36] * Offset to sprite image (no mask allowed)
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[+40] * Sprite type (must be &301680B5)
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Whereas for normal sprites the sprite image would be a series of colour words
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of the format RrGgBb00, alpha-blended sprites use the empty byte to specify
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the alpha value, ie RrGgBbAa.
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The alpha values represent the blending level on a linear scale where 0x00
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represents that the source pixel is totally transparent and 0xff that it is
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totally opaque. It should be noted that as a standard 32bpp sprite (eg as
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created with !Paint) will have the alpha channel set to 0x00 by default no
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output will be visible when plotting as an alpha-blended sprite.
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SWIs provided
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¯¯¯¯¯¯¯¯¯¯¯¯¯
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Tinct provides four SWIs to plot sprites and one to convert sprites to their
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32bpp equivalent. All values supplied to Tinct must be in OS units, and the
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current OS clipping rectangle is used.
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The sprite pointers provided are equivalent to calling OS_SpriteOp with
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bit 9 of the reason code set. To plot a sprite by name, the sprite should
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first be found by using OS_SpriteOp with reason code 0x18 and using the
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returned sprite address.
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Tinct_PlotAlpha (0x57240)
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¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
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Plots an alpha-blended sprite at the specified coordinates.
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-> R2 Sprite pointer
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R3 X coordinate
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R4 Y coordinate
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R7 Flag word
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Tinct_PlotScaledAlpha (0x57241)
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¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
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Plots a scaled alpha-blended sprite at the specified coordinates.
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-> R2 Sprite pointer
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R3 X coordinate
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R4 Y coordinate
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R5 Scaled sprite width
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R6 Scaled sprite height
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R7 Flag word
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Tinct_Plot (0x57242)
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¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
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Plots a sprite at the specified coordinates with a constant 0xff value for
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the alpha channel, ie without a mask.
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-> R2 Sprite pointer
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R3 X coordinate
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R4 Y coordinate
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R7 Flag word
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Tinct_PlotScaled (0x57243)
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¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
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Plots a scaled sprite at the specified coordinates with a constant 0xff value
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for the alpha channel, ie without a mask.
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-> R2 Sprite pointer
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R3 X coordinate
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R4 Y coordinate
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R5 Scaled sprite width
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R6 Scaled sprite height
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R7 Flag word
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Tinct_ConvertSprite (0x57244)
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¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
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Converts a paletted sprite into its 32bpp equivalent. Sufficient memory must
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have previously been allocated for the sprite (44 + width * height * 4).
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-> R2 Source sprite pointer
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R3 Destination sprite pointer
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Flag word
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¯¯¯¯¯¯¯¯¯
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All the SWIs provided by Tinct for plotting use a common flag word to
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describe the manner in which the plot is performed. Each bit controls a
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particular characteristic of the plotting:
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0 Forcably read the screen base (only use if hardware scrolling)
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1 Use bi-linear filtering when scaling sprites
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2 Dither colours in 16bpp and below
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3 Invert dither pattern
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4+ Reserved (must be 0)
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Although bi-linear filtering is only relevant during scaled plotting, this
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situation occurs when the EigFactors of the mode are not equal. As such, an
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application should always set their preferred flags to ensure consistency. |