Scratch

There are many different implementations of gravity for different games.
Having one built-in would reduce the flexibility in designing new games.

No there should not be a gravity button. People should learn how to implement gravity themselves. Scratch is simple enough with out it having every script premade for you.

I've never used the "if on edge, bounce", because I rarely use turtle graphics (I use glide, go to, and change x,y much more than I use "move n steps").  The block has never made much sense to me.  It is one of those that should be relegated to an optional library so that people could see how it is defined.  (I'm still not sure exactly what its definition is---I know if changes the direction, but I'm not sure what algorithm it uses for choosing the new direction.)

you can program an if on edge, bounce yourself? Why it is pre-programmed in a block?

Its a command for young (at heart) minds :-)

To make a sprite "bounce" (it actually implements a wave type reflection) off the sides without having to do things like:-  if on edge - if (top or bottom edge) - turn in (direction-180) - else turn in (180-direction) - endif -move 10 steps - endif.

(Code just for illustration - I can't remember which way round you have to code top/bottom vs left/right edge detection !  :-)  )

I REALLY REALLY want it expanded so you can "bounce" off any sprite and not just the edge but every time I vote for it Kevin votes against, so I can never get enough support :-)

One day, he'll come round :-)

regards

Simon
PS Jens - your on my Christmas card list this year :-)

Last edited by SimpleScratch (2007-12-13 01:48:39)

Bouncing correctly off a sprite is not as easy as bouncing off an edge, because doing it right depends on the shape and orientation of the sprite.  If you are interested in seeing some code for bouncing off of straight lines, circles, and the insides of circles, see the School Pinball game:  http://scratch.mit.edu/projects/kevin_and_abe/54377

The bounce is done by the ball, but it has to get orientation information from the object it is bouncing off of.  There are comments in the code (thanks to version 1.2 beta) that explain the variables, messages, and methods.

As Kevin mentioned, "Gravity" may mean different things to different Scratchers, so a Gravity "button" doesn't really make sense. 

Gravity can mean acceleration in negative y (down on the screen) or it might mean acceleration toward another sprite.  To simulate the gravity of a planet, the programmer would want gravity to vary (inversely) with the square of distance to the planet.  Also, depending on what is desired, Gravity would likely only act on certain sprites in a game or simulation.  The makes the meaning (and usefulness) of a Gravity "button" questionable.

For ideas on implementing "gravity", you would do best to search in the galleries or projects.  If you are stumped, post a project that you'd like to add gravity to, and descibe what you would like gravity to do.  That will allow someone to download your project and help you out.

I've done several projects with gravity, below are a couple of projects.  Note that there are other ways to program gravity, I'm posting these links to illustrate two different "meanings":

http://scratch.mit.edu/projects/EdnaC/48110

http://scratch.mit.edu/projects/EdnaC/37711

-Mr Ed

Maybe it will cause problems in the corners of the scratch field...

Yes, allowing top.bottom,etc would be good. You can program it yourself, (i've done it in a flash game, you have to calculate in what corner of the screen you are, if touching edge), but the script for that was (at that moment) very hard for me to find out.

Hmm... Ofcourse, if I am programming a flash game, i really think t's hard enough    So I want to be sure my game is working well with that objects, i don't want to be sure every mentionable shape would work 

I want both—for kids to enjoy Scratch, doing things quickly, and for them to learn how to do it themselves and not have to rely on pre-written code.

I'm looking forward to the day when Scratch has libraries of user-definable blocks, and only a bare minimum of primitives, so that kids can see how magic like "if on edge, bounce" can be done.

Definable blocks have been near the top of the request list for some time.  I believe that the Scratch team is looking into how to design a good user interface for them for some time, but I don't think that they have started coding anything for them yet.  (I'm not on the Scratch team, so I'm just guessing about what they are doing.)