The overlap of computer science and theoretical quantum physics grew a little today when the journal Nature published a very cool new paper from scientists in Zurich.
What entropy means to software people is usually much different than what it means thermo-dynamics people. Turns out the concepts can be unified via a deeper understanding of quantum mechanics.
If you are a normal person you probably fell asleep reading the last two sentences. If you're an abnornal person and are interested in things like the ultimate limits of super-fast computers, it's worth a read.
A Surface application I’ve been working on has just gone live to the public in Manhattan, NY. You can check it out at Bloomingdale’s in the Clinique area.
All the real products you see in the photo can be used interactively. For example you can place a tube of moisturizer on the table and have it sent to your Facebook wall or e-mail.
Everything “feels” great to use – the gestures, discoverability, and design I all like personally. Although the last statement may sound self-serving it’s really to the credit of the designers and Clinique product team. By the end the process they really “got it” and have enabled a special experience.
I’ve found that creating water effects that respond interactively and smoothly is not really something easy to do in WPF or Silverlight.However because a project required it I trudged forward managed to get it working pretty nicely – buttery smooth and low CPU utilization.
The results can be seen in the video, but the reasons it wasn’t easy may be interesting to WPF/Silverlight developers so I’ll mention why below. Can you guess what the technical obstacles are?
(user interaction 17 seconds in)
(user interaction 17 seconds in)
Graphics architecture
The first hurdle is that WPF and Silverlight primarily use a “display list” or “retained mode” graphics architecture.This is usually not a bad thing, and allowed developers to break away from the WM_PAINT model which is used in WinForms and Win32 before that.Those older systems encourage an “immediate mode” architecture while WPF and Silverlight encourage a retained mode, but will tolerate immediate mode in certain cases.
WPF and Silverlight
You might have nightmares about WinForms and Win32 and wish all manner of death upon them.Death to immediate mode as well – long live WPF/Silverlight and retained mode graphics!
Just one small problem – immediate mode graphics are well suited to certain things like games and simulations, and no one wants give those up.In fact Direct3d itself is an immediate mode architecture which underlies most graphics in Windows 7 including WPF itself.I’m speaking in general here and there are always exceptions.For example I’ve written simple games and simulations in WPF, but many times it is not an ideal approach.
Animation vs. Simulation
To get more specific on why WPF/Silverlight are not great at simulating water, it’s basically about loops.Many simulations for water and other phenomena run a programming loop over and over again throughout the simulation.The reason is that future states of the simulation depend on the output of previous states of the simulation.The problem is that animations are different than simulations.
In WPF you can easily animate a ball along a flat line. WPF determines the position of the ball using a function with time as the input:
Time elapsed 1 second - Move ball to 25% along the line
Time elapsed 2 second - Move ball to 50% along the line
Time elapsed 3 second - Move ball to 75% along the line
You could also choose any random time and the ball would be placed properly because the new position does not depend on the previous position.It’s always just a function taking any time as an input, and spitting out a position as output.
Most animation systems work in a similar way, which is why regardless of what applications artists and animators choose it usually has a big timeline along the bottom.
So if that's how animation works, how is simulation different?
In simulation of water our function to calculate the new position takes not only time as an input but also the previous position of the water. This feedback loop is the key difference.
Since WPF does not encourage loops that let you feedback the output into the next input, we are kind of stuck.
What about CompositionTarget?
One ray of hope is the CompositionTarget. This is a WPF component specifically designed to enable controlling your own rendering loop.I consider CompositionTarget going a bit off the reservation because it’s not really a mainstream technique used by most apps, however it’s fairly clean and that’s ok if it can get us past the loop barrier. While this lets us have precise control over the rendering loop, it still does not allow any way for output of the shaders to feed back into the loop.
We could just not use shaders and write the code procedurally, but then the question becomes how much performance do shaders afford our simulation?
How fast are shaders in WPF?
VERY fast. In fact to get nice performance and realistic water, custom shaders are essentially required. It's an order(s) of magnitude difference. Using shaders should be no problem because WPF supports shader model 3.0.In fact in a previous blog post I showed a single drop water effect using WPF shaders and the code is simple XAML with no problems.But again, that was an animation not a simulation.The drops could not affect each other and you could not drag your finger through the water and create a wave as in the video above.
To get some real coolness we need a simulation, which means using the output from the shader as the input for the next time the shader is run.The problem is when you capture the output of shaders as data in WPF (rather than just display the output), you lose hardware acceleration and performance goes to heck in a handbasket. One item on my wishlist for WPF 5.0 would definitely be enabling a form of shader trees where the output can be modified and looped back as input.
To keep pixel shaders running in hardware in this fashion within a WPF app requires Direct3d, which in turn requires D3DImage.D3DImage is the class that can allow separate Direct3d C++ code to output into an image brush within a WPF app.It also allows (requires really) the Direct3d code to have it’s own rendering loop, which allows us to feed shader output back to the input for the simulation.
With Silverlight its worse because of the lack of GPU acceleration and because the Direct3d integration is not an option.I’ve seen a few liquid/particle simulations in Silverlight but they are quite slow, and tend to peg CPU utilization.
So the bottom line is to create interactive effects like this in WPF really requires CompositionTarget, custom shaders, and a separate Direct3d helper DLL.The first two techniques alone will do the same thing at about a quarter of the speed.
Every year I hope to never code in C++ again, but it looks like that time is not here quite yet.
A lot of contributions to OSS are either writing code to add
new features, or users requesting new features.
Both of these roles are critical but I've noticed there are few OSS
participants formally identified as part of a "triage" team, yet it's
quite common in the commercial software world.
Triage is essentially helping to decide what work should not be done, so that
precious resources get applied to the best possible features and fixes (or
least what work should have lower priority).
The term comes from the medical emergency rooms where doctors triage the
patients into groups of different priority. If you've broken your arm,
you'll being waiting behind the person who needs immediate surgery, even if you
arrived an hour earlier.
Certainly triage happens in OSS, a lot of times by development leads.
Tough decisions happen all the time. However scouring a huge bug list for
items that should be shifted up or down in priority, or questioning obscure
items that might really not be as important, doesn't seem to always happen as
much as it could. How could it with only extremely busy developers held
responsible?
Once example of this not being emphasized in OSS is that many projects have an
option to "Vote for this issue as important", while not having an option
to "Vote for this issue as not important" (allowing votes to be cast
as 0 to 5 stars would help this).
So for projects where I contribute or at least follow the progress, I'm trying
to make sure I keep an eye out for helping with triage where it seems productive.
Some project leaders may not want anti-requests or triage help, and that's of
course their choice. However I just think we should remember that an
anti-request could be the catalyst to get your actual feature request enough
resources to be completed sooner.
