I don't usually like mode 2 games as I can't use the arcade pixels but usually with a reduced palette and movement is too fast or 25fps.
BombJack in the arcades moves horizontally 1 pixel one one frame and then 2 on the next meaning that it isn't very smooth anyway!
This would be 1*30 + 2*0 = 90 native pixels per second, on the beeb MODE 2 moving one double pixel every frame would be the equivalent of 2*50 = 100, so very close.
The other sprites move at fractional rates anyway, so will never by 50/60Hz smooth.
The other thing with MODE 2 is that one option is to draw the "background" using four colours and the "sprites" using three colours which allows sprites to be ORed on and the screen to be ANDed with either #&F0 or #&0F (your choice) to remove sprites (2nd fastest general sprite clear routine after just storing background colour).
So, with near 1 50fps MODE 2 pixel per frame of the main character and a background image needing to be kept, this is a good choice for a MODE 2 sprite demo that could be made into a game.
The second advantage of MODE 2 after having more colours is that you only need two copies of the gfx to cover all horizontal positions which means 1/2 the memory required than MODE if your store the shifted copies.
With 5 large backgrounds to store and quite a lot of sprites and animation, it isn't going to fit on a 32K beeb, so first thing required is to support a bank of SWRAM, which is simple if you don't use the OS as once you have control, you can swap it in and just run with 48K, although the screen has to go in the lower 32K of that, but it can be anywhere in that lower 32K.
So far, I have compressed the backgrounds by converting them to four colour (inc red) images, tiling them and compressing the tile maps but not the tiles (because they are too big to decompress on the fly when needed).
The backgrounds take up about 13+K, but borrowing a trick from Sarah, as they only need 2 bits per pixel, 4 bits per bytes, the sprites which currently are allocated 8KB go into the same memory as they only need the other bits in each byte - Yah Sarah
The sprites could be extended to 12K as nothing else is likely to go in that memory but I haven't yet done that.There are also 63 levels to define which use various combinations of backgrounds and bomb/platform layouts.
The sprite routines are written to draw sprites at each horizontal position, mirrored (X>=128), flipped upside down (sprite>=128) and disabled (Y=0). They are drawn in four bands down the screen, each band waits for the beam to be at the bottom of it and:
1st quarter clears any sprites wholy in it.
2nd quarter clears any sprites wholy in it and draws any sprites wholy in the 1st quarter.
3nd quarter clears any sprites wholy in it and draws any sprites wholy in the 2nd quarter.
4th quarter clears any sprites wholy in it and draws any sprites wholy in the 3nd or 4th quarter.
This allows a good balance between simple code and maximum time available to draw things without flicker.
This current demo only loops, drawing the 63 screens with their platforms and then adds the bombs in the order that they will become active. This is a quick reminder, but could be removed if it is unpopular. The SWROM is nearly full, but main memory has over 15K free and could easily be more byt removing the blank edges of the screen!
If you can draw a better background, then the easiest way is to draw them in MODE 5, 104x192 pixels (top two rows+ are always background colour). I have reserved red on each screen for the bombs, but they could be different colours on each background if it looks better. They also needs to compress well, so the number of tiles in a screen needs to be no more than 256 and the total for the five screens about 1000 (each screens tiles are separate for simplicity). The colours are: background, darker, lighter, bomb (currently always red).
There are at least two other BombJack games in development, so don't be too disappointed if I don't make this into a game. One of the others in RobC's VideoNuLA version, which will be amazing when (if) finished and I'm afraid that I have forgotten the other (please post here).
As usual any feedback or questions about how I do things or how could things be done are very welcome
) so I've played with the colours and contrast.
