So for anyone looking into old school graphics programming, bit planes are pretty confusing when you don't understand why they exist.
Two big reasons. First, it's about running memory chips in parallel to increase bandwidth. Image data was hard to get to the screen fast enough with hardware in that era.
Second it allowed for simple backwards compatibility. Programs were used to writing directly to video memory, and in an EGA card the start of the video memory was valid CGA data. The rest of the colour data was in a separate bit plane.
Yes, I too remember the memory savings being the key aspect of bitplane design choices in the early and mid 80s. The increased granularity in varying the trade-off between memory for graphics vs program code being the secondary aspect. The downside to bitplanes vs chunky pixel formats was having to write to four different memory addresses to fully change one 16 color pixel.
One case where bitplanes could be faster was high-res bitmapped text. As long as your text was monochrome (all in one bitplane), you could write an 8 pixel wide character with one byte. This was a big deal when it came to scrolling a screen full of bitmapped text.
Memory savings and by extension memory bandwidth for low color modes, as you indirectly point out with the text example.
The early Amigas had a memory bandwidth of ~7MB/s, and the chip RAM bandwidth was shared with the custom chips... Anything that'd reduce needed memory transfers was a big deal.
Indeed. As per this timing diagram, Denise accesses each 16-bit word of each bitplane sequentially. Any bitplanes you turn off, the more cycles available for blitter... or CPU!
Fun fact! The Amiga Workbench is 4 colour hires by default, because hires is impressively businessy... but 8 or 16 colour hires would lock out the CPU most of the time, as the chipset would have to dip into the 68000's even cycle RAM accesses and stall it. 4 colour hires lets the CPU (on a chipmem-only system) run at full speed!
Two big reasons. First, it's about running memory chips in parallel to increase bandwidth. Image data was hard to get to the screen fast enough with hardware in that era.
Second it allowed for simple backwards compatibility. Programs were used to writing directly to video memory, and in an EGA card the start of the video memory was valid CGA data. The rest of the colour data was in a separate bit plane.