By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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Original Blog Entry
Static images that just slide around the screen work OK, but we can do better—so for this update we'll add a couple new images so we can animate the players as they run around the arena.
While you can have as many frames of animation as you'd like, the code is most efficient if the number of frames is a power of 2 (2, 4, 8, 16, etc). The code that cycles through 4 frames is this:
Example: inc frame lda frame and #3 ; limits values from 0-3, if A was 4 it becomes 0 after the and sta frame
Just change the #3 to #7 (8 frames), #15 (16 frames) and so on. If you wanted to cycle through a non-power of 2 count, say for example 5 frames, the code would look like this:
Example: ldx frame inx cpy #5 bne save ldx #0 save: stx frame
For Collect we're going to use 4 frames. You might be wondering why there's only 3 humanoid images—it's because we're going to use 1 of the images twice:
To animate the players, we'll need to keep track of which frame they're showing, so let's add 2 new RAM variables:
; indexes for player animation sequences Animation0: ds 1 ; stored in $B3 Animation1: ds 1 ; stored in $B4
Then modify PositionObjects so it will animate the images when it preps the variables for the 2LK, but only when the player is in motion:
PositionObjects: ... ; select image to show for Player0 lda ObjectX ; get current X location for player 0 cmp SavedX ; compare with prior X location bne Animate0 ; if different, animate player 0 lda ObjectY ; otherwise check current Y location cmp SavedY ; against prior Y location bne Animate0 ; and animate player 0 if they're different lda #0 ; if X and Y didn't change then select 0, the beq SaveFrame0 ; stationary image, and save it Animate0: inc Animation0 ; increment to select the next frame lda Animation0 ; load it and #3 ; limit to 0-3 (if it was 4, it's now 0) SaveFrame0: sta Animation0 ; save it tax ; Transfer A to X ; Player0Ptr = HumanGfx + HUMAN_HEIGHT - 1 - Y position lda ShapePtrLow,x ; select image as specified in X sec sbc Temp sta Player0Ptr lda ShapePtrHi,x ; select image as specified in X sbc #0 sta Player0Ptr+1 ... ShapePtrLow: .byte <(HumanGfx + HUMAN_HEIGHT - 1) .byte <(HumanRunning0 + HUMAN_HEIGHT - 1) .byte <(HumanRunning1 + HUMAN_HEIGHT - 1) .byte <(HumanRunning0 + HUMAN_HEIGHT - 1) .byte <(BoxGfx + HUMAN_HEIGHT - 1) ShapePtrHi: .byte >(HumanGfx + HUMAN_HEIGHT - 1) .byte >(HumanRunning0 + HUMAN_HEIGHT - 1) .byte >(HumanRunning1 + HUMAN_HEIGHT - 1) .byte >(HumanRunning0 + HUMAN_HEIGHT - 1) .byte >(BoxGfx + HUMAN_HEIGHT - 1)
The code for player 1 is almost the same, though it adds a test so the box image will be displayed for one player game variations:
; select image to show for Player1 bit Players bpl UseBoxImage ; if 1 player game then draw the box lda ObjectX+1 ; get current X location for player 1 cmp SavedX+1 ; compare with prior X location bne Animate1 ; if different, animate player 1 lda ObjectY+1 ; otherwise check current Y location cmp SavedY+1 ; against prior Y location bne Animate1 ; and animate player 1 if they're different lda #0 ; if X and Y didn't change then select 0, the beq SaveFrame1 ; stationary image, and save it Animate1: inc Animation1 ; increment to select the next frame lda Animation1 ; load it and #3 ; limit to 0-3 (if it was 4, it's now 0) SaveFrame1: sta Animation1 ; save it tax ; Transfer A to X .byte $2C ; $2C = BIT with absolute addressing, trick that ; causes the ldx #4 to be skipped over UseBoxImage: ldx #4 ; select the Box Image ; Player1Ptr = BoxGfx + HUMAN_HEIGHT - 1 - Y position lda ShapePtrLow,x ; select image as specified in X sec sbc Temp sta Player1Ptr lda ShapePtrHi,x ; select image as specified in X sbc #0 sta Player1Ptr+1
It works, but the players move so fast they look Superman running on a greased treadmill. See collect_20140714_toofast.bin in my blog entry. To fix that, we'll revise it to use an image over multiple frames. For testing, we'll make the left player use each image for 2 frames while the right uses each image for 4:
Animate0: inc Animation0 ; increment to select the next frame lda Animation0 ; load it and #7 ; limit to 0-7 (if it was 8, it's now 0) SaveFrame0: sta Animation0 ; save it lsr ; divide by 2 for 0-3 - this means we show the same ; image twice in succession tax ; Transfer A to X ... Animate1: inc Animation1 ; increment to select the next frame lda Animation1 ; load it and #15 ; limit to 0-15 (if it was 16, it's now 0) SaveFrame1: sta Animation1 ; save it lsr ; divide by 4 for 0-3 - this means we show the same lsr ; image four times in succession tax ; Transfer A to X
See collect_20140714_speedtest.bin in my blog entry.
Both look OK, though I think the left player looks a little better, so the final version will use each image twice. One minor thing happens when the game is over—if the players were in motion, the animation keeps on going even though the players are no longer in motion.
To fix that, we'll add a Game Over check (same logic was added for Player1) that will select the stationary image:
; select image to show for Player0 bit GameState bpl StopAnimation0 ; if game is inactive, stop animation lda ObjectX ; get current X location for player 0 cmp SavedX ; compare with prior X location bne Animate0 ; if different, animate player 0 lda ObjectY ; otherwise check current Y location cmp SavedY ; against prior Y location bne Animate0 ; and animate player 0 if they're different StopAnimation0: lda #0 ; if X and Y didn't change then select 0, the beq SaveFrame0 ; stationary image, and save it
The ROM and the source are at the bottom of my blog entry.
Other Assembly Language Tutorials
Step 14: Add Animation
This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).
This book only assumes a working knowledge of BASIC. It was designed to speak directly to the amateur programmer, the part-time computerist. It should help you make the transition from BASIC to machine language with relative ease.
The 6502 Instruction Set broken down into 6 groups.
Nice, simple instruction set in little boxes (not made out of ticky-tacky).
This book shows how to put together a large machine language program. All of the fundamentals were covered in Machine Language for Beginners. What remains is to put the rules to use by constructing a working program, to take the theory into the field and show how machine language is done.
An easy-to-read page from The Second Book Of Machine Language.
A useful page from Assembly Language Programming for the Atari Computers.
Continually strives to remain the largest and most complete source for 6502-related information in the world.
By John Pickens. Updated by Bruce Clark.
Below are direct links to the most important pages.
Goes over each of the internal registers and their use.
Gives a summary of whole instruction set.
Describes each of the 6502 memory addressing modes.
Describes the complete instruction set in detail.
Cycle counting is an important aspect of Atari 2600 programming. It makes possible the positioning of sprites, the drawing of six-digit scores, non-mirrored playfield graphics and many other cool TIA tricks that keep every game from looking like Combat.
Atari 2600 programming is different from any other kind of programming in many ways. Just one of these ways is the flow of the program.
The "bankswitching bible." Also check out the Atari 2600 Fun Facts and Information Guide and this post about bankswitching by SeaGtGruff at AtariAge.
Atari 2600 programming specs (HTML version).
Links to useful information, tools, source code, and documentation.
Atari 2600 programming site based on Garon's "The Dig," which is now dead.
Includes interactive color charts, an NTSC/PAL color conversion tool, and Atari 2600 color compatibility tools that can help you quickly find colors that go great together.
Adapted information and charts related to Atari 2600 music and sound.
A guide and a check list for finished carts.
A multi-platform Atari 2600 VCS emulator. It has a built-in debugger to help you with your works in progress or you can use it to study classic games.
A very good emulator that can also be embedded on your own web site so people can play the games you make online. It's much better than JStella.
If assembly language seems a little too hard, don't worry. You can always try to make Atari 2600 games the faster, easier way with batari Basic.
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