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+====== Individual sprite 'boxes' for collision detection ======
+by Achim
+The regular way of detecting a sprite to sprite collision is to define a box around the sprite, which means you add an offset to the sprite's y and x values to get y2 and x2.
+In most cases the '[[base:simple_software_sprite_to_sprite_collision|one size fits all]]' approach will work, as long as all sprites in the game have a similar size. But with different sprite sizes (e.g. a big space ship and a small bullet) it's better to work with individual boxes for accurate results.
+===== Setup =====
+Set up sprite tables (for eight sprites):
+<code>
+spritey:	.byte	$00, $00, $00, $00, $00, $00, $00, $00
+spritex:	.byte 	$00, $00, $00, $00, $00, $00, $00, $00
+spritemsb:	.byte	$00, $00, $00, $00, $00, $00, $00, $00
+</code>
+The main program will use these tables to move the sprites and the irq will read the values to update the VIC registers every frame.
+Next step is an offset table with four offset values for figuring out y1, y2, x1 and x2.
+Here's an example sprite:
+{{:base:spritebox.gif?200|}}
+Spritey and spritex declare the upper left corner of the sprite.\\
+y coordinates of red box: y1= y+3px and y2= y1+12px\\
+x coordinates of red box: x1= x+8px and x2= x+16px  (use x again to figure out x2 for easier msb handling.)
+<code>
+offsettable:	.byte	03, 12, 08, 16,		//the two y and two x offset values
+	                ...    			//and so on for all sprites...
+</code>
+Finally you need tables for y1, y2, x1 and x2:
+<code>
+spritey1:	.byte	$00, $00, $00, $00, $00, $00, $00, $00
+spritex1:	.byte 	$00, $00, $00, $00, $00, $00, $00, $00
+spritemsb1:	.byte	$00, $00, $00, $00, $00, $00, $00, $00
+spritey2:	.byte	$00, $00, $00, $00, $00, $00, $00, $00
+spritex2:	.byte 	$00, $00, $00, $00, $00, $00, $00, $00
+spritemsb2:	.byte	$00, $00, $00, $00, $00, $00, $00, $00
+</code>
+===== Calculation =====
+In this example the y-reg has to point at the correct position of 'offsettable'
+and x-reg at the correct position of 'spritey', 'spritey1' etc.
+<code>
+		lda spritey,x             //fetch y
+		clc
+		adc offsettable,y         //add first offset for y1
+		sta spritey1,x
+		adc offsettable+1,y       //and second offset for y2
+		sta spritey2,x
+		lda spritex,x
+		clc
+		adc offsettable+2,y
+		sta spritex1,x
+		lda spritemsb,x
+		adc #$00
+		sta spritemsb1,x
+		lda spritex,x
+		clc
+		adc offsettabel+3,y
+		sta spritex2,x
+		lda spritemsb,x
+		adc #$00
+		sta spritemsb2,x
+</code>
+This has to be looped for all sprites.
+===== Detection =====
+The detection is similar to the 'one size fits all' approach mentioned
+above. Let's assume the first slot (in spritey1, spritey2 etc.) is reserved for the player's
+sprite, the other slots are used for enemy sprites.
+<code>
+	ldx #$00
+loop:	lda spritey2
+	cmp spritey1+1,x
+	bcc skip		//player above enemy
+	lda spritey2+1,x
+	cmp spritey1
+	bcc skip		//enemy above player
+	lda spritex1+1,x
+	sec
+	sbc spritex2
+	lda spritemsb1+1,x
+	sbc spritemsb2
+	bcs skip		//enemy on player's left
+	lda spritex1		//player on enemy's right?
+	sec
+	sbc spritex2+1,x
+	lda spritemsb1
+	sbc spritemsb2+1,x
+	bcc hitbysprite		//no, boxes hit each other
+skip:	inx
+	cpx #$07
+	bne loop
+	rts
+hitbysprite:
+	inc $d020
+	rts
+</code>
+Using the 2*x trick makes life easier: Delete all msb tables, use 8bit additions to figure out x1 and x2 and use comparisons instead of subtractions for collision detection.