While trying to get 3 stars on the more complex levels (the ones with a lot of destructible entities) I noticed that even after playing the same level over 50 times, doing always the same thing, I've never seen the same final result. Something always goes differently.

Does the same input always result in the same output, or is there a random factor in there?

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    How can you be sure the input you're giving is 100% identical?
    – user9983
    Commented Dec 29, 2011 at 16:34
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    Perfect input can be given via emulation of a phone, special programs or, simply, the desktop version of the game with the mouse, as agent86 has done.
    – Zelda
    Commented Dec 29, 2011 at 17:47
  • regarding "input", most fingers are bigger than 1 pixel wide. I suppose the OS converts your finger big input to a xy-coordinate that may even be float (mean value of the pixels your fingertip covers)
    – tnavidi
    Commented Nov 2, 2023 at 13:02

5 Answers 5


The algorithm is deterministic, but it's arguable that the overall result of launching a bird is effectively non-deterministic because it relies on (extraordinarily sensitive) user input.

To test the algorithm and the effect user input has on it, I used the following procedure:

  • Load the Google Chrome version of Angry Birds
  • Load level 1-1
  • Pull the mouse back to a specific X-Y coordinate. (This was past the point of "full draw" for the bird.)
  • Release the bird, and wait for the game to reach a steady state.

To measure my X-Y coordinate, I opened a background Chrome window, and navigated it to this page. I aligned the two windows so that I could see the X and Y positions of the mouse while still able to cause something to happen when I released the bird. I aligned my mouse cursor with the lower left hand corner of the box around these two numbers, and then released. I noted the position of my mouse cursor then (as the display only updates when the mouse is not released and the cursor is over that window) and then noted the resulting score in Angry Birds. If you're having issues with exact mouse positioning, you might try an accessibility option offered by your OS, for instance, Mouse Keys on Windows.

I got the same score, so long as my cursor was in the exact same spot as it was on previous iterations.

Prior to coming up with an enhanced test rig that allowed me to measure my cursor's position to the pixel, I ran this experiment about 10 times, using a reference point in the background image of the game. I got scores by firing the first bird that varied between 8,000 and 11,000 points. Using the enhanced testing strategy, if I was off by a single pixel, I could expect score differences of anywhere between 150 and 500 points.

I will note that the cursor was beyond the edge of the game area, outside the browser window, far beyond the maximum distance the bird can be drawn back, and I still noticed differences when moving my cursor even a single pixel.

Further, I took screencaps for an extreme case where my cursor was on a different monitor than the game window, and ran several iterations where I kept my cursor in the same location, or moved it slightly. By my calculations (based on measuring distance from screencaps) the game was able to create a different result (score 32960 vs 31520) based on a one pixel difference in height (544 vs 545 pixels) over a distance of approximately 1,284 pixels of width. This works out to be an angular difference of 0.03 degrees.

I'd say that for any practical purposes, you can expect to employ the same general strategy on the same level repeatedly and expect wildly varying results. The level of sensitivity in the controls is so extreme that there's no practical way to get it to do the same thing twice. This is exacerbated by the fact that if you're playing on a touchscreen, it would be extraordinarily difficult to reproduce the same movements down to the individual pixel.

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    If you want to try this experiment, you can install my userscript. It adds boxes that track the exact cursor position to the Angry Birds page. A preview of the script is available here. Commented Dec 29, 2011 at 22:44
  • Dear agent86, you wrote "I got scores by firing the first bird that varied between 8,000 and 11,000 points." Is it possible to run the test with the current version and see if the scores are systenatically higher?
    – Gil Kalai
    Commented Dec 19, 2012 at 23:21
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    @GilKalai You can't compare different levels like that, it entirely depends on where the test was aimed. Commented Jan 4, 2013 at 17:57

Angry Birds uses Box2D library for physics.

Is Box2D deterministic? For the same input, and same binary, Box2D will reproduce any simulation. Box2D does not use any random numbers nor base any computation on random events (such as timers, etc).

However, people often want more stringent determinism. People often want to know if Box2D can produce identical results on different binaries and on different platforms. The answer is no. The reason for this answer has to do with how floating point math is implemented in many compilers and processors. I recommend reading this article if you are curious: http://www.yosefk.com/blog/consistency-how-to-defeat-the-purpose-of-ieee-floating-point.html


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    This is great (thanks for actually checking with the source); for the sake of completeness, do you have a reference that Angry Birds uses Box2D? And where did you pull this quote from?
    – user3389
    Commented Dec 29, 2011 at 20:56
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    @MarkTrapp geek.com/articles/mobile/… and en.wikipedia.org/wiki/Box2D Commented Dec 29, 2011 at 21:06
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    And I bet physics FPS varies as well, due to numerous factors (not enough CPU, overheating, page faults, etc), thus affecting the 'accuracy' of the simulation. :) Commented Dec 30, 2011 at 1:35
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    @agent86 I would change your wording to "[...]but per-platform and per-Box2D release each simulation will run the same way each time." (that "same binary" part in the first paragraph seems to indicate that). Commented Dec 30, 2011 at 18:38
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    @Jake Yes, it is deterministic but with the limitations mentioned in the second paragraph.
    – Pubby
    Commented May 13, 2012 at 5:25

Some levels are subject to random explosions shortly after the level starts, and some bricks can fall on their own before you've ever thrown a bird. These events occur rarely but, by the time you've three-starred every level in every game (as I have!), you'll probably see it a few times.

I would expect that this occurs because of timing issues, possibly related to threading. I'm quite sure that if you loaded every level a dozen times and waited thirty seconds you'd end up with a non-zero score at least once. In light of Pubby's answer, I can only assume that the Angry Birds app does not always provide the same input to the Box2D functions at the start of a level.


I played on Facebook using a macro recorder. I started the recorder, went to angry birds window, then zoomed out, and played the first two birds. I then stopped the recorder. EVERY time, the mouse went to the exact pixels and released with the exact same power etc. It had to, because the mouse movement recorded does the same thing every time, pixel for pixel. Each and every time I got a different score, and not every time the same number of pigs killed etc. This game IS inconsistent


I did 10 tests, pulling to the same point every time (also Chrome Angry Birds, level 1, as in agent86's tests).

Red Crosshair of test spot

I never received the same score twice, despite being careful to pull to the exact same spot, and doing it about as soon after start of level. I believe Angry Birds is very much not deterministic.

This is rather easy to explain: it was designed to work on small touch screens. If it was deterministic, users could easily repeat moves due to the limited precision of the input device. This could be bad, in that their device could literally not have the precision necessary to aim at the exact angle and power they want. By having it slightly jiggle the results, and owing to the input of a 'finger', users would not notice the lack of precision, and they would also not be frustrated by never being able to make a desired shot.

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    I think you're wrong. The touch screen actually makes it extremely hard to get the same input each time. For example, the iphone has 614400 pixels. Each of those would produce a differing output.
    – Daenyth
    Commented Dec 29, 2011 at 18:16
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    @Daenyth: are you sure that the input resolution precision is 1 pixel? It may be, but what I'm saying is that it is not necessarily. Commented Dec 29, 2011 at 20:13
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    @Daenyth not necessarily each of them. Once you're beyond the maximum distance, there may be a set of pixels at the same angle from the center that could reasonably be assumed to cause identical
    – Random832
    Commented Dec 29, 2011 at 22:02
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    @AndreasBonini, Further testing seems to indicate that it's so precise that a single pixel can cause differences in the results, even over crazy distances - see the latest edits to my answer.
    – agent86
    Commented Dec 31, 2011 at 1:29
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    This is an example of chaos (tiny changes in input leading to huge changed in outcome) rather than non determinism Commented Jan 27, 2014 at 13:38

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