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This is my first post, so I apologise in advance if I have done anything wrong here in asking my question. I've looked all over the net for a specific answer, but can't find one, so here goes.....

I'm writing a game based on Surfaceview and so far, all is going well, however, I want to move my main sprite for example by 1 pixel on a 160DPI screen as a baseline (so basically 1 DIP as 1 pixel = 1 DIP on a 160DPI screen correct?)

I'm using the using the following forumla:

private static final float spritemovestep = 1f;
final float scale = getResources().getDisplayMetrics().density;
MoveX = (int) (spritemovestep * scale + 0.5f);

And then... something like

SpriteX=SpriteX+MoveX

First question - is this correct?

If it is, can someone explain what the +.05f is actually for, I've read that it's to 'round up to the nearest number' but....

if spritemovestep = 1, then on a 120DPI screen (which returns .75 as the scale I think) it would work out as: 1 x .75 + .5? which would be 1.25? So what is the .5 for?

Also what is the result when it's cast to an int value?

On some, the final result seems to be '0' on a low density screen so the sprite isn't moving at all.

Also some sprites which are supposed to be moving at different speed are moving at the same speed at certain densities.

I'm sure I'm being silly and missing something here but I just can't understand how this is supposed to work. If I want to move my sprite by 1 DIP/physical pixel on a MDPI screen how can it move less than 1 pixel on a LDPI screen?

Also, what is this formula I keep seeing:

px = dp * (dpi / 160) - When is this used?

Would really appreciate if someone could answer my questions.

Thanks all

skaffman
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Mooonbuggy
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1 Answers1

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The +0.5f is to round up to the nearest nukber, as you said. Ideally, when the number is scaled down for ldpi, a value of 1 becomes 0.75, which, when cast to an int is expressed as less than 1 or ~=0. By adding the rounding figure, this number is raised to 1.25 which, when cast as an int yields <2 or ~=1. This way, your sprite should be drawn with a minimum movement of 1. The only reason sprites that move at different speeds would move the same speeds is if they are so close that, when rounded, they wind up being the same size using the scale you gave. Altogether, your equation is very similar to others ive seen. Im making a game that uses surfaceview for the company i work for as well, and while i cant go into details on the code, your issue is one that i struggled with for sone time. Im not sure how your physics updates, but perhaps thats something you should check into, specifically, how it counts ticks for your game timer. It may be that your application is reading its ticks as being too close together to reach the point where it would hit the point of moving the 1.25 or 1 after casting to int, and therefore your sprite appears not to move. I briefly experienced that problem and at first was looking at my velocity until i found that the error was in the timer. One other thing i noticed is that your algorithm collects the density. On a mdpi device, does this return 1 or 160? That could make a big difference, but im not sure, as the equation i used was different. The other equation you found is a paraphrase of the equation listed in the development guide at android.developer.com to describe how the os converts pixels into dip. The reson people tend to quote that is to provide a reference to help others build their own algorithm for scaling appropriate for the jeeds of their app. Hopefully that helps, as its really the best answer i can give at this time. Sorry for any typing errors, im sending this from my phone

thisguy
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