Original Chip Set - Denise

Denise

Denise controls the video timings, but can also synchronise to an external video signal. Denise is programmed to fetch planar video data from 1 to 5 bitplanes and translate that into a colour lookup. The number of bitplanes is arbitrary, thus if 32 colours are not needed, 2, 4, 8 or 16 can be used instead. The number of bitplanes (and resolution) can be changed on the fly, usually by the copper. This allows for very economical use of RAM. There is also a sixth bitplane, which can be used in three special graphics modes:

In Extra-HalfBrite (EHB), if a pixel is set on the sixth bitplane, the brightness of the regular 32 colour pixel is halved. Early versions of the Amiga 1000 sold in the United States did not have the Extra-HalfBrite mode.

In Hold-and-Modify mode (HAM), each 6-bit pixel is interpreted as 2 control bits and 4 data bits. The 4 possible permutations of control bits are "set", "modify red", "modify green" and "modify blue". With "set", the 4 data bits act like a regular 16-colour display look up. With one of the "modify"s, the red, green or blue component of the previous pixel is modified to the data value, and the other two components are held from the previous pixel. This allows all 4096 colours on screen at once and is an example of lossy image compression in hardware.

In Dual Playfield mode, instead of acting as a single screen, two "playfields" of 8 colours each (3 bitplanes each) are drawn on top of each other. They are independently scrollable and the background colour of the top playfield "shines through" to the underlying playfield.

There are two horizontal graphics resolutions, "lowres" with 140 ns pixels and "hires" with 70 ns pixels. This makes the display 320 or 640 pixels wide without overscan. Denise supports very wide overscan; there is no need for a border around the graphics as other computers suffered from. Vertical resolution, without overscan, is 200 pixels for an 60 Hz NTSC Amiga or 256 for a 50 Hz PAL Amiga. This can be doubled using an interlaced display.

Denise can also lay up to eight 16 pixel wide sprites per scan line (in automatic mode) on top, underneath, or between playfields, and detect collisions between sprites and the playfields or between sprites. These sprites have 3 visible colours and one transparent colour. Optionally, adjacent pairs of sprites can be "attached" to make a single 15 colour sprite. Using Copper or CPU register manipulations, each sprite 'channel' can be reused multiple times in a single frame to increase the total sprites per frame. Sprite position registers may also be changed during a scanline, increasing the total sprites on a single scanline.