Sprite priority on display.

[Daedalus]

Originally Posted by Havie

Could you show me this in code as I don't understand what you mean...sorry, being a bit thick.

Basically, the DisplayControls command appears to carry out an Exclusive-OR with the arguments for BPLCON2 and BPLCON4 and their defaults, but not with BPLCON3. Exclusive-OR (XOR) is a simple logical function that reverses all the bits involved. So, carrying it out twice on the same value will give you back the original value, meaning you can eliminate the effects of the XOR on BPLCON2 and 4 by carrying out a pre-XOR on them.

Something like this:

; Xor %100001 with the default $224 to get the rear playfield behind all sprites, and the front playfield behind channels 0 & 1 only
; $1C00 sets 2nd playfield palette to start at 16
; Xor $23 with the default $11 to set sprite bank 3 for odd & joined sprites, and bank 2 for even sprites
DisplayControls 0, Xor(%100001, $224), $1C00, Xor($23, $11)

[jotd]

I just want all sprites to be above the bitplanes (no dual playfield)

BPLCON2 was set to 0, and sprites are behind the bitplanes.

Tried to set to $1F made no difference. Someone help?

[DanScott]

$24

[jotd]

bplcon2 <= $24: that works. Thanks! All sprites are on the foreground.

I wonder why the documentation is so hard to understand / lacks simple basic examples like this. That's the second time someone answers with an hex value and I put it in my code and it works exactly as I intended. Thanks again.

[Daedalus]

Just to explain the magic number: some examples are sadly lacking from the Blitz documentation, but the register is universal across languages so you can read more detail about it here. For a non-dual-playfield display, bits 5-3 set the priority, with %100 being what you're looking for to put all sprites above the playfield. So you want a value of %00100000, which is $20 (or %00100100 to also cover dual playfield displays, which is $24).

The additional problem is that Blitz XORs the default value with the given value, so to apply a value, you need to bear this in mind as simply giving the desired value won't always work. That's why I pre-XOR the values in my examples, so I would do it this way for sprite priority:

DisplayControls 0, Xor(%00100100, $224), $1C00, 0

The information is there, it's just not as clear as it should be unfortunately...

[jotd]

thanks! I see that a lot of things can be done with sprites/playfields, and it can depend on the sprite number as well!

[rcman]

Daedalus:

I am always so impressed with you. Your wealth of knowledge is amazing.

Is there not a autobiography on you? Your accomplishments, reward etc? I for one would read that.


RC

[Daedalus]

Haha, cheers! That's just a misspent youth, nothing more

[roondar]

On the topic of sprite priorities, are there some good examples in either games of demos of sprites used while having "odd" or unusual priority schemes?

I've often wondered what could be achieved this way and some examples always kickstart my imagination.