SOURCE CODE ()

	' We suggest the use the most coloured
	' resolution (up to 16 colors).
	
	BITMAP ENABLE(16)
	
	' This is the number of stars to be displayed.
	' Currenly there is a maximum number of 32 sprites multiplexed.
	
	CONST starCount = 24

	' This data structure will hold, in one place, the information 
	' related to each individual star we'll draw. Specifically, 
	' we'll keep the (x,y) coordinates, the animation frame 
	' (frame), and the reference to the multiplexed sprite 
	' displayed (sprite).
	
	TYPE starType
		x AS POSITION
		y AS POSITION
		sprite AS MSPRITE
		frame AS BYTE
	END TYPE
	
	' This directive allows you to disable the control over image
	' height constraints, where they are present. In fact, the
	' conversion of images from the modern format to the native format
	' of the target must guarantee some specific constraints. However,
	' where it is ensured that the dimensions of the graphic resources
	' are consistent, it is possible to disable this type of control. 
	' This is in order to be able to load graphic resources that do not 
	' need to respect these constraints, such as resources linked 
	' to sprites.
	
	DEFINE IMAGE FREE HEIGHT
	
	' This pragma allow to delay the refresh of the changes made
	' to multiplexed sprites. They will be stored and reflected 
	' on the screen only when the instruction MSPRITE UPDATE 
	' will be executed.
	
	DEFINE MSPRITE ASYNC
	
	' Let's change the border and background color.
	
	COLOR BORDER BLUE	
	CLS BLUE
	
	' Let's wait for a key press. This trick will
	' allow to initialize the TIMER variable to a really
	' random value, that depends on the key press delay.
	
	INK WHITE
	VHCENTER "PRESS ANY KEY"
	WAIT KEY RELEASE
	
	' Let's initialize the random number generator
	' using the time passed from the execution start.
	
	RANDOMIZE TIMER
	
	' Load from the host the (degrated quality of the) image 
	' of the opening titles of the videogame.
	'
	' Note that the original image has been converted to 
	' match at best each target, and moved the result into 
	' each folder with the same target name. In particular:
	'
	'	c64 			: 160x200 pixel, 16 colors
	
	background := LOAD IMAGE("vdg_orville.png")

	' Load from the host the (degrated quality of the) image 
	' of the animated sprites of the rotating stars.
	
	star := LOAD ATLAS("vdg_orville_star.png") FRAME SIZE (24,21)

	' Finally, load from the host the music track, in MIDI
	' format. It will be converted to the native format, as well.
	 
	music := LOAD MUSIC("vdg_orville.mid")

	' Let's create an array that will hold separate 
	' information for each star.
	
	DIM stars(starCount) AS starType
	
	' Now let's initialize each individual star...
	
	FOR i = LBOUND(stars) TO UBOUND(stars)
	
		' The horizontal starting position is from 0 to 319.
		
		stars(i).x = RND(320)
		
		' The vertical starting position starts from 52 
		' and go down of 8 pixel for each star. Note that
		' the vertical screen resolution could be not high
		' enough to keep all the sprites. 
		
		stars(i).y = 52+i*8
		
		' Assign the sprite resource to this sspecific star.
		' We use the IMAGE( ) function to extract the image
		' from the loaded ATLAS (using the FRAME 0 for the 
		' first one). Then, we use the MSPRITE instruction to
		' convert the bitmap into a sprite resource, and using
		' the IGNORE COLOR LIGHT GREEN we are saying that
		' that color will be used as "transparency" for the
		' sprite.
		
		stars(i).sprite = MSPRITE( IMAGE( star FRAME 0 ) IGNORE COLOR LIGHT GREEN )
		
		' Each star starts from a random frame (0..3).
		
		stars(i).frame = RND(4)

		' We can enable the sprite.
		
		SPRITE stars(i).sprite ENABLE
		
	NEXT i
	
	' Put the titles on the screen.
	
	PUT IMAGE background AT 0, 0
	
	' Let's start the music.
	
	MUSIC music
	
	' This is an infinite loop, which will update 
	' the stars' positions at each cycle.
	
	DO
	
		' For each star inside the array...
		
		FOR i = LBOUND(stars) TO UBOUND(stars)
		
			' Increment the horizontal position, up to the
			' end of the screen.
			
			ADD stars(i).x, 1+RND(4)
			IF stars(i).x > 350 THEN stars(i).x = 0
			
			' Based on the current frame, we are going
			' to convert the bitmap from the specific
			' frame of the ATLAS into a sprite. Note that
			' we are going to indicate the previous
			' sprite, in order to "replace" the image.
			
			SELECT CASE stars(i).frame
				CASE 0
					stars(i).sprite = MSPRITE( IMAGE( star FRAME 0 ), stars(i).sprite IGNORE COLOR LIGHT GREEN )
				CASE 1
					stars(i).sprite = MSPRITE( IMAGE( star FRAME 1 ), stars(i).sprite IGNORE COLOR LIGHT GREEN )
				CASE 2
					stars(i).sprite = MSPRITE( IMAGE( star FRAME 2 ), stars(i).sprite IGNORE COLOR LIGHT GREEN )
				CASE 3
					stars(i).sprite = MSPRITE( IMAGE( star FRAME 3 ), stars(i).sprite IGNORE COLOR LIGHT GREEN )
			ENDSELECT
			
			' Increment the frame, up to the
			' end of the ATLAS' frames.
			
			INC stars(i).frame
			IF stars(i).frame = FRAMES( star ) THEN stars(i).frame = 0

			' Move the star to the specific position.
			
			SPRITE stars(i).sprite AT stars(i).x, stars(i).y
			
		NEXT i
		
		' Update every sprite at once.
		MSPRITE UPDATE
		
	LOOP