AA (Advanced Amiga) Graphics Features

V39 (Workbench 3.0) introduced the AA chip set (aka AGA), which redefined Amiga graphics capabilities: 24-bit color, new display modes, palette sharing, and RTG-compatible APIs. This article covers what changed and how to use the new features.

What AA brings

  • Up to 24-bit color — 8 bits per gun, 16.8 million colors. SetRGB32() takes 32-bit per-gun values (upper 8 bits used).
  • New display modes — DblNTSC, DblPAL (flicker-free), plus higher-resolution ECS-compatible modes.
  • Palette sharing — multiple apps share a limited set of hardware color registers via ObtainPen()/ReleasePen().
  • Interleaved bitmaps — BMF_INTERLEAVED flag for faster bitmap access.
  • Mode promotion — interlaced screens automatically promoted to flicker-free modes when a multiscan monitor is available.
  • Retargettable graphics (RTG) — RastPort functions designed to work with future non-Amiga graphics hardware.

The critical design goal per the source: "no features have been added to the new graphics system which cannot be kept for the future." Parameters like bits-per-pixel, resolution, and color palette size are variable or very large — code should not assume 4-bit-per-gun or 4096-color limits.

Color management on AA

void SetRGB32(struct ViewPort *vp, ULONG n, ULONG r, ULONG g, ULONG b);
LONG ObtainPen(struct ColorMap *cm, ULONG pen, ULONG r, ULONG g, ULONG b, ...);
void ReleasePen(struct ColorMap *cm, ULONG pen);
ULONG ObtainBestPens(struct ColorMap *cm, ...);

Pens are shared resources. Without ObtainPen(), another application can change your colors. Always ReleasePen() when done. On a non-AA system, only 32 or 64 pens exist; on AA, up to 256.

New screen tags (V39)

Tag Purpose
SA_Colors32 32-bit-per-gun color array (replaces SA_Colors)
SA_Pens Pen array for pen sharing
SA_SharePens Allow other screens to share pens
SA_Exclusive Exclusive screen mode
SA_Interleaved Use interleaved bitplanes
SA_DClip Custom display clip
SA_BackFill Backfill hook for background painting
SA_AutoScroll Enable auto-scroll when mouse hits screen edge

Mode promotion and coercion

Mode promotion: if the user has installed a DblNTSC or DblPAL monitor driver (in DEVS:Monitors/) and has a multiscan monitor, interlaced screens are automatically displayed using double-height non-interlaced modes — flicker-free. Enabled via IControl Prefs (on by default in 3.1).

Coercion (programmatic mode control):

ULONG CoerceMode(struct Screen *screen, ULONG modeID, ULONG flags);
ULONG BestModeIDA(struct TagItem *tags);  /* find best mode for criteria */

BestModeIDA() tags: BIDTAG_DesiredWidth, BIDTAG_DesiredHeight, BIDTAG_Depth, BIDTAG_MonitorID.

Color wheel and gradient slider (V39 BOOPSI classes)

Two new BOOPSI gadget classes for color selection:

/* Color wheel — HSB-based color selection */
APTR wheel = NewObjectA(NULL, "colorwheel.gclass", tags);
SetAttrs(wheel, WHEEL_Hue, hue, WHEEL_Saturation, sat, TAG_DONE);
GetAttr(WHEEL_RGB, wheel, &rgb);  /* get RGB result */

/* Gradient slider — select along a gradient */
APTR gs = NewObjectA(NULL, "gradientslider.gclass", tags);

The color wheel works in HSB (hue, saturation, brightness) space but converts to/from RGB. See Extras/BOOPSI/ and NDK_3.1/Examples1/colorwheel/ for examples.

Double buffering

struct ScreenBuffer *AllocScreenBuffer(struct Screen *screen, struct BitMap *bm, ULONG flags);
ULONG ChangeScreenBuffer(struct Screen *screen, struct ScreenBuffer *sb);
void FreeScreenBuffer(struct ScreenBuffer *sb);

Enables flicker-free animation by swapping between two screen bitmaps synchronized to vertical blank.

Compatibility gotcha

The AA chips are register level compatible with old/ECS at boot up time. However, when new AA modes are enabled and displayed, and a game then takes over the screen display without informing the OS, some registers may be in incompatible settings.

This reinforces the programming philosophy: use the OS, don't bang hardware. Games that bypass the OS for screen display may fail on AA hardware.

See Also

Sources: Commodore-Amiga, Inc., "Overview of V39 Graphics" and "Intuition 3.0" and "Color Wheel and Gradient Slider" tutorials (1991-1993). Raw: raw/tutorials/v39-aa-features.md Updated: 2026-08-04