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[Solved] 2D transparent sprite animations leaving residue pixels behind if they're intersecting each other.

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#1 asperatology   Members   


Posted 13 October 2013 - 06:26 PM

I have created a 2D spritesheet consisting of a character drawn in 3 frames for sprite animation. While drawing (or blitting) the sprite twice or more, near each other in proximity, the transparent portion of the sprite frames overlaps the already-drawn sprite behind it, causing it to leave behind pixel residues.

My main goal is to draw (or blit) 2D sprites with animations multiple times without overlapping one another. This character's main role in the game is act as a unit that the player can call upon multiple times, so overlapping and erasing a portion of the sprite's bitmap is not a good thing.

Here are some pictures to depict what I meant:

These are two units near each other, and they both can be animated.


This is what happens when they are near each other, when the correct way is to not erase the overlapped portions of the unit.


And here is the drawing code:
@Override	public void render(int[] screen) {		for (int yy = 0; yy < h; yy++) {			int y0 = yy + yPosition;			if (y0 > GameComponent.HEIGHT || y0 < 0)				continue;			for (int xx = 0; xx < w; xx++) {				int x0 = xx + xPosition;				if (x0 > GameComponent.WIDTH || x0 < 0)					continue;				int src = (yy + column * 16) * spriteSheetH + (xx + row * 16);				int tgt = (y + y0) * GameComponent.WIDTH + (x + x0);				int col = this.pixels[src];				if (col != screen[tgt])					screen[tgt] = col;			}		}	}
Any way to fix this? Any tips you can give me? Thanks in advance.

Edited by tom_mai78101, 17 October 2013 - 02:08 PM.

#2 Pink Horror   Members   


Posted 13 October 2013 - 09:56 PM

Where in your code is the part that is supposed to blend two colors together? All I see is an int from 'pixels' overwriting an int from 'screen'.


One tip I would give you is to use a debug background like bright pink, so it is clear when you are or are not overwriting the background.


How do you mark a pixel as transparent, anyway? Is it meant to be a key color or do you have an alpha value? Whatever it is, because you are rendering by writing the copy yourself, if you want it to blend, you have to do the math in your code.


(If there is anything in there that is meant to handle transparency and I overlooked it, I apologize in advance.)

Edited by Pink Horror, 13 October 2013 - 09:57 PM.

#3 asperatology   Members   


Posted 13 October 2013 - 11:17 PM

1. There are no blending colors in the code.
2. The integers stand for the ARGB values, in respective color codes.

0xFF000000 means the color black with an alpha value of 0xFF.

I handle direct image manipulation in just this code alone, nothing else was made or done, including the updates.

The background color is set to dark navy blue, which is what you are seeing in the image. The background for the image itself is white. I just added the alpha vajue by bitwising OR the color white with the minimum alpha value, so the white is shown as transparent.

That is all, using Android to post.

#4 haegarr   Members   


Posted 14 October 2013 - 01:05 AM

Alpha blending is a process where the source image has a multi-level mask (just the so-called "alpha") that is used to weight the original target color and the source color to yield in the new target color. Assuming that the alpha value of 255 means full opaque source color and a value of 0 means full transparent source color, then it looks like so:

   colorTgt := ( 1 - alphaSrc / 255 ) * colorTgt + ( alphaSrc / 255 ) * colorSrc

Here "color" means each of the R, G, B values separately.


In your case it seems that alpha is either full opaque or full transparent. Setting this into the formula above shows that

   colorTgt(alphaSrc = 0 ) = ( 1 - 0 / 255 ) * colorTgt + ( 0 / 255 )* colorSrc = colorTgt

   colorTgt(alphaSrc = 255 ) = ( 1 - 255 / 255 ) * colorTgt + ( 255 / 255 )* colorSrc = colorSrc

so that the piece of code can be written as:
int col = this.pixels[src];
if ((col&0xFF000000) != 0) {
   screen[tgt] = col;

This is called "alpha masking". The above code snippet assumes that the alpha is stored in the most significant bits of the pixel.


Doing actual alpha blending (instead of alpha masking) requires some caution be get it free of artifacts.

Edited by haegarr, 14 October 2013 - 01:08 AM.

#5 asperatology   Members   


Posted 17 October 2013 - 02:08 PM

Solved the problem by continuing the loop.

I am on Android, which doesn't allow me to format the text here. Basically, I do this:

1. Check to see if pixels[src] is 0, or check for specific value conditions (alpha is 0 or 255, etc.).
2. If true, continue. Else, set pixel[tgt] as pixel[src].

Thanks for taking your time.

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