Well it was a dedicated coprocessor which was synched with the "VBL" (vertical blank line).
So you could do all kind of nifty tricks, like changing the colors of your 5-bitplanes (32 colors) between lines, creating an image with much more than 32 colors.
Using 32 colors per line was easy : you could even change in the middle of a line but that was getting complicated.
One easy trick with the copper was to change the background color at each vertical line. Because the background color did extend to the far left and the far right of the screen, you could create a "copper line" and several copper lines (typically containing a gradient) would create a "copper list".
People don't realize just how smooth these 2D games running at 50 or 60 frames per second and synched with the vertical blank lines were.... 25 years ago!
The Atari ST was quite a beast too but it didn't have these great coprocessors the Amiga had.
(You may know some of this already, but for the benefit of everyone else...)
The original Amiga chipset only had 12-bit colour, so instead of 8 bits per channel there were 4, and you had 4096 colours, not 16.7M. One thing I did was create a copper bar (the "copper line" gradient you mention) where every second line flickered between the colour above it and the colour below it. This let me create 31 blues instead of 16. It looked pretty good at 60Hz.
The Amiga hardware was designed for 32 display colors - which doesn't seem like much today, but at the time was up against Herculese monocrome, CGA, and B&W Macs.
The system could be hacked to display 64 colors fairly easily. Getting all 4096 was a bit more complicated and required using HAM mode (hold and modify).
The technique allowed the 32 display colors to be reset for each scan line. With careful attention an artist could create a better image by adjusting the pallet scan line by scan line. IIRC one lost the ability to use the hardware sprites and HAM mode was never used for games, just static images.
No, that's just a normal 5 bitplane display mode with a copper trick. Real original 6 bpl HAM didn't need copper, it depended on current beam position left side pixel. For each pixel right side you could either set all RGB to one of 16 color palette OR modify red, green or blue value, but otherwise 'copy' left side pixel color.
So say you had white (15,15,15) pixel left side. Current pixel 'color'-data is "modify red 7", thus pixel becomes (7, 15, 15). Green and blue color components were held from left side pixel.
It was important to select those 16 paletted reset colors carefully for maximal effect. Otherwise you'd get a lot of ugly 2 pixel horizontal streaks when multiple color components changed from horizontal pixel to another quickly. You could of course use copper to change that 16 color palette for each scanline, too.
The big problem for Amiga programmers was that the Amiga was sold in NTSC and PAL versions, which ran at different framerates (50 and 60hz) with different resolutions. While PAL Amigas could play NTSC games (and most TVs could render the screen properly) I don't remember it working the other way around (although I could be wrong - any American miggy owners care to correct?).
Anyway, consequently things like funky horizontal copper manipulation relied on the Amiga either running in NTSC or PAL mode, but not both. This is why a lot of people would use the copper for gradients on the earlier Amiga 500 games, that and the fact that a lot of earlier Amiga 500 games were ports of ST code.
NTSC Amigas could play PAL games, but the bottom of the screen was overscanned/truncated off the bottom of the screen, and the gameplay and music was usually 20% faster.
So you could do all kind of nifty tricks, like changing the colors of your 5-bitplanes (32 colors) between lines, creating an image with much more than 32 colors.
Using 32 colors per line was easy : you could even change in the middle of a line but that was getting complicated.
One easy trick with the copper was to change the background color at each vertical line. Because the background color did extend to the far left and the far right of the screen, you could create a "copper line" and several copper lines (typically containing a gradient) would create a "copper list".
People don't realize just how smooth these 2D games running at 50 or 60 frames per second and synched with the vertical blank lines were.... 25 years ago!
The Atari ST was quite a beast too but it didn't have these great coprocessors the Amiga had.