[ASM BEGINNER] Sprite0 does not overlay

[Ozzyboshi]

Hello EAB coders.
I am currentlly studying the italian ramjam course ( http://corsodiassembler.ramjam.it/ ).
At chapter 7 is stated that sprites are pointed by SPRXPT register (for example $dff120 is sprite0) and they allow to draw simple images OVER bitplanes.

Based on this assumption i created a little test program where only 1 bitplane is activated and pointers BPL1PTH-BPL1PTL point to an area of memory which I inizialized with $DD value in every bytes, total of 40*256 to cover the whole LOWRES PAL screen.

Oddly the sprite is visible only where there there are 0 values on the bitplane (color 0), on the contrary, for bitplanes bits equal to 1, color palette 1 is used instead the one set up for the sprite (from color 17 up to color 19).

Can anybody tell me where I am wrong? According to my text book this cant happen since the sprite is drawn over bitplanes.
Where am I wrong?

My code is available at

https://gist.githubusercontent.com/O...d/spritetest.s

I used devpac 3.04 as an assembler on a stock A600.
I attached also a png screenshot where, if you look closely at the top center of the screen there is my sprite hidden by black bars.

Thanks in advance for your support.

[ross]

Check BPLCON2 value.

[DanScott]

BPLCON2 controls the sprite / playfield priority... try writing (IIRC) #$24 into BPLCON2

ie. in your copperlist dc.w $104,$0024

[ross]

Originally Posted by DanScott

BPLCON2 controls the sprite / playfield priority... try writing (IIRC) #$24 into BPLCON2

ie. in your copperlist dc.w $104,$0024

Dan, do not give him directly the solution otherwise he will not learn

[Ozzyboshi]

Ok you guys nailed it, with setting dff104 to hex 24 works as I wanted but... why?

In other words you told me to set PF2P2 and PF1P2 ON.
Since I use one playfield only I expected that setting PF1P2 on was enough but.. actually it's PF2P2 that it's forcing the sprite to be on top.

Then I found this document:
http://amigadev.elowar.com/read/ADCD.../node0159.html

that states:
Be careful:
-----------
PF2P2 - PF2P0, bits 5-3, are the priority bits for normal (non-dual)
playfields.

According to this, since I use a single playfield mode, all I really need is to set PF1P2, is it correct?

Originally Posted by DanScott

BPLCON2 controls the sprite / playfield priority... try writing (IIRC) #$24 into BPLCON2

ie. in your copperlist dc.w $104,$0024

[ross]

Originally Posted by Ozzyboshi

According to this, since I use a single playfield mode, all I really need is to set PF1P2, is it correct?

No, PF2P2 - PF2P0, bits 5-3, are the priority bits for normal playfields (like you have written above).
Also doesn't hurt setting valid priority for PF2PRI and PF1P2 - PF1P0.
You can experiment strange value to make special effect but this is an advanced topic.

[malko]

Originally Posted by Ozzyboshi

Hello EAB coders.
I am currentlly studying the italian ramjam course ( http://corsodiassembler.ramjam.it/ ). [...]

Thanks for the link ! Amazing lecture !
Back in the 80' a really well known French humorist (of Italian origin from his father) said in a sketch that he was able to speak some foreign language... "under torture" .
For such RamJam reading I am sure I can read Italian .
Edit: If you read the sketch : "Lisieux" is a place in France but sounding may be like "les yeux" (the eyes), so when it's said "il vous reste Lisieux pour pleurer" you have to understand "you still have your eyes to cry". It's kind of black humour

[ross]

Il est temps de changer Canton

Un saluto ai vecchi amici in RamJam.

It's always nice to see new users approaching assembly/metal programming.

[malko]

I have other UK books to finish first.
Anyway this IT one is definitively in my list
(but damn, +1800 pages... it's a huge one for a non IT speaker guy... Fortunately FR & IT have a common Latin base. If I forgot to be dumb, I will manage to understand a bit )

[Ozzyboshi]

Ok let me try again , I read some stuff and now I think I have a better understanding about sprite/bitplanes priorities:

$DFF104 (BPLCON2) among other things lets you define if the sprites must be printed under or over playfields.
To do this it uses bits from 0 to 5 (6 in total), bits 0-1-2 are fiags that contain the priority against playfield1, the others 3-4-5 against playfield2.

Assuming that:
- Playfield 1 uses odd bitplanes (1-3-5) and Playfield 2 uses even bitplanes (2-4-6).
- My amiga has 8 sprites registers

In my asm code i set $dff100 (BPLCON0) in a way that uses one bitplane only setting bit 12 (BPU0) to 1.Dual playfield mode is disabled because the tenth bit of bplcon0 is 0.

Since dual playfield mode is disabled, to achieve sprites over playfield I oddly must set PF1PX bits and PF2PX.Yesterday i was assuming that PF1PX were enough because I am using bitplane 1 only which is ODD and so It must be referred to playfield 1.

On the contrary in dual playfield mode i would have set PF1PX for declaring sprite priority related to odd bitplanes (playfield 1) and PF2PX for even bitplanes (playfield 2).

Now the question is, how to set PF1PX and PF2PX?
First of all I noticed that sprites are always shown over the playfield zeros, so if the bitplane is filled with zeros the sprite must be completely visible.
If the bitplane's bits are not zero, PFXPX bits of bplcon2 are questioned.
if PFXP(0-2) is 000 - playfield is over all the sprites
if PFXP(0-2) is 001 - playfield is over sprites from 2 to 7 (but not 0-1)
if PFXP(0-2) is 010 - playfield is over sprites from 4 to 7 (but not from 0 to 3)
if PFXP(0-2) is 011 - playfield is over sprites from 6 to 7 (but not from 0 to 5)
if PFXP(0-2) is 100 - all sprites are always over the playfield
if PFXP(0-2) is 101 - I don't know
if PFXP(0-2) is 110 - I don't know
if PFXP(0-2) is 111 - I don't know

Now I realize why you told me to write #$24 (binary 100100).
In this way you told me to put all sprites over the playfield, actually in this case, since i am using only one sprite and this prites belongs to the first couple, you could have suggested something like

dc.w $104,$0009

I'd like to know from you if all of this makes sense or not.If not I have to start over my research again.
Feel free to fill in the "I don't know" I put on the PFXPX use cases.
Thanks a lot for your help.

Originally Posted by ross

No, PF2P2 - PF2P0, bits 5-3, are the priority bits for normal playfields (like you have written above).
Also doesn't hurt setting valid priority for PF2PRI and PF1P2 - PF1P0.
You can experiment strange value to make special effect but this is an advanced topic.

[Ozzyboshi]

Dont be scared of the 1800 pages, they are very repetitive and contains a lot of code.
I am a native italian speaker if you have trouble translating in your language feel free to contact me, we can use english as a bridge language, my french is very poor.

Originally Posted by malko

I have other UK books to finish first.
Anyway this IT one is definitively in my list
(but damn, +1800 pages... it's a huge one for a non IT speaker guy... Fortunately FR & IT have a common Latin base. If I forgot to be dumb, I will manage to understand a bit )

[malko]

Thanks !