So one example of your large sprite, here I color coded a few areas to show some simple compression schemes you could apply directly in HuC. So while the PCE sprites cells are 16x16 in hardware, that doesn't mean you couldn't store them in smaller components like 8x8. Matter of fact, it's pretty easy to convert four 8x8 sprites into a single 16x16 when writing it to the hardware. But for the sake of simplicity and keeping the planar format, I chose 8px wide as the smallest width.
So the simplest area of compression is of course just to omit blank 16x16 tiles. Those are marked in red (a group of 8x8 tiles making a 16x16 block). But obviously it doesn't always work with all sprites and sizes.
The darker blue translucent areas show part of the image that can be represented as 2bit color instead of 4bit. That's an automatic 50% saving in those areas. Some of the transitioning areas of the bag in that image, have a limit of 7 colors and could be stored in 3bit format for a 25% savings per cell area (which I didn't highlight). Notice that I used 8x8 sectioning in the highlighting.
The light blue and orange-ish areas show repeated data. Since this is planar, and a byte can represent 8px across, that data can be represent in numerous ways (RLE, bitmask for repeat patterns, etc). You can head sections of tile groups with headers, indicating which tiles are compressed and what type of decompression to use. I didn't highlight the whole image with all of columns of repeating 8px wide sections along 8x8 cells, but just enough to demonstrate.
Darker-Pink is similar to light blue areas, except the whole tile can be represented as a single row of 4bit 8x line and a simple RLE for the column length.
The purple one is just an example of where a whole 16x16 cell is wasted to show a few pixels. The art could be changed slightly, and you'd save 128bytes right there.
And just look at all those 8x8 green tiles (which are invisible pixels). Those could easily be represented as a single header byte (blank 8x8 tile). There are all sorts of compression schemes that you can throw at it, and having a diverse set of simple schemes can help out quite a bit. These types of schemes are also very fast to decompress to vram;
many hucard games use compression schemes like these. If all you did was compress for blank 8x8 tiles and nothing else, you'd save 1,800 bytes alone in that pic (20% rom saving in space for the image).
