Saturday, 29 July 2017


I might be slow, but at least I'm inconsistent. A few weeks ago, I thought, "I know, I'll read Stephen King's Dark Tower novels again, seeing as I'm a bit of a fan and the movie is coming out soon. It won't take long; There are only eight books, and probably not much more than 4000 pages." The next thing I'm aware of is struggling to remember the password to get back into this blog.

In the meantime, Ciaran Anscomb has been working hard and has got his excellent Dunjunz conversion ready for release. This is an achievement to be proud of as finishing a project can sometimes be the hardest part. Check it out; it's everything a game should be.

Right, I can't put it off any longer. Strap in and prepare yourselves for some awful puns. Truly awful.

Let there be sprite

As this is a shoot-em-up, we need cannon fodder to shoot at, enemies flying around and suchlike, and for that we will need some sprites.

I'm not sure of a precise definition for a sprite, but perhaps a reasonable description would be a graphical object that moves around separately to the background. It's a bit like a tile, but with a life of its own and behaviour that depends on what type of object is being represented.

This suggests that a sprite should have variable and constant data associated with it in addition to the image data. For example I've currently got the following pieces of information making up a sprite:

  • Pointer to image data
  • Screen coordinates
  • Velocity
  • Scoring and sound effect information
  • Pointers to routines that implement behaviour

The image data pointer can be constant or changed frame by frame to give animation. There are more than a few ways of drawing sprites, each with a different trade-off between memory use and speed. I would like to draw at arbitrary pixel coordinates which means bit shifting will be involved. At the slow end of the spectrum we could pixel shift the image data as it's being drawn, or at the fast end we could have pre-compiled sprites, where the image data is embedded into immediate instructions.

I've opted for a middle-of-the-road method that is based on pre-shifted image data. By that I mean the four possible bit-shifted versions of the image already exist in memory at the time of drawing.

I've chosen a sprite size of 12 x 12 pixels, requiring 3 x 12 = 36 bytes per image. When converted to the pre-shifted form, it takes 48 x 4 = 192 bytes, because an extra byte of width is needed to fit a shifted image, and we need four images, each with a different amount of shift. The extra memory used is the price paid to gain some speed.

Simply copying the sprite data onto the screen isn't going to work very well in this game, because the background would be overwritten by the parts of the sprite image that should have remained transparent. A way to fix this is to use a mask i.e. an image that is ANDed onto the screen to make a hole into which we can OR the sprite image.

A set of pre-shifted masks and images

Unfortunately, using logical operations means we are restricted to using the a and b registers to transfer the image data to the screen, though a small optimisation is to use ADD instead of OR, allowing us to combine two 8 bit operations into one 16 bit operation. Drawing one row of sprite looks a bit like this:

    pulu d     ; get two bytes of mask
    anda ,x    ; AND with screen
    andb 1,x   ;
    addd 46,u  ; OR with two bytes of image data
    std ,x     ; store onto screen
    pulu d     ; get next two bytes of mask
    anda 2,x   ; etc
    andb 3,x
    addd 46,u
    std 2,x
    leax 32,x  ; move to next screen row

The u register is pointing to the image data which is made up of 48 bytes of mask followed by 48 bytes of sprite. x is pointing to where we need to draw in the screen buffer. Nothing too exciting going on, but it gets the job done.

Sprite place at the sprite time

To determine where to draw the sprite, we need to calculate a screen buffer address from the coordinates. To simplify the address calculation, I'm using scaled coordinates. The x-coord is scaled by 64 and the y-coord is scaled by 32. That means I can form an address just by throwing away the bottom five bits of the y-coord, adding the top byte of the x-coord, then adding the screen buffer base address:

    ldd SP_YORD,y  ; y part of coord
    andb #$e0      ; remove sub-pixel bits
    adda td_fbuf   ; screen start address
    tfr d,x
    lda SP_XORD,y  ; x part of coord
    leax a,x

The y register is pointing to the sprite variables and the calculated address is returned in x. Note that I'm only adding the top eight bits of the buffer base address. This is fine as long as the bottom eight bits are zero.

That gives us the address, but we also need to select one of the pre-shifted frames to show the sprite in the correct pixel column. We need to determine the shift value 0-3 and then multiply that by 96 to give us an offset to add to the sprite image address.

Noting that the shift value is contained in bits six and seven of the x-coord, i.e. already multiplied by 64, we can get the required result by multiplying those bits by 1.5 (Because 64 x 1.5 = 96):

    ldb SP_XORD+1,y
    andb #192
    leau d,u
    leau b,u

Having to use clra and leau d,u is a bit unfortunate and is there to avoid interpreting the top bit of b as a sign bit. One day I may look into re-arranging the pre-shifted data and replace the last four instructions with the more efficient:

    leau b,u
    leau b,u

Or even better, if the registers can be re-allocated without incurring a penalty somewhere else:


The speed of sprite

Movement is achieved very simply by regularly adding a velocity to the coordinates. The moving background makes things slightly more interesting because we want to specify the sprite velocity relative to the background, not the screen. The way to deal with that is to add the background velocity to the sprite velocity. The background velocity is determined once per frame and affects all moving objects equally. The coord update looks like this:

    ldd SP_XORD,y   ; horizontal component
    addd SP_XVEL,y  ; sprite horizontal velocity
    addd scroll_x   ; background horizontal velocity
    std SP_XORD,y

    ldd SP_YORD,y   ; vertical component
    addd SP_YVEL,y  ; sprite vertical velocity
    addd scroll_y   ; background vertical velocity
    std SP_YORD,y

A Hard Day's Sprite

I'd imagine that most people who have experimented with software sprites will have seen what happens when a sprite goes off the edge of the screen. Going off the left or right edges usually means harmlessly reappearing on the other side, but going off the top or bottom can spell disaster as the sprite has moved out of display memory and possibly into memory being used for something else. It's something to be avoided.

In some games the issue can be avoided by ensuring the sprite coords always remain within bounds. However, in my game I would like sprites to cross the screen edges, moving in and out of view just as the background does. That means the sprites must be clipped at the screen edges.

In ROTABB, I had a screen buffer that was larger than the viewport. The sprites were rendered into the buffer and then the central portion of the buffer was copied to the screen, automatically clipping any sprites overlapping the edges of the viewport. Unfortunately, that doesn't sit very well with my new scrolling engine which relies on being able to copy big chunks of data without worrying about where the screen edges are. Plus I need to at least try to keep memory use down, meaning the buffers should be no larger than necessary. I'm going to have to clip sprites the hard way.

Clipping at the bottom of the screen isn't too bad. If the y-coord is greater than the screen height less the sprite height, then the sprite needs to be partially drawn. Subtracting the y-coord from the screen height in fact gives us the number of sprite rows to draw. If it's zero or negative then the sprite is completely off screen and doesn't need drawing.

Clipping at the top is a little more complicated in that we need to calculate how many rows to draw and we also need to calculate an offset for the image data, because the drawing no longer starts at the top row of the sprite. If the y-coord is negative then we need to clip. If it is less than or equal to the negated sprite height then the sprite is off screen. Negating the y-coord tells us how many rows of image to skip, and subtracting that value from the sprite height tells us how many rows to draw.

Sprite of hand

Clipping at the left and right edges is something altogether different. We need to selectively draw one, two, three or four bytes wide, depending on how much of the sprite is visible. I'm happy to say that Steve Bamford had already figured this one out and shared his clever solution involving a lookup table. All we need to do is take the horizontal offset of the screen address and use this to look up the address of a sprite drawing routine from a table. (Taking care to treat the offset as a signed value, because it becomes negative as the sprite crosses the left edge.)

The different sprite routines look after drawing the different widths of sprite and not a single x-coord comparison is required, because the lookup table can be book-ended with addresses that point to a routine that deals with the sprite moving completely off screen. Very elegant. Thanks, Steve!

A sprite for sore eyes:
Clipping in action on all four sides of the screen

Sprite at the end of the tunnel

I've got some more distractions coming up in August but hopefully it won't be too long until the next post, even though the path of the spriteous man is beset on all sides by the inequities of life and the tyranny of work. Or something.

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