Chapter 11: Graphics Programming¶
Note
This chapter is still under construction.
Interfacing with CGIA¶
Display List¶
The Color Graphic Interface Adaptor (CGIA) of the X65 microcomputer utilizes a display list system for defining and controlling video output. This approach is inspired by Atari’s ANTIC processor, with additional flexibility to support text, bitmap, multicolor, and advanced graphical modes.
Overview of the CGIA Display List¶
A display list is a series of instructions stored in memory that defines how the CGIA renders each scanline. Instead of the CPU handling screen rendering directly, the Video Processing Unit (VPU) interprets the display list, fetching graphics data, colors, and character definitions on a per-line basis. This method allows for a dynamic and efficient beam-chased video output without CPU intervention.
Display List Instructions¶
The CGIA display list consists of two main categories of instructions:
Control Instructions (0-7) – Used for line fills, jumps, memory loads, and register manipulation.
Mode Row Instructions (8-F) – Define how the CGIA should render specific scanlines.
For instructions requiring additional data, these follow the instruction code.
Control Instructions (0-7)¶
0 – Insert empty lines filled with the background color.
Bits 6-4: Number of empty lines.
Bit 7: Trigger a Display List Interrupt (DLI).
1 – Duplicate the last raster line multiple times.
2 – Jump to another location in the display list.
If DLI bit is set, execution waits for Vertical Blank before jumping.
3 – Load new memory pointers.
Bits 4-7 determine which offsets will be updated:
4: Load Memory Scan (LMS) – Points to screen data.
5: Load Foreground Scan (LFS) – Points to foreground color data.
6: Load Background Scan (LBS) – Points to background color data.
7: Load Character Generator (LCG) – Points to character shape definitions.
4 – Load an 8-bit value into a register. Each plane has 16 multipurpose registers, indexed 0 - 15.
Bits 7-4: Register index.
5 – Load a 16-bit value into a register. Plane registers are treated line 8 x 16-bit values, indexed 0 - 7.
Bits 6-4: Register index.
Mode Row Instructions (8-F)¶
Mode row instructions define the type of graphics displayed on a scanline. The lower three bits (0-2) determine the mode, while bit 3 differentiates them from control instructions.
8 – Reserved (TBD)
9 – Reserved (TBD)
A – Text/Tile Mode – Uses character-based graphics with a foreground and background color map.
B – Bitmap Mode – Direct pixel-based graphics.
C – Multicolor Text/Tile Mode – Character-based graphics with 4-color cells.
D – Multicolor Bitmap Mode – Direct pixel graphics using 4-color cells.
E – Hold-and-Modify (HAM) Mode – Similar to Amiga HAM, where pixel colors can be modified based on previous pixels, enabling a larger color range.
F – Affine Transform Chunky Pixel Mode (MODE7) – Inspired by SNES MODE7, allowing rotation and scaling of graphics for pseudo-3D effects.
Additionally, bit 7 in any mode row instruction triggers a Display List Interrupt (DLI), allowing for scanline-specific effects, color changes, or sprite updates.
Using the Display List¶
A typical display list sequence begins with a Load Memory Scan (LMS) instruction, setting the screen’s base address. Subsequent instructions configure row modes, colors, and additional graphics settings. The final instruction often loops back to the beginning, ensuring continuous display refresh.
By leveraging the CGIA’s display list system, developers can efficiently mix text, tiles, bitmaps, and advanced graphical effects in a single frame without CPU overhead. This design enables smooth scrolling, parallax effects, sprite multiplexing, and dynamic screen updates, making the X65’s graphics system a powerful tool for retro-style computing and game development.
Plane Registers Interpretation¶
Each CGIA plane has 16 associated registers, but their interpretation depends on the selected plane type and graphics mode. The last display list mode instruction determines how the plane registers are utilized. The register structure varies based on whether the plane is handling background graphics, HAM mode, affine transformations, or sprites.
Background Plane Registers¶
flags (8-bit) – Configuration flags.
border_columns (8-bit) – Number of border columns.
row_height (8-bit, unsigned) – Height of each row.
stride (8-bit, unsigned) – Memory stride for addressing screen data.
scroll_x, scroll_y (8-bit, signed) – Fine scrolling values.
offset_x, offset_y (8-bit, signed) – Base offsets for plane positioning.
shared_color ×2 (8-bit each) – Two shared colors.
Background Plane Flags¶
Bit 0 – Color 0 is transparent.
Bits 1-3 – Reserved.
Bit 3 – Border is transparent.
Bit 4 – Double-width pixel mode.
Bits 5-7 – Reserved.
PLANE_MASK_TRANSPARENT %00000001
PLANE_MASK_BORDER_TRANSPARENT %00001000
PLANE_MASK_DOUBLE_WIDTH %00010000
HAM Mode Registers¶
flags, border_columns, row_height (8-bit) – Same as background mode.
base_color ×8 (8-bit each) – The 8 base colors used for HAM modifications.
Affine Transform Mode Registers¶
flags, border_columns, row_height (8-bit) – Same as before.
texture_bits (8-bit) – Defines texture width and height in bits (size is power of two).
u, v (16-bit, signed) – Texture coordinates for the upper-left corner.
du, dv (16-bit, signed) – Texture step increments per pixel.
dx, dy (16-bit, signed) – Transform coefficients for scaling and rotation.
Sprite Plane Type Registers¶
active (8-bit) – Bitmask indicating which sprites are active.
border_columns (8-bit, unsigned) – Number of border columns.
start_y, stop_y (8-bit, unsigned) – Vertical range where sprites are visible.
By utilizing dynamic plane register interpretation, the CGIA enables highly versatile graphics rendering, allowing the X65 to seamlessly mix background layers, sprites, advanced color manipulation, and affine transformations in real time.
Implementing HAM Mode Graphics¶
Hold-And-Modify (HAM) Mode¶
HAM mode is a unique graphical technique that allows for an expanded color range by modifying the color of each pixel based on the preceding pixel. This technique, inspired by the Amiga HAM mode, enables an expanded color range without requiring a full 8-bit per-pixel framebuffer.
HAM Encoding Format¶
HAM mode operates using 6-bit commands, where 4 screen pixels are packed into 3 bytes:
[CCCDDD] - C: Command bits, D: Data bits
Commands¶
000 – Load a base color from an 8-color palette (DDD specifies which color).
001 – Blend the current pixel with another color from the 8-color palette.
CCS – Modify a single color channel:
01S – Modify the Red channel.
10S – Modify the Green channel.
11S – Modify the Blue channel.
S – Sign bit (0 = positive delta, 1 = negative delta).
DDD – Delta value (000 represents +1).
Rendering HAM Mode¶
The CGIA processes HAM mode by starting with a base color and applying per-pixel modifications. This means that each pixel is calculated based on the previous pixel, allowing for smooth color transitions across the screen. However, this also means that artifacts may appear when picture colors are changing rapidly.
Optimizing HAM Graphics¶
Due to its serial dependency, HAM mode benefits from careful palette selection and strategic placement of base color indices to minimize unwanted color blending. Developers can improve visual quality by:
Careful base colors selection to minimize artifacting.
Using blending instructions to smoothly transition between colors.
Leveraging display list registry set instructions and/or interrupts (DLI) to alter base-colors mid-frame.
Applications of HAM Mode¶
HAM mode is particularly useful for high-color images, gradients, and advanced shading effects. Unlike traditional indexed-color modes, HAM can generate a broader range of colors while keeping memory usage low.
By integrating HAM Mode (MODE6) into the X65 graphics pipeline, developers can achieve highly detailed visuals with minimal additional memory overhead, making it a powerful tool for static images.