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How to draw a "trail" on a Canvas using a Bitmap


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#1 Recon12   Members   -  Reputation: 103

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Posted 01 April 2014 - 02:46 PM

Hello,

 

I am programming a game and right now I am currently animating a sprite that is moving up and down the screen. After the sprite is drawn on screen, I am to draw a permanent bitmap in place of where the animated sprite just was, sort of like a trail. However, I'm not sure how to permanently draw unto a Canvas. I want the character who is moving down the screen to leave a path behind him that is drawn from a bitmap. Is there any way to do this? I am programming using the Android SDK with Java on Windows 7.



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#2 ferrous   Members   -  Reputation: 3107

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Posted 01 April 2014 - 06:23 PM

Two main ways to do this spring to mind.

 

The easy, cheezy way is to keep an array of previous positions, and then just loop and draw the sprite at those positions at decreasing opacity.  You have to manage the indices to start from, but it's pretty simple stuff.  If you're sprite is animated, you have to draw it at the frame at which it was captured at, though you can probably get away without doing that.  If you've ever played any of the old ninja gaiden games, you've seen this method.  Or the Mouse trails setting on a windows PC.

 

A slightly more technical way is to keep a bitmap, and draw on it, and also draw it at decreasing opacity.  This is basically a form of motion blur, and if you search using those terms you should see some techniques for it.


Edited by ferrous, 01 April 2014 - 06:24 PM.


#3 Stainless   Members   -  Reputation: 1476

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Posted 02 April 2014 - 01:53 AM

A third technique is to keep a bitmap as above, draw a full screen quad over the top with an alpha blend, then draw the new sprite in.

 

The end effect is the trail will gradually fade out.

 

You could also play with other effects.

 

Have two render targets old and new. Clear the new one, draw the old one onto the new one, but one pixel smaller or bigger (feedback effect) then add the new trail. Display the new one and then swap.

 

Try alpha blending with a colour instead of white.

 

Rotate the old image one degree

 

Run a blur filter over it

 

Loads of ideas you can play with.


Edited by Stainless, 02 April 2014 - 01:54 AM.


#4 Glass_Knife   Moderators   -  Reputation: 7246

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Posted 02 April 2014 - 07:48 AM

Two main ways to do this spring to mind.

 

The easy, cheezy way is to keep an array of previous positions, and then just loop and draw the sprite at those positions at decreasing opacity.  You have to manage the indices to start from, but it's pretty simple stuff.  If you're sprite is animated, you have to draw it at the frame at which it was captured at, though you can probably get away without doing that.  If you've ever played any of the old ninja gaiden games, you've seen this method.  Or the Mouse trails setting on a windows PC.

 

A slightly more technical way is to keep a bitmap, and draw on it, and also draw it at decreasing opacity.  This is basically a form of motion blur, and if you search using those terms you should see some techniques for it.

 

It sounds like this is what they are looking for.  If you create a BufferedImage and draw to it every frame with the position of the sprite, but you don't clear the old image from the image, then it will leave a trail or previous images.  This doesn't work on the Canvas because the screen is cleared every frame.

 

Come to think of it, the really hacky way would be to just override the paint() method and not call super.paint() which would keep the graphics area from being cleared.  But I don't think that's what the assignment is trying to teach.


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#5 ferrous   Members   -  Reputation: 3107

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Posted 02 April 2014 - 11:51 AM

A third technique is to keep a bitmap as above, draw a full screen quad over the top with an alpha blend, then draw the new sprite in.

 

The end effect is the trail will gradually fade out.

 

 

Actually, for my second technique, I was thinking of the first technique you mentioned here, but you explained it better than me.

 

 

Though I guess the OP didn't mention if they wanted to have things gradually fade, that was an assumption on my part.


Edited by ferrous, 02 April 2014 - 11:53 AM.





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