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Tinct
=====
The ability to plot alpha-blended sprites is only provided by certain
versions of RISC OS provided by RISC OS Ltd (ie RISC OS Select), and
currently not by the latest versions provided by Castle (ie RISC OS 5)
 There is currently no separate module available from RISC OS Ltd, nor a
version of Select that will function on an Iyonix. As such, users of machines
that will not run RISC OS 4 (and thus Select) cannot display images with
variable transparency (eg PNGs).
 This module provides the necessary functionality to display alpha-blended
sprites both scaled and otherwise. It also provides functions for dithering
and performing bi-linear filtering to improve their appearance.

 
Technical information
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To ensure future compatibilty, this module does not patch the OS in any way
and works in a totally legal way. It also does not write to itself in any
way, so is suitable for running from ROM.
 Redirection to sprites is supported, although due to the overheads involved
with caching the colour translation tables it is not recommended that this is
done. There are some exceptions to this, however, as output to a 32bpp mode
does not cause the tables to be re-calculated, and redirecting to a sprite
that has the same characteristics as the current mode and palette has a
minimal overhead.
 Tinct comes in two different variants - one that is slightly faster and
only suitable for memory architectures that support LDRH and STRH (denoted by
a '_h' filename suffix), and one that is suitable for all machines (denoted
by a '_b' filename suffix).


Format of a sprite with 8-bit alpha channel
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The sprite format used by Tinct differs from those used by RISC OS Select,
and no facilities to manipulate or create the required format is provided.
 All sprites used by Tinct must be 32bpp, and cannot have a mask.
 The basic format of the sprite is shown below, with the restrictions to the
standard sprite format marked with an asterisk (*):

	[+0]	Offset to next sprite
	[+4]	Sprite name, up to 12 characters with trailing zeroes
	[+16]	Width in words - 1
	[+20]	Height in scan lines - 1
	[+24] *	First bit used (must be 0)
	[+28] *	Last bit used (must be 31)
	[+32]	Offset to sprite image
	[+36] *	Offset to sprite image (no mask allowed)
	[+40] *	Sprite type (must be &301680B5)

Whereas for normal sprites the sprite image would be a series of colour words
of the format RrGgBb00, alpha-blended sprites use the empty byte to specify
the alpha value, ie RrGgBbAa.
 The alpha values represent the blending level on a linear scale where 0x00
represents that the source pixel is totally transparent and 0xff that it is
totally opaque. It should be noted that as a standard 32bpp sprite (eg as
created with !Paint) will have the alpha channel set to 0x00 by default no
output will be visible when plotting as an alpha-blended sprite.


SWIs provided
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Tinct provides four SWIs to plot sprites and one to convert sprites to their
32bpp equivalent. All values supplied to Tinct must be in OS units, and the
current OS clipping rectangle is used.
 The sprite pointers provided are equivalent to calling OS_SpriteOp with
bit 9 of the reason code set. To plot a sprite by name, the sprite should
first be found by using OS_SpriteOp with reason code 0x18 and using the
returned sprite address.


Tinct_PlotAlpha (0x57240)
¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
Plots an alpha-blended sprite at the specified coordinates.

->	R2	Sprite pointer
	R3	X coordinate
	R4	Y coordinate
	R7	Flag word


Tinct_PlotScaledAlpha (0x57241)
¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
Plots a scaled alpha-blended sprite at the specified coordinates.

->	R2	Sprite pointer
	R3	X coordinate
	R4	Y coordinate
	R5	Scaled sprite width
	R6	Scaled sprite height
	R7	Flag word


Tinct_Plot (0x57242)
¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
Plots a sprite at the specified coordinates with a constant 0xff value for
the alpha channel, ie without a mask.

->	R2	Sprite pointer
	R3	X coordinate
	R4	Y coordinate
	R7	Flag word


Tinct_PlotScaled (0x57243)
¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
Plots a scaled sprite at the specified coordinates with a constant 0xff value
for the alpha channel, ie without a mask.

->	R2	Sprite pointer
	R3	X coordinate
	R4	Y coordinate
	R5	Scaled sprite width
	R6	Scaled sprite height
	R7	Flag word


Tinct_ConvertSprite (0x57244)
¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
Converts a paletted sprite into its 32bpp equivalent. Sufficient memory must
have previously been allocated for the sprite (44 + width * height * 4).

->	R2	Source sprite pointer
	R3	Destination sprite pointer


Flag word
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All the SWIs provided by Tinct for plotting use a common flag word to
describe the manner in which the plot is performed. Each bit controls a
particular characteristic of the plotting:

	0  Forcably read the screen base (only use if hardware scrolling)
	1  Use bi-linear filtering when scaling sprites
	2  Dither colours in 16bpp and below
        3  Invert dither pattern
        4+ Reserved (must be 0)

Although bi-linear filtering is only relevant during scaled plotting, this
situation occurs when the EigFactors of the mode are not equal. As such, an
application should always set their preferred flags to ensure consistency.