Introduction to Geminus
Geminus aims to extend the graphics capabilities of your RISC OS computer by
allowing the use of additional screens, rotatable LCD panels and, later,
DVI outputs. It can also accelerate rendering and screen redraws making the
desktop feel much slicker. These features can be purchased individually,
Geminus is a software layer that sits between the RISC OS 5 kernel and the
graphics card drivers, allowing it to extend and optimise many operations.
Additional features will be released over time, demoes will be available
and users may purchase only those features which they desire.
If you are only interested in the acceleration code, click
here to skip the sections on multi-monitor and
screen rotation features.
For faster JPEG decoding see here.
For the Red/Blue colour swapping feature (supporting unmodified NVIDIA
graphics cards) click here.
Installing a second graphics card
The primary graphics card, ie. the one which will be used when Geminus is not
running, will be the lowest one when fitted in the tower case. Since IYONIX pcs
are supplied with their graphics card in a 64-bit slot, this will likely be
your original card.
We believe there is an issue with the use of multiple recently-supplied (JATON)
graphics cards in the Iyonix and another, unrelated issue with the use of older
cards (as shipped with pre-production machines) in 32-bit slots. To avoid both of
these issues, we recommend that you install the second graphics card in a 32-bit slot
(either of the two righthand slots that have shorter, white sockets on the
Also some users may find that it helps to swap over their original graphics card
and the USB card/terminator pair if they are installing their new graphics card
in the slot closest to the existing cards. This can give slightly better clearance
between the cards and eliminates any chance of the USB card's metal pins contacting
the new graphics card.
Lastly, RISC OS 5.09 and 5.10 are modified slightly with the setup of your
graphics card(s) when they are programmed into the flash ROM so it is inadvisable
to change the card positions after programming. This decision was taken by Castle
to resolve startup problems but, since it can prevent the machine from booting
with alternative cards/positions, we sincerely hope that are more workable solution
will be found.
When you have downloaded Geminus you will have a zip file that contains an application
called !GemConfig. This application can be run in the normal manner by double-clicking
on its icon but it is also designed to be installed as a Configure plug-in by
dragging it into the directory
!Boot.Resources.Configure. If you do
this a Geminus icon should appear in the Configuration window (viewed by double-clicking
on !Boot) allowing you to configure Geminus in the same way that you configure other
aspects of your computer.
If you wish Geminus to start up automatically when you switch
on your machine, you should copy the Geminus module from inside !GemConfig to the
Note the inclusion of an exclamation mark so that it precedes the 'Configure' directory
thus ensuring that Geminus is loaded before the machine tries to enter a Geminus screen
mode, should you later configure it do that.
To configure Geminus, open the Configuration window by double-clicking on !Boot and
then Geminus (Or just double-click on the !GemConfig application instead). The window
that opens has four panes -
The first pane ('Info') just shows a few details about the version of Geminus that
you're running and the features that you have purchased. You can also learn more
about other features that are available or under development, and visit the website
to purchase them or learn more about their progress.
The 'Modes' pane is used to define your screen modes, but first you must tell Geminus
the type(s) of the monitor(s) you are using, so that it can use the correct Monitor
Definition File(s) (hereafter referred to as MDFs). An MDF describes the capabilities
of the monitor and it's important that you use the correct file for your monitor, where
one is available.
First, you should tell Geminus the location of your 'primary' monitor. This is the
one that RISC OS appears on when you first switch on your machine, before Geminus is
running. You may have this screen positioned on either the left or the right, and then
choose the appropriate option in the 'Monitors' pane so that Geminus can create
suitable mode definitions.
You then need to tell Geminus the type of each screen that you are using, by choosing
a monitor type or MDF from the menu that opens to the right of each section:
Red/Blue colour swapping
If you have purchased the Red/Blue colour swapping feature (or 'PC RGB format')
then you also need to tell Geminus the RGB order of each graphics card/monitor. For
the standard NVIDIA graphics card shipped with the IYONIX pc, you should leave the
'PC format RGB output' option unticked. For cards that you source yourself or which
are purchased through Spellings Ltd as 'unmodified', this option should be ticked
because the card expects the Red and Blue components to be in standard PC order, not
the order used within RISC OS.
If you have not purchased this feature, the 'PC format RGB output' tickboxes will
be shaded and have no effect.
We strongly recommend that people using the red/blue colour swapping feature
also purchase the acceleration feature because this makes a significant difference
to the speed of the desktop. There is an unavoidable performance overhead in swapping
the colour components in all direct screen accesses made by the OS and applications.
Geminus, however, is aware of the existence of PC format screens and is therefore
able to access them directly and with much greater efficiency, eg. when plotting
sprites it will simply use a modified palette rather than modify the value of
each pixel that is plotted.
Certain applications, notably video players, will exhibit much lower performance
on colour-swapped screens until they have been modified to accomodate them via
the Geminus API. Cino, the DVD player application, is aware of Red/Blue swapped
screens and can thus modify its own output appropriately, giving equal performance
on either screen type. In fact, since the NVIDIA cards are designed for use with
PC RGB order, the reproduction of colours is better on screens that employ
Geminus's colour swapping capability.
Now that it knows the monitor type(s) being used, it can suggest some suitable mode
definitions, so click on 'Modes' to open the modes pane which will be empty when you
first use Geminus. You will see a button labelled 'Suggest modes' which, when clicked,
displays a window showing some likely screen modes. You can select any number of modes
from this list using the SELECT and ADJUST mouse buttons in the normal way.
To add any of these modes into your configuration, simply drag them into the mode
list in the main window. If you then select a mode in this list by clicking on it,
you will see a graphical representation of this mode in the top half of the pane.
This picture shows you the position and resolution of the monitors that the mode uses,
and the portion of your desktop that will appear on each of the monitors. The red line
on each monitor indicates its bottom edge.
By double-clicking on any monitor in this picture you can change its resolution,
rotation and frame rate using the pop-up menus as shown below:
To start with, we suggest creating a mode that uses all of your monitors in
landscape mode and - if they are rotatable - another that uses them in portrait
mode, eg. if you have two LCDs each capable of 1280 x 1024, then amongst the
suggested modes you should see two modes akin to the following:
- Landscape Dual 2560 x 1024 at 75Hz
- Portrait Dual 2048 x 1280 at 75Hz
Once you have added the desired modes to the list in the Modes pane, you can
try them out by selecting a mode and clicking on 'Try.'
If you select a mode and click on the 'Try' button, Geminus will switch
the desktop into that screen mode and display a countdown timer with the
message "Do you wish to stay in this new mode?" If you click 'No' or do not
respond within the 8 second time limit, Geminus will automatically revert
to the previous screen mode, allowing you to recover if - for some reason -
the screen mode is unusable.
The most likely cause of a mode that doesn't produce a stable, usable
picture, is that you have a mode definition in your MDF which the monitor
cannot handle. If this is the case, you should edit or remove that definition
from your MDF to avoid confusion, or - alternatively - tell Geminus to use
a different MDF for that monitor.
Once you have a set of modes that you are happy with, just click on
the 'Set' button and Geminus will remember the modes and settings for you.
Geminus makes its modes available to you via an icon on the right hand side
of the icon bar. A separate icon and window are provided in addition to the standard
DisplayManager because Geminus is much more flexible and can be used to define many
different screen modes all having the same resolution. Wherever possible Geminus will
also make its modes available via the normal DisplayManager icon.
In Geminus's own window, a pictorial representation of the selected screen
mode is provided, allowing you to easily identify the screen mode, including
the resolution and orientation of the monitors that it uses.
To select a screen mode, simply click SELECT on the desired mode, or use the cursor
keys if the window has the input focus. Then choose the colour depth from the pop-up
menu, if required, and click on the Change button.
Booting into a Geminus screen mode
If you want the machine to boot into a screen mode provided by Geminus (ie.
not available via the DisplayManager) then, with the current release of Geminus,
you will need to manually edit the file
to specify the screen mode that you want to use, eg.
WimpMode X2560 Y1024 C16M EX1 EY1
in place of the existing WimpMode command. Note that the EX1 EY1 suffix
should be used with most modern modes/monitors to ensure that windows
and text are displayed with the desired aspect ratio.
Note that you will need to install
Geminus in the PreDesk directory in order to boot into a Geminus screen mode.
The Settings pane of the GemConfig application allows you to alter a few of
the more obscure features of Geminus; you shouldn't need to use this often.
DMA and AAU hardware accelerations
Geminus can take advantage of some specialised hardware present in the IYONIX pc,
specifically the DMA channels (already implemented) and the Application Accelerator Unit
(may be implemented in a later version of Geminus).
The use of DMA makes scrolling and dragging between screens much faster, especially
on rotated screens and it should therefore be enabled unless this is found to
conflict with other new IYONIX software that takes advantage of this hardware.
Maximum desktop size
Geminus needs to be told the maximum size of your desktop screen memory so that
it can allocate a range of memory addresses for the virtual screen that it creates.
Note that no actual memory is used for the virtual screen now (earlier versions of
Geminus did reserve memory for this purpose, limiting the amount of other software
that could be used).
This is the total size of all the screens you are using when driven at their maximum
resolution and colour depth; normally, with two screens, you should find the default
setting of 16MB is more than adequate. If, however, you have more screens or they are
capable of very high resolutions then you may want to increase this limit to 24MB.
If you use Geminus with rotatable screen and rotate them often then
you may find that your screens are in the wrong orientation when you switch
on the computer. To help you change mode without using the mouse, which
can be awkward, Geminus allows you to define a 'hot key' that opens its Mode
changing window. You can then use the keyboard to choose a suitable mode
and press Return to enter it.
The settings pane also holds a list of applications that require special
treatment by the acceleration code, if that feature is available.
See here for further details.
Hints and Tips
If you use Geminus with rotatable LCD displays then you'll probably
want to use Geminus in just two modes - native landscape and native portrait.
A handy way of switching between these two modes is to define two 'coloured
keys' in the Keyboard settings of Configure so that they issue the appropriate
*WimpMode commands, as illustrated below:
Oweing to the way that Geminus splits the screen display across multiple screens,
the width of each screen image is constrained to a multiple of 1KB, ie. a multiple of
- 256 pixels in 16 million colour modes
- 512 pixels in 32 thousand colour modes
- 1024 pixels in 256 colour modes
In practice this constraint isn't too onerous, because in 16 million colour modes,
most of the standard resolutions are available (1024 x 768, 1280 x 1024, 1536 x 1152,
2048 x 1536).
Geminus is able to accelerate the desktop by using the NVIDIA graphics card
hardware more extensively than RISC OS 5 does, and by reducing the amount of data
that is transferred over the PCI bus bottleneck. This is achieved by remembering
sprites and window contents in the off-screen memory of the NVIDIA card being
used so that they can be redrawn straight from the card's memory. This is
The NVIDIA card currently being shipped with the IYONIX pc, or available as an
extra from Spellings, has 64MB of on-board video RAM yet the screen display can only
consume 12MB of that memory even with a high resolution mode such as
2048 x 1536 x 32bpp. This leaves a lot of wasted memory that Geminus can put to
good use cacheing frequently-plotted sprites and window contents.
Geminus contains its own sprite plotting code which uses the extra instructions
and architectural features of the XScale CPU to accelerate plotting, especially on
rotated and/or colour-swapped screens.
DMA-accelerated sprite plotting
The OS sprite plotting code makes no attempt to accomodate the architecture
of the IYONIX pc, which really requires prefetching of sprite data and burst
writes to the PCI bus. When developing the video output stage of the Cino DVD
player it was discovered the IOP321's DMA channel is capable of performing
these transfers much more rapidly than the XScale CPU and Geminus puts this
hardware to good use when plotting large sprites of the same bit depth as the
Since most IYONIX pc users will be using 32bpp modes, the effect of
DMA-accelerated plotting will be apparent when viewing large JPEGs in !ChangeFSI,
large 32bpp sprites in !Paint and all large images in !NetSurf if the image
settings are 'Use OS' in NetSurf's Choices window.
NetSurf and Firefox use a module called Tinct (written by Richard Wilson) to render
images in their windows. When Sprite plotting is enabled for these applications
Geminus will also intercept calls to the Tinct module and use DMA to accelerate the
plotting of those images, leading to a smoother display in those web browsers,
particularly with large images.
Cacheing of sprites is achieved by simply rendering the whole sprite to off-screen
storage using Geminus's sprite plotting code (which is therefore also capable of the
necessary rotations and colour swaps) and the visible portion of the sprite is then
copied to the actual screen display using a hardware operation (or two hardware
operations if the sprite must be masked). Subsequent plots of the same sprite will
only perform the hardware operation requiring very little CPU involvement and
PCI bus traffic.
The benefits of Geminus's sprite cacheing are especially noticable when
working with large textured windows such as Filer windows or sprite files holding
a lot of sprites in !Paint.
By far the slowest graphical operations that you will encounter on the RISC OS
desktop are applications redrawing the contents of their windows. Sometimes this is
because the application code hasn't been tuned extensively. Sometimes, as in the case
of ArtWorks, it is simply because the complexity of the displayed image is essentially
unbounded, so no sooner does a faster machine or more highly optimised rendering code
become available than an artist creates a more photorealistic or complex image.
Geminus is able to remember the contents of windows using the off-screen memory
so that when you later uncover the window by closing a window that was in front of it
or by dragging another window across it, Geminus can render the uncovered area
immediately using the graphics hardware rather than the much slower alternative of
asking the application to redraw the unchanged contents of its window.
This makes a big difference to the fluidity of the desktop when working with
large or complex vector graphics in ArtWorks. A future version of Geminus may be
able to accelerate the initial rendering too, and not just subsequent redraws of
the same area.
Horizontal and vertical lines
Geminus can employ the NVIDIA's hardware acceleration for rendering horizontal
and vertical lines (eg. window borders and DrawFile rendering) to give a speed
increase over the OS rendering routines. This will give a further boost in a future
version of Geminus that can overlap graphics operations with CPU activity.
Inverting a rectangular area of the screen is a very slow operation on the
IYONIX pc because it requires that a large amount of data be read over the PCI bus
from the NVIDIA card, inverted by the CPU and then written back. The very poor
performance of this operation is apparent when selecting rows and columns in
!Paint, for example.
Geminus can use the NVIDIA hardware to perform this operation literally
thousands of times faster, making these selections much slicker and far more
pleasant to use.
Since Geminus provides its acceleration code, and particularly the cacheing
of sprites and window contents, transparently for all applications rather than
as a new OS API extension, it has the potential to give problems with some
applications, typically displaying images that are out of date and have not
changed when the application they should.
To circumvent such problems, Geminus allows you to specify a list
of applications that require special treatment, and also the ability to disable
sprite plotting ,
and redraw cacheing
globally if required.
Applications can be added to the list of special cases by choosing from
the pop up menu to the right of 'Add application.' This menu shows the
applications that are currently running, so you must have already started
the application that you wish to add to the list.
You can then alter the on/off state of each of the three main accelerations
as described above and click on 'Set' to apply the changes.
The 'All' and 'Clear' buttons manipulate which of the applications are
selected (you can select individual applications in the normal manner by
clicking with Select or Adjust over the application names)
The demo version of the Geminus acceleration code allows you to quickly see
the benefits of acceleration by Adjust-clicking on the Geminus iconbar icon to
turn the accelerations on and off. When temporarily disabled in this way the
Geminus icon will be crossed out thus:
This disables all of the following: sprite plotting, sprite cacheing,
redraw cacheing, line drawing and inversions, JPEG decoding and plotting.
Geminus can intercept JPEG-plotting calls to the SpriteExtend module from
applications such as:
- Ovation Pro
- Pinboard (backdrop)
- ArtWorks 2
It then uses its more highly optimised JPEG decoding and plotting routines
to provide rendering that is up to three times faster than the SpriteExtend
routines. This can improve the usability of any image viewer such as Thump
when manipulating a lot of large JPEGs.
The JPEG decoding/plotting can be disabled (or re-enabled) in the
Settings pane of !GemConfig in the event that it causes
problems with any of the applications that you use. If you find an application
that it doesn't work with, please report the problem to us.
In addition to being faster than the SpriteExtend code, it is designed
to allow music playback to continue whilst decoding and rendering large JPEGs
(the SpriteExtend code executes in SVC mode and thus blocks ShareFS network
traffic and audio playback from disc/network whilst it is rendering). By
running in USR mode it also limits the effect of any coding errors. Note that
for technical reasons if Geminus's decoder does fail it will disable itself
until the next mode change.
Rotated and transformed JPEGs
The JPEG code in the OS's SpriteExtend module provides SWIs to perform
rotated and transformed plotting of JPEGs, but this functionality was never
implemented. Geminus implements those SWIs and provides, for the first time,
full rotation and skewing of JPEGs in applications such as !Draw, illustrated
Some more advanced applications such as Ovation Pro and ArtWorks 2 are
already able to rotate/skew JPEGs in this way by working around the incomplete
API of the SpriteExtend module, however doing so is both slower and uses more
Note: Some applications, including !Draw, will check for the ability
to transform/rotate JPEGs only once when they start up or first render a JPEG,
so you will need to make sure that Geminus is running with its JPEG feature
enabled before starting them. OpenVector, which has recently been adapted to
exploit this capability of Geminus, can adapt dynamically.
With !Draw you will see a message akin to the following if the feature
Lastly, although Geminus will render JPEGs in modes with 256 colours or fewer,
it employs only a simple algorithm and does not (yet) implement dithering or
error diffusion. 16bpp and 32bpp modes are considered to be much more useful nowadays
and have received correspondingly more development effort. Geminus will therefore step
aside and let the OS render the image if transformation is not being requested,
hence there will be no speed increase in these low-colour modes for applications
that just employ scaling.
Loading of JPEGs into ChangeFSI is difficult to accelerate because the
way that it works is inherently inefficient. It invokes an open source utility
called 'djpeg' which converts the JPEG to a large bitmap file and saves
it to disc. ChangeFSI then loads this bitmap into memory (rather slowly)
Geminus can only hope to accelerate the JPEG decoding, via a modified
djpeg utility which will be made available, but this is not the slowest
stage of the above process so the speed up will not be as great as with
the JPEG viewers such as !Thump and !SwiftJPEG. ChangeFSI, however, is
intended for processing images rather than merely viewing them.
A number of additional features are planned for release in later versions of Geminus,
all of which have already been proven as prototype code but need completing and fully
testing before they can be released.
Because the nVIDIA graphics card supplied with the IYONIX pc does not support
screens with fewer than 256 colours, these screen modes have to be emulated.
This is only an issue for older, 26-bit software and this feature is therefore
currently included in Aemulor Pro. Moving this emulation into Geminus,
by combining it with the emulation required to support rotated screens,
provides a considerable performance increase and it is therefore anticipated
that a later version of Geminus will replace Aemulor Pro's screen emulation.
You can find more information about Geminus on the support
forums, and on other products published by Spellings Computer Services Ltd
Geminus is Copyright (C) Adrian Lees 2004-6 and
is published by Spellings Computer Services Ltd