X.org mouse acceleration proposal Author: Simon Thum (simon [dot] thum [at] gmx de) Date: 12/06 - 04/2007 Intent - improve heavy-load behaviour (no jumping pointer) - enable accuracy and fluid motion with every device - provide a better 'feel' for X pointing devices Postulate When the pointer gets accelerated with respect to device motion, it becomes important for the user to be able to predict that acceleration. Otherwise, the user is unable to intuitively use his device. Thus, it is presumed that the most critical part is doing a sophisticated estimate about the velocity of the device in a users hand, as this is the reference the users brain has to build its own knowledge about the applied translation from device to screen. Because this is an intuitive process, easing it just 'feels better'. Current problems adressed 1) On slow movements, current code infers 3 'velocities': 1, 1.41, and 2. As 1 remains unaccelerated, we're left with just 2 acceleration levels. It is hard to foresee what exactly will happen. Worse, when using the simple accelerated/unaccelerated scheme, acceleration either comes to effect or it doesn't, providing no predictability around the threshold. 2) If a system is under load, a device may accumulate its movement delta while the system does not query the device, causing irrational high cursor movement. 3) Some people own very responsive devices, creating a need to reduce speed on precise tasks or even in general. 4) With the simple acceleration scheme, acceleration is more sensitive on diagonal movements than on axis-aligned ones. These problems result in a reduced ability for our intuition to predict a matching hand movement for desired screen movement, causing more correctional moves than neccessary. Put simply, the mouse 'feels bad'. Method 1) A better approximation of velocity is done. Given available data, we calc dots per millisecond. A correction is applied to account for slow diagonal moves which are reported as alternating horizontal/vertical mickeys. These would otherwise be guessed 40% too fast. Dots per milisecond is quite intractable in integers. Thus, we multiply by a configurable factor to arrive at values where the usual X controls, threshold and acceleration, can be applied with some sense. This velocity is then weighted with an exponentially dropping function, i.e. the longer a movement signal is back in time, the less influence it has on the current estimate (Technically this is an IIR filter, which is actually much cheaper in CPU terms than it may sound). Typically during begin and end of a mouse stroke, weighting is not approriate. Therefore, a coupling is employed to override velocity if weighting seems inapproriate. After some short inactivity time, the background data is reset (called non-visible state (reset) in the patch). This is to ensure no two distinct strokes affect each other. 2) Acceleration functions are made continuous. This avoids sudden jumps in the amount of acceleration, enhancing intuitivity furter. 3) Reported values are slightly flattened This aims to improve evolving-speed movements such as painting with a mouse typically requires. It happens just below device precision and only if acceleration is actually performed. It can be independently turned off. 4) For too responsive devices, there are two possibilities (together if desired): 1) a constant deceleration can be applied 2) acceleration function(s) can be allowed to decelerate on slow movements (adaptive deceleration) Benefits Mostly, the polynomial acceleration becomes more usable. It can be used with higher acceleration coefficients (acc > 2), still providing enough control. But also the classic acceleration should become less jumpy since it now graduates (rather) soft towards accelerated motion. Users with too precise devices can slow them without loosing precision, independent of hardware driver support. Even more important, an acceleration function may decelerate on slow movements, giving (sub)pixel precision without sacrificing on pointer speed. The code is more robust towards different devices: One could imagine a mouse reporting very often, but only 1 dot per event. Old code would not accelerate such a device at all. While this is a theoretical case, there is robustness against jitter in device event frequency (as could be caused by system load, for example). By introducing a coefficient in xorg.conf (VelocityScale or ExpectedRate) you can make two attached devices feel similar or intentionally dissimilar. Users disliking all this can switch it off, retaining constant deceleration if desired. Configuration The defaults should suffice if you had no problems before, and feel quite similar. The settings discussed here are the coarse and the very subtle settings, not the usual xset or GUI panel controls you might know. However, setting treshold to 0 is strongly recommended, to use the more intuitive polynomial acceleration. Acceleration should be about 1.5 to 2.5 then. The settings described below are device-specific and need to be set in the approriate "InputDevice" section in xorg.conf (or what your X reads). Usually it looks like: Section "InputDevice" Identifier "Mouse0" Driver "mouse" [...] EndSection For example to enable the adaptive deceleration feature, put in a line reading Option "AdaptiveDeceleration" "2" or similar. A few tips If you have a feeling your mouse moves far too fast, ConstantDeceleration is your friend. Set to 2 or higher to divide speed accordingly. This will not discard precision (at least only on nv-reset, see Method or below). If you like the speed but need some more control at pixel-level, you should set AdaptiveDeceleration to 2 or more. This allows to decelerate slow movements down to the given factor. You might want to keep nv-resets away by setting VelocityReset to e.g. 300 ms, and maybe tweak VelocityScale to give good results. If you are picky about a smooth kick-in of acceleration, for example to ease doing art, I suggest tweaking VelocityScale and using adaptive deceleration. Maybe increasing velocity coupling also helps it. Options AdaptiveDeceleration [real] Allows the acceleration function to actually decelerate the pointer, resulting in enhanced precision on slow moves. Default is 1, which deactivates adaptive deceleration. Setting 2 or higher allows for respective deceleration. Adaptive deceleration should not affect your normal mouse useage; if it does, VelocityScale is probably too low. ConstantDeceleration [real] Constantly decelerates the mouse by given factor. Using ConstantDeceleration should be preferred over corresponding device driver options (if any) since these throw away data which could be better used to predict device velocity. Default is 1 (no deceleration). VelocityScale [real] or ExpectedRate [real (Hz)] In short, this controls sensitivity of acceleration. This setting is designed to be device-dependent, i.e. you set it once to match your device, then modify behaviour using the classical controls. It is important to note there is no 'right' factor, only one that bears the nice property of matching to X controls just like the old code did. Rationale: Device deltas are being divided by delta milliseconds before being weighted, so they are about 10 times too small compared to a device reporting every 10 ms. Because the reporting rate is usually unknown in advance, this is the only way to scale up to 'normal' values as well as identify slow movement. Default is 10, or 10x, which is suitable for devices reporting around 100hz. If your mouse reports x times per second, set to (1000/x). You can use ExpectedRate to set according to this formula. WeightingDecay [real] Tweaks the weighting applied to approximate velocity. Higher values exhibit more integrating behaviour, introducing some lag but also may feel smoother. Less is more responsive, but less smooth. However, any lag will only show up if velocity coupling is disabled or too weak. Default: 15 ms. VelocityReset [integer] Specifies after how many milliseconds of inactivity non-visible state (i.e. subpixel position) is discarded. This affects three issues: 1) Two mouse strokes do not have any effect on each other if they are [VelocityReset] miliseconds from each other. This would be neglible though. 2) Velocity estimate remains correct within this time if the pointer/X is stuck for a short moment, not querying the pointer device. 3) slow movements are guessed correctly if all device movement events are inside this time from each other. An increment might be neccessary to fully take advantage of adaptive deceleration. Default 200 ms. VelocityCoupling [real] Specifies coupling, a feature ensuring responsivity by determining if the weighted velocity is a valid one. Weighted estimate is deemed valid if it differs from current either below 1.0 (hardcoded) or below this factor. If it is deemed invalid, velocity will be overridden by the current measurement and any weighted data is discarded. 0 disables, so only weighted velocity is used. This may exhibit some lag, depending on WeightingDecay. >0 Higher setting means it is more likely that weighted velocity is deemed valid, i.e. actually a lower value tightens coupling. Default is 0.2, or 20%. Softening [boolean] Tweaks motion deltas from device before applying acceleration to smooth out rather constant moves. Tweaking is always below device precision to make sure it doesn't get in the way. Also, when ConstantDeceleration is used, Softening is not enabled by default because this provides better subpixel information. However you can set this "off" or "false" if you don't like it. EstimateVelocity [boolean] Setting this to "off" retains the simpler, old-fashioned algorithm for determining velocity. It doesn't behave exactly like the old algorithm but similar enough most people wouldn't note a difference. AccelerationScheme [int] Select Scheme. This is to really switch off the code discussed here. 0 means previous method, 1 is the one discussed here, which also is the default. Acceleration functions/profiles/schemes (for the inclined programmer) Acceleration functions translate device velocity into an acceleration to be imposed on the pointer. Xorg offers two functions: Simple and polynomial. They are selected somewhat strange through the threshold in xset (or some gui): threshold = 0 means polynomial, simple otherwise. The simple acceleration function is now continuous, and the polynomial maintains f(1) = 1. They are designed to mimic previous behaviour. Just copying old functions would not provide much benefit: The patch would make the point when acceleration is performed be more predictable, but not cause the pointer to cease jumping around that point. If you like to play with the functions, a few nice properties: 1) f(1) = 1 a fixed point, to enable exchanging functions 2) continuous very nice-to-have since we would otherwise throw away our data (probably causing jumps) 3) continuous over derivative(s) nice to have for smoothness 4) f'(min_acceleration) = 0 Ensures a soft kick-in of acceleration 5) f( < 1) < 1 enables adaptive deceleration - although it is possible to hold all of the properies, included functions only hold 1, 2, and 5. - acceleration functions are not meant to enforce constant or adaptive deceleration. This is done in a seperate step. A profile is an intermediary of which there is currently only one, the classic profile. It switches between the two functions simple and polynomial just as the old code did (explained above). If you want to do freaky new functions, you best put them in an own profile, see ClassicProfile() for reference. The profile is currently not configurable in xorg.conf since there is only one, but it is easyly exchangeable. A scheme is on top of the hierarchy and if you think this is all bullshit and your algorithm rocks you should do an own scheme. Currently, 'plain old X' and the scheme discussed here are implemented. Problems / Todo I am not an experienced X dev, so some points are left. More complex algorithms have more knobs, and currently they can only be set in the server config. If you have suitable values however, change should only be needed when the device changes. Better would be access via xset and/or API. The Patch is only for xorg; kdrive or xgl have not been touched. Since it is now in dix, this is not a big problem. Hotplug devices haven't been tested and given the suboptimal integration probably will fall back to old algo. The main problem I encountered was finding a place where access to config data and access to valuators is given during init. If you know that place, write me. Interaction with synaptics driver I noticed two important things to consider when using the synaptics driver (or any other driver doing substantially more than decoding mickeys): 1) It seems this driver implements its own acceleration, which can be switched off. Two ways of acceleration certainly don't do good. This can be accomplished by setting 2 options, 'MaxAcc' and 'MinAcc' to the same. 2) I chose MaxAcc = MinAcc = 1, which seemingly made the native touchpad resolution available to X, which was far too responsive. I had to apply a ConstantDeceleration of 6 to work with it. This also makes the full device precision available to guess velocity. Also, I found increasing WeightingDecay on the touchpad to feel more comfortable. Reference http://lists.freedesktop.org/archives/xorg/2005-September/010211.html https://bugs.freedesktop.org/show_bug.cgi?id=138 https://bugs.freedesktop.org/show_bug.cgi?id=2927