<palign="center">A <b>high-performance</b> library for writing <b>tile-based</b> games for the Apple IIgs personal computer in 65816 assembly langugage. Unlock the full, 16-bit potential of the last machine of the Apple ][ line.</p>
# Introduction
The Generic Tile Engine (GTE) project is a tile engine built to exploit the unique hardware capabilities of the Apple IIgs personal computer. It supports the Apple IIgs super hires graphics mode (320x200, 16/256 colors) and provides two full-screen independent scrolling layers along with software sprites. The API and core functionality of the library is inspired by the graphics hardware of the NES/SMS/SNES/Genesis console era.
The library iscurrently implemented as a set of source files that must be compiled into a GS/OS application. A set of example project can be found under the `demos` folder. Each demo folder uses a `package.json` file to define the build targets and a build of each application can be created by executing a `npm run build` command.
Each demo application has a top-level `App.s` that references the `src/Core.s` file which includes all of the GTE source
files into a separage OMF Segment.
## Dependencies
GTE uses the [merlin32](https://brutaldeluxe.fr/products/crossdevtools/merlin/) assembler to compile its source into GS/OS OMF files and [Cadius](https://brutaldeluxe.fr/products/crossdevtools/cadius/index.html) to copy those files onto a ProDOS disk image. The paths to these tool can be set in the `package.json` file.
An empty 2MG disk image is included in `emu/Target.2mg` and is used as the default location for copying demo applications. This image can be mounted in any IIgs emulator.
Starting up GTE only requires a single subroutine call to `GTEStartUp`. This subroutine will take care of starting up the necessary Toolboxes and allocating any necessary memory. GTE heavily leverages memory for its speed. A total of 4MB is RAM is recommended, with a 2MB as the minimum.
Once the engine is initialized, the play field must be set up. The play field defines a rectangular area of the physical graphics screen that is managed by the Tile Engine.
The size of the play field can be set directly by passing the width and height in the `x` and `y` registers. Also, there are 9 predefined screen sizes that correspond to well-known Apple IIgs software titles and hardware of the era which can be selected by the `x` register argument.
By default, the play field will be centered on the graphics screen. If a custom placement of the play field is desired, then the `SetScreenRect` subroutine can be used directly to set a specific area of the graphics screen as the managed area.
| Play Field Id | Width | Height | | Size (bytes) | Percent of Full Screen |
A simple `SetPalette` subroutine is provided in order to set any of the IIgs' 16 palettes.
```asm
ldy #PALETTE_NUMBER ; 0 - 15
lda #^PaletteData ; High Word of palette color array
ldx #PaletteData
jsl SetPalette
PaletteData dw $0000,$007F,$0090,$0FF0
dw $000F,$0080,$0f70,$0FFF
dw $0fa9,$0ff0,$00e0,$04DF
dw $0d00,$078f,$0ccc,$0FFF
```
## Tilemaps
Up to two tile layers are supported in GTE. Each layer can have its own tile map and origin set, independent of the other. This allows for a true parallax scrolling effect.
## Background 0
In order to enable a tile map on the first background, the width, height and pointer to tile data must be set by initializing the appropriate values on the GTE direct page. The direct page locations are defined in the `Defs.s` file and can be included in an application's main source file.
```asm
lda #NUMBER_OF_TILE_COLUMNS ; Set the tile map dimensions
sta TileMapWidth
lda #NUMBER_OF_TILE_ROWS
sta TileMapHeight
lda #TileMapBG0 ; Set the pointer to the tile map data
sta TileMapPtr
lda #^TileMapBG0
sta TileMapPtr+2
```
Once the tile map has been initialized, the camera view into the layer is set by defining the upper-left corner of the screen. The resolution of the tile map coordinates are byte-aligned, so each tile has a width of 4 and height or 8 even though each tile is 8x8 pixels.
```asm
lda #TileMapLeft
jsl SetBG0XPos
lda #TileMapTop
jsl SetBG0YPos
```
## Background 1
The second background is initialized in exactly the same manner as the first background.
```asm
lda #NUMBER_OF_TILE_COLUMNS ; Set the tile map dimensions
sta BG1TileMapWidth
lda #NUMBER_OF_TILE_ROWS
sta BG1TileMapHeight
lda #TileMapBG1 ; Set the pointer to the tile map data
sta BG1TileMapPtr
lda #^TileMapBG0
sta BG1TileMapPtr+2
lda #TileMapLeft
jsl SetBG1XPos
lda #TileMapTop
jsl SetBG1YPos
```
## Sprites
There are four subroutines that are available to provide sprite support in GTE: `AddSprite`, `MoveSprite`, `UpdateSprite` and `RemoveSprite`. GTE supports up to 8 sprites.
There is a single `Render` subroutine that applies all of the frame changes and efficiently renders to the super hires screen. It bears repeating here that most of the GTE functions operate in a deferred manner; any expensive operation that involved updating internal data structures is delayed until the `Render` function in called.
