In this, part 2, we will be looking at scrolling maps, a pivotal element of top-down 2D computer role play games of yore.
I think something that is lost on many modern gamers, who didn’t grow up in the 80’s. The majority of games on the early 8-bit systems were limited to a single screen of play. Really good games may have multiple screens but when you moved off the edge it would load a new screen.
So it was utterly FANTASTIC to see a game screen that was a view-port on a much larger world. When I first saw Ultima II, I was in total shock. There was (to my viewpoint) no limit to what could be beyond the borders of the screen! It both was thrilling and increased my curiosity and expectations of what the game could have.
A scrolling map was also well beyond what most BASIC languages could do in those days. Some were better than others at high-speed video display, but anything close to full screen was a challenge no matter the platform. So knowledge of assembly language was needed to implement one.
So how to render a map that is bigger than the screen? Let’s dive in…
The first thing to figure out how you are encoding your maps internally. How much memory are you devoting internally to a map? Is the data compressed on disk and must be uncompressed when loaded? How many unique tiles can be present on a map? Are tiles global or specified for the map?
Early Ultima’s like II and III had 64×64 size maps. Both had less than a byte’s worth of tiles (64 and 128) so they would use up 4K of RAM to store the map, uncompressed and using a full byte per tile. Ultima IV uses 32×32 size maps and some clever coding to load a continuous world map. As you walk around, new 32×32 chunks are loaded. This creates some challenging edge cases (literally) when you approach corners and the game will need to load up to 3 new chunks to get the data needed. Ultima V goes to 16×16 chunks for the world map.
The issue with doing continuous map loads on early 8-bit systems is most of them aren’t that efficient with loading data continuously. Until hard drives came along, loading even a few kilobytes from floppy disk could take a second or so. And it got worse on systems like the Commodore 64 where the continuous music would suddenly get stuck on a single note as it loaded fresh data. My own experiments with a “big map” showed the problem, as every time I approached an edge it would take seconds to load fresh data.
So I decided for my own maps to just have singleton maps that are a maximum of 4K in size, and if you left them you’d just load a new one as a self-contained area. For tiles, I have 128, and multiple character sets. So for a world map, there is a “world” tile set that includes mountains and other features only found on world maps. The leftover bit is used as a lighting mechanic; it indicates if this tile is “lit” or not naturally on the map.
The nice thing about a map buffer is it can take any shape you want. Just because I have 4096 tiles doesn’t mean it’s automatically 64×64. By specifying a height and width parameter for each map, I can have them in many different sizes. In practice I found that 32×32 was a decent size for most towns and dungeons. 48×48 was nearly perfect, just big enough to have a lot of interesting areas. 64×64 was almost TOO big, there was a few cases where I split maps into multiple maps because a super large map wasn’t actually ideal, especially if they had a lot of mobs (mobile objects) on them.
Some game maps are compressed on disk so they take less room. With a lot of older CRPG’s this makes good sense; you don’t have a lot of unique tiles and you tend to have long horizontal stripes of them, which screams “compress me!” The most common form of compression is RLE, or run-length-encoding. RLE defines the data as either a singleton (one tile) or a count of whatever comes next of values to repeat. There’s usually also a terminator value as well to indicate map processing should end.
For example, if you have 64 unique tiles, the top two bits could be used to indicate a few different ideas:
- The top two bits are control bits. If both are 0, it’s a terminator for the map data. If 01, this is a singleton tile. If 10, it’s two consecutive tiles (or whatever the most common count of consecutive tiles is in your maps.) If 11, use the tile value as a count and that’s how many of the next tile to produce.
- The top two bits are count bits. Value 0 means it’s a terminator for the map data. Otherwise produce 1-3 of the given tile.
Let’s look at an example, here is a 7×7 map, showing an island. Uncompressed, if you used a full byte per tile, it would take up 49 bytes.
With method #1 above, it would take two bytes for any length of greater than 2 to store. Crunching the numbers, it would take 25 bytes (including a terminator byte) to store. Almost 50% compression!
With method #2 above, while we are limited to a maximum of 3 repeated tiles, it actually comes in at 21 bytes, over 50% compression. Nice! And the algorithm to decompress is a little simpler as well.
Of course, this example only has two tile types, and the map is rather straight-forward. Any CRPG map is likely to have a lot richer of a data set. And there is a point where RLE won’t be as effective. My own maps utilize a lot of “two pair” tiles where they are the same type (grass, for instance) but are in fact different tiles. This breaks RLE, which expects a lot of the same tiles to be repeated in a horizontal line. For such maps, using a more complex pattern-oriented compression technique like LZW and Huffman makes more sense.
The main value of compression is to reduce disk size, though. Is disk size really a problem to solve? In the old days, the answer was unhesitatingly yes. Most 8-bit systems had 160-180K disks, and every disk was an expense to replicate and put in a box. Data compression saved money. In the modern era, though, with digital distribution and USB sticks that hold more memory than the entire production run of a computer system’s RAM added together, it’s not as big of a deal. Even retro enthusiasts these days tend towards modern storage solutions like emulated disk systems or even cartridges with megabytes of space available. So I figured, why bother?
Towards the end of my development work, I did consider that it would be better to have dynamic tiles. In other words, store a key list of the unique tiles used on the map, assign them unique values, then the map data itself using those values. It’s a nice idea, as each map then basically has it’s own unique tile set, and you wouldn’t have a lot of needless replication. But it adds a lot of overhead towards map loading, and would require a complicated editor for maps. Something to consider for the future…
Okay, so you have your map loaded in memory. How to get it on screen?
Well, the first thing is to determine the number of tiles you want to appear on the screen. Depending on how many pixels per tile, how large the screen, etc. And is your “avatar” character always at the center? If so, an odd number of tiles per side makes sense for balance.
Using a ‘view-port buffer’ is a best approach. Don’t try and pull each tile individually from your map data, use an in-between buffer to store it. Using map height and width and a position offset, it’s not hard to create a double loop to copy out map data to your view-port buffer. But what do you do when your view-port goes over the edge of the map?
Different games handle it different ways. If you want your map to just stop scrolling at edges, Gauntlet-style, that’s easy to do; you just make sure offsets never go past a certain value. This does create the sense of reaching a “map edge” though, and may remove the player a little from immersion.
Other games just have a default “overflow” tile that is used, and interacting with it will pop the character off the map. This also works but still clearly illustrates that you’ve reached a map edge.
For my own game, I had two versions. One is “repeat the edge tile” which takes whatever tile was along that edge and repeats it indefinitely. This creates a less obvious map edge. The other version is “wrap-around”, which creates a continuous map by wrapping to the other side. I use this one sparingly, usually for maps that are either “warped magic” in nature or in a more clever fashion to create non-square style caverns.
One other feature I introduced with my own map system is slanted maps. Since map projection is up for interpretation, I can slant map data by row to make a more natural style map for coast lines that aren’t going in cardinal directions. Very useful, and very difficult to debug!
One additional task is to place mobs (mobile objects) on your map. This would be your monsters, points of interest, and other items that aren’t part of the static map. I store these in a separate data set, so they must be placed in the view-port area if they are visible. This puts a practical limit of how many mobs per map, since every extra calculation is costing you processing time.
So now we have our view-port, with our set of tiles. Now (finally) it’s time for line of sight!
Line Of Sight
Back in the early 90’s I tried to create the LOS algorithm in BASIC, using sine and cosine functions. I thought of it as I was flinging light out from the center and traversing a circle, and that if I struck a barrier that blocked line of sight, everything after it on the path would be dark.
So… it kind of worked. Except that even on a 11×11 size screen and going 1 degree at a time, it still didn’t cover all the squares. Plus given it was in BASIC it ran VERY slowly, taking ten minutes to complete.
Years later, I got a hold of the Ultima III LOS algorithm, and was able to see my error. I was thinking in reverse! What you do is trace a path from the tile you want to check LOS on and traverse back to the center.
The algorithm is pretty simple:
- Hide all tiles in the view-port, except the center tile, where your avatar is.
- For each tile in the view-port, plot a path back to center. The original algorithm uses two arrays to achieve an offset. It moves diagonally towards the center until it hits a cardinal direction, then continues the rest of the way on the cardinal.
- If at any point you encounter a “blocking” tile, move on to the next tile.
- Otherwise, if you reach center, uncover the target tile and move to the next tile.
Simple indeed, but processor-intensive. Besides processing the view port in start-to-end order without taking the 2D nature of the data into account, it also ends up reprocessing a LOT of tiles unnecessarily. If a tile is blocked, then wouldn’t any tiles between it and the blocking tile also be blocked? And if so, why calculate those at all?
I introduced some optimizations to reduce unnecessary calculations and it worked pretty well. However, the LOS algorithm in Ultima III was rather heavy; a single tile creates a huge swath of diagonal shadow behind it. I noticed that Ultima IV on the PC seemed to have a better algorithm so I took a copy of the source from XU4 (sadly now an extinct project) to analyze.
The algorithm is VERY different. It starts by tracing in the cardinal directions from the center outward. If it encounters a blocking tile, it blocks only the cardinal tiles behind it.
After the four cardinal plots, it then does four quadrant calculations, which start on the edge and go towards the center on a horizontal or vertical direction first then diagonal. This has the effect of not blocking so pervasively. It’s a much more efficient algorithm because it’s actually taking the structure of the view-port into consideration.
So, remember that light bit on the tiles? That’s used to determine dark and light areas. But, if the player has a light source going, how to determine if a square is lit or not?
I use a light map, which has concentric circles of numbers, matching the size of the view port. The center area is value 0, and slowly increases as it goes outward. A light source has a strength (or radius) value, which the light map is subtracted from. If the value is less than zero, it’s not lit. If it’s equal or greater, it’s lit.
The light map is always applied after LOS has been calculated. A square that’s already blocked remains blocked.
This was a late-development feature, which came about when I was out on a ridge one day looking over a magnificent set of waterfalls in the distance. I realized that despite being in a forest and having a deep valley of forest between us, I could still see the falls clearly. And that got me thinking, what about elevation and LOS?
The elevation map is a separate data set from the map’s data, as not every map uses elevation. The data is also stored in a compressed format (run-length encoding!) so it doesn’t take up much disk space. There are four levels of elevation, from 0 to 3.
The math is easy. Whatever elevation your avatar is at, everything below it is not a blocking tile. Everything at same level respects the LOS calculations. And everything above your level is considered a blocking tile automatically.
I mostly use elevation on world maps, but a few special maps use it. I think my favorite in this regard is a sewer dungeon, with both upper and lower levels. The biggest problem to solve with elevation was coming up with clear boundary delineation.
Below is ROA’s map view algorithm, in TMS9900 assembly. It uses multiple buffer maps for each stage and then combines them at the end for the finished mapview.
* Map Building Routine * Extract map from map buffer into VMAP BLDMAP MOV R11,*R10+ LI R0,MOBVIS LI R1,32 BLDMP0 CLR *R0+ * Clear mob visibility array DEC R1 JNE BLDMP0 LI R3,2 BLWP @PAGE1 * Set >2000 to page 2 (map buffer) LI R3,1 BLWP @PAGE2 * Set >3000 to page 1 (elevation) MOVB @SLANT,R1 * Get orientation into R1 SRL R1,8 MOV @DIRY(R1),R9 * Set R9 to -1 (left), 0 (none), or 1 (right) MOV @DSLANT(R1),R8 * Set R8 to 0 (left), -6 (none), -12 (right) MOV @Y,R1 * Get Y value into R1 MOV @X,R2 * Get X value into R2 AI R1,-6 * Set starting y position A R8,R2 * Add slant start to x position LI R3,13 * Row count LI R4,13 * Column count CLR R5 * buffer index MOVB @EDGES,@EDGES * Check if edge or repeating map JNE BLDMPW * Build map with edge BLDMPE MOV R1,R6 * Copy R1 to R6 MOV R2,R7 * Copy R2 to R7 BL @EDGCHK * Check if over edge MOV R0,R0 JEQ BLDME1 * Edge correction COC @W1,R0 * Vertical? JNE EDGEM2 MOV R6,R6 JLT EDGEM1 MOV @VWIDTH,R6 DEC R6 JMP EDGEM2 EDGEM1 CLR R6 EDGEM2 COC @W2,R0 * Horizontal? JNE BLDME1 MOV R7,R7 JLT EDGEM3 MOV @HWIDTH,R7 DEC R7 JMP BLDME1 EDGEM3 CLR R7 * Get tile BLDME1 MOV R7,R0 * Copy R7 to R0 MPY @HWIDTH,R6 * Multiply Y by hortz width A R0,R7 * Calculate map index into R7 MOVB @MAPBUF(R7),@VMAP(R5) * Copy tile MOVB @MAPENV(R7),@EMAP(R5) * Copy elevation level MOVB @VMAP(R5),@LMAP(R5) * Copy light level SOCB @B128,@VMAP(R5) * Set high bit on active tile SZCB @B127,@LMAP(R5) * Filter light array to top bit only MOVB @B1,@LOSMAP(R5) * Set LOS map to blocked INC R5 * Increment the buffer pointer INC R2 * Increment the column index DEC R4 * Decrement the window width count JNE BLDMPE INC R1 * Increment the row index A R9,R8 * Add slant change to R8 MOV @X,R2 * Move X back into R2 A R8,R2 * Add slant to x position LI R4,13 * Reset the window width to 13 DEC R3 * Decrement the window height count JNE BLDMPE JMP BLDMP2 * Jump to next routine * Build map with wrapping BLDMPW MOV R1,R6 * Copy R1 to R6 MOV R2,R7 * Copy R2 to R7 BL @EDGCHK * Check if over edge MOV R0,R0 JEQ BLDMW1 * Wrap correction COC @W1,R0 * Vertical? JNE WRAPM2 MOV R6,R6 JLT WRAPM1 S @VWIDTH,R6 JMP WRAPM2 WRAPM1 A @VWIDTH,R6 WRAPM2 COC @W2,R0 * Horizontal? JNE BLDMW1 MOV R7,R7 JLT WRAPM3 S @HWIDTH,R7 JMP BLDMW1 WRAPM3 A @HWIDTH,R7 * Get tile BLDMW1 MOV R7,R0 * Copy R7 to R0 MPY @HWIDTH,R6 * Multiply Y by hortz width A R0,R7 * Calculate map index into R7 MOVB @MAPBUF(R7),@VMAP(R5) * Copy tile MOVB @MAPENV(R7),@EMAP(R5) * Copy elevation level MOVB @VMAP(R5),@LMAP(R5) * Copy light level SOCB @B128,@VMAP(R5) * Set high bit on active tile SZCB @B127,@LMAP(R5) * Filter light array to top bit only MOVB @B2,@LOSMAP(R5) * Set LOS map to blocked INC R5 * Increment the buffer pointer INC R2 * Increment the column index DEC R4 * Decrement the window width count JNE BLDMPW INC R1 * Increment the row index A R9,R8 * Add slant change to R8 MOV @X,R2 * Move X back into R2 A R8,R2 * Add slant to x position LI R4,13 * Reset the window width to 13 DEC R3 * Decrement the window height count JNE BLDMPW * Retrieve data into state and sensing arrays BLDMP2 MOVB @VMAP+84,@CTILE * Set current tile value MOVB @EMAP+84,@CELEV * Set current elevation level MOVB @B247,@VMAP+84 * Set player graphic for permissible space MOVB @B1,@LOSMAP+84 * Set center of LOS map to visible SETO @SURRND * Clear the surrounding tile contents SETO @SURRND+2 SETO @SURRND+4 SETO @SURRND+6 MOVB @VMAP+97,@SURRND * Copy the down tile MOVB @VMAP+83,@SURRND+2 * Copy the left tile MOVB @VMAP+71,@SURRND+4 * Copy the up tile MOVB @VMAP+85,@SURRND+6 * Copy the right tile * Mob processing LI R3,4 BLWP @PAGE2 CLR @WORK2 * Clear WORK2 (in map mob count) MOV @MOBCNT,R0 * Copy total mob count to R0 JEQ BLDMP3 * If 0, skip to next phase MOV @MOBADR,R1 CLR @WORK * Clear @WORK (Mob #) LI R6,WORK2+2 * Set R6 to WORK2+2 BM2 MOVB *R1,R2 * Copy mob type into R2 JEQ BM2B * If zero, skip, no counter decrease CB @B23,R2 * Check if inert JEQ BM2B * If so, skip but decrease counter JMP BM2C BM2A AB @B1,@WORK DEC R0 * Decrement mobs processed JEQ BLDMP3 * If finished, move on JMP BM2 BM2B AB @B1,@WORK AI R1,8 * Go to next mob DEC R0 * Decrement mobs processed JEQ BLDMP3 * If finished, move on JMP BM2 BM2C MOV *R1+,@MAPMOB * Get mob data MOV *R1+,@MAPMOB+2 MOV *R1+,@MAPMOB+4 MOV *R1+,@MAPMOB+6 BLWP @MOBWIN * Calculate window positon MOV R3,R3 JLT BM2A * Not visible, skip placement INC @WORK2 * Increase mob count MOV R3,*R6+ * Copy index to WORK2 array MOVB @MAPMOB+1,*R6+ * Copy pattern to WORK2 array MOVB @WORK,*R6+ * Copy mob index MOVB @MAPMOB+1,@VMAP(R3) * Copy pattern to VMAP for LOS calculations LI R4,4 LI R2,MOBSEN * Load mob sense data BM2D C R3,*R2+ * Check if position is next to player JNE BM2E MOV *R2+,R5 * Get address into R5 MOVB @WORK,*R5 * Copy mob to state array BM2E INCT R2 BM2F DEC R4 * Loop all four locations JNE BM2D JMP BM2A * Update sense counter for traps/secrets BLDMP3 CLR R1 * Clear R1 for sense counter LI R0,SURRND+1 * Set R0 to SURRND array, mob area LI R2,4 * Set R2 to 4 (4 directions) BM3A CLR R3 MOVB *R0+,R3 * Copy mob # to R3 JLT BM3B * if negative, skip SRL R3,5 * Make 8-step index A @MOBADR,R3 MOVB *R3,R4 * Copy mob type to R4 SB @B16,R4 * Subtract 16 from mob ID JLT BM3B * If less than zero, not a hidden mob SRL R4,8 * Shift value to make index MOVB @SENSEV(R4),R5 * Copy from character array JEQ BM3B * If 0, skip SOCB R5,R1 * Set bit BM3B INC R0 * Increase to next tile position DEC R2 * Decrement counter JNE BM3A MOVB R1,@SENSEC+1 * Copy R1 to SENSEC (Counter) * Check party light level, map onto tiles LGTMAP MOV @MAGEYE,R0 * Check if magic eye is active JEQ LGT1 BL @MEVIEW * Fully open map B @PCME4A LGT1 MOVB @LIGHT,R0 * Check if map is fully lit JEQ LGT1A * If so, skip to LOS algorithm MOVB @PARTY+30,R0 ANDI R0,>2000 * Check for Radiant Pharos JEQ LGT1B LGT1A BL @MEVIEW * Fully open map JMP LOS LGT1B CLR R1 * Set buffer index LI R5,169 * Set buffer counter LGT2 MOVB @ALIGHT(R1),R2 * Copy window position light value into R2 SB @LGTLV+1,R2 * Subtract the current light strength from window value JGT LGT3 * If greater, unlit MOVB @B128,@LMAP(R1) * Otherwise, mark the tile lit LGT3 INC R1 DEC R5 JNE LGT2 * Map line-of-sight on the map LOS LI R8,4 * Number of directions to process CLR R9 * Direction index CLOS1 LI R7,6 * Count of tiles LI R0,6 * Column position LI R1,6 * Row position CLOS2 CLR R12 * Set tile to copy to closed tile BL @POSCLC * Calculate position MOVB @LOSMAP(R3),R2 JEQ CLOS3 CI R3,84 JEQ CLOS2A BL @CHKTLE * Fetch tile's opacity level, also check elevation ANDI R5,>8000 * Check if a blocking tile JNE CLOS3 CLOS2A LI R12,>0100 * Set tile to open * Set tile CLOS3 MOV @MAPLOS(R9),R2 * Copy direction vector index A @DIRX(R2),R0 * Add direction vector to column A @DIRY(R2),R1 * Add direction vector to row BL @POSCLC * Get buffer index MOVB R12,@LOSMAP(R3) * Copy visible/blocked value to buffer DEC R7 JNE CLOS2 * Loop through path INCT R9 * Change direction DEC R8 JNE CLOS1 * Loop through rows * Diagonal LOS DLOS LI R8,4 * Number of directions to process CLR R9 * Direction index DLOS1 LI R6,6 LI R0,6 DLOS2 LI R7,6 LI R1,6 DLOS3 BL @POSCLC * Collect four tile indices in FRAM array MOV R3,@FRAM MOV R0,@FRAM+8 MOV R1,@FRAM+10 MOV @MAPLOS+8(R9),R2 A @DIRX(R2),R0 A @DIRY(R2),R1 BL @POSCLC MOV R3,@FRAM+2 MOV @FRAM+8,R0 MOV @FRAM+10,R1 MOV @MAPLOS+10(R9),R2 A @DIRX(R2),R0 A @DIRY(R2),R1 BL @POSCLC MOV R3,@FRAM+4 MOV @MAPLOS+8(R9),R2 A @DIRX(R2),R0 A @DIRY(R2),R1 BL @POSCLC MOV R3,@FRAM+6 MOV @FRAM+8,R0 MOV @FRAM+10,R1 * Check three surrounding tiles CLR R12 MOV @W3,@FRAM+8 LI R2,FRAM DLOS4 MOV *R2+,R3 MOVB @LOSMAP(R3),R4 JEQ DLOS5 BL @CHKTLE ANDI R5,>8000 JEQ DLOS6 DLOS5 DEC @FRAM+8 JNE DLOS4 JMP DLOS7 DLOS6 LI R12,>0100 * Set tile DLOS7 MOV @FRAM+6,R3 MOVB R12,@LOSMAP(R3) MOV @MAPLOS+8(R9),R2 A @DIRY(R2),R1 DEC R7 JNE DLOS3 MOV @MAPLOS+10(R9),R2 A @DIRX(R2),R0 DEC R6 JNE DLOS2 AI R9,4 DEC R8 JNE DLOS1 * Final opening of permitted space PCMEND CLR R1 * Set buffer index LI R0,169 * Set buffer counter MOVB @CELEV,R2 * Copy elevation to R2 PCME1 MOVB @LOSMAP(R1),R3 * Check LOS JEQ PCME3 MOVB @VMAP(R1),@LMAP(R1) * Set the tile onto the map JMP PCME4 PCME3 MOVB @SPACE,@LMAP(R1) * Make the tile black (invisible) PCME4 INC R1 DEC R0 JNE PCME1 * Place visible mobs PCME4A MOV @WORK2,R0 * Check mob count JEQ PCME7 * If zero, skip LI R1,WORK2+2 PCME5 MOV *R1+,R2 * Get mob index MOV *R1+,R3 * Get pattern CB @LMAP(R2),@SPACE * Is the space blacked out? JEQ PCME6 MOVB R3,@LMAP(R2) * Set mob on map ANDI R3,>00FF * Get mob # MOVB @B1,@MOBVIS(R3) * Set mob visibility to 1 PCME6 DEC R0 * Decrement count JNE PCME5 PCME7 LI R0,>F700 SB @BOAT,R0 MOVB R0,@LMAP+84 * Copy the party icon to the center B @SUBRET * Magic eye/Pharos view MEVIEW LI R0,169 * Set all tiles to lit/visible LI R1,VMAP LI R2,LMAP MEVW1 MOVB *R1+,*R2+ DEC R0 JNE MEVW1 RT * Calculate mob position in viewing window * Returns window index (0-169) in R3, -1 = not in window MOBWIN DATA VWS,MOBWN0 MOBWN0 MOVB @SLANT,R2 * Get orientation into R2 SRL R2,8 MOV @DIRY(R2),R0 * Set R7 to 1 (left), 0 (none), or -1 (right) NEG R0 * Negate R0 MOV @W7,@WMOB * Store 7 in WMOB MOV @WN7,@WMOB+2 * Store -7 in WMOB+2 MOV @MAPMOB+2,R2 MOV R2,R4 SRL R2,8 * Set R2 to mob Y ANDI R4,>00FF * Set R4 to mob X S @Y,R2 * Subtract player y from mob y CI R2,7 * Check right vector JLT MOBWN1 MOVB @EDGES,@EDGES JEQ MOBWN5 S @VWIDTH,R2 * Subtract wrap offset MOBWN1 CI R2,-7 * Check left vector JGT MOBWN2 MOVB @EDGES,@EDGES JEQ MOBWN5 A @VWIDTH,R2 * Add wrap offset CI R2,6 * Check left vector again JGT MOBWN5 MOBWN2 MPY R2,R0 * Multiply Y offset by R0 S R1,@WMOB * Adjust positive boudnary for X S R1,@WMOB+2 * Adjust negative boundary for X MOV @WMOB,@WMOB+4 DEC @WMOB+4 S @X,R4 * Subtract player x from mob x C R4,@WMOB * Check down vector JLT MOBWN3 MOVB @EDGES,@EDGES JEQ MOBWN5 S @HWIDTH,R4 * Subtract wrap offset MOBWN3 C R4,@WMOB+2 * Check vector JGT MOBWN4 MOVB @EDGES,@EDGES JEQ MOBWN5 A @HWIDTH,R4 * Add wrap offset C R4,@WMOB+4 JGT MOBWN5 MOBWN4 MPY @W13,R2 * Multiply by 13 (window width) AI R3,84 * Add center offset A R4,R3 * Add X delta A R1,R3 * Add shift delta MOV R3,R3 JLT MOBWN5 CI R3,168 JGT MOBWN5 MOV R3,@>0006(R13) * Copy back to calling routine RTWP MOBWN5 SETO @>0006(R13) RTWP * Check tile at index in R3 CHKTLE MOVB @LMAP(R3),R4 * Check light level JEQ CHKTL2 * If not lit, is automatically blocking CB @CELEV,@EMAP(R3) * Check current elevation against target tile JEQ CHKTL3 * If equal, continue to opacity test JLT CHKTL2 * If less, go to block CHKTL1 CLR R5 * Clear R5 (open) JMP CHKTL4 CHKTL2 SETO R5 * Set R5 (closed) JMP CHKTL4 CHKTL3 MOVB @VMAP(R3),R4 * Copy tile to R4 low byte SRL R4,8 ANDI R4,>007F MOVB @TILES(R4),R5 * Copy tile code into R5 high byte CHKTL4 RT * Check for map edges * R1 = Y, R2 = X * R0 returns 0 if no violation, 1 if vertical, 2 if horizontal, 3 if both EDGCHK CLR R0 * Set to 0 C R2,@HWIDTH * Check X against horizontal width JL EDGCH1 INCT R0 EDGCH1 C R1,@VWIDTH JL EDGCH2 INC R0 EDGCH2 RT * Determine index on map POSCLC MOV R1,R2 MPY @W13,R2 A R0,R3 RT
Despite the numerous calculations going on, the map view creation is pretty fast, enough that I had to introduce some artificial delays. I did notice that the more mobs on the map the more impact it had on performance. For that reason, there is a limit of mobs per map, and I actually broke up some maps into more than one map to remove performance problems.
So that’s it with maps and part 2! I’m not sure what part 3 will cover yet, I’m open to feedback.