Studies - The effect of "flame".

(C) Ivan Roshchin, Moscow, 1997

 A few graphical effects


          1. "Flame"


   There are many variations of this effect (see, for example, 
music demo FSRD, INSANE, S4B, NOUMENON, EYE ACHE

2, RAGE, etc.). I realized until now never meet: the screen is
Some pictures (say, a screen saver program), and in the back
background (and this sites pictures
painted black) as
times and is seen "flame". How does it
looks like, you can see by running "ZX FORUM 4 (effect used in 
the main menu of the the electronic edition).


   Thus, the text of the program (if it is
development, I used fragments of procedure, "Flame", taken
from an article in the St. Telitsin ZX REVIEW
05/06 1997):
140.
BUF1 EQU # 8000; Here are memorized

                   ; Attributes of the image.
BUF2 EQU # 8300; array, resp.

                   ; One printed image
                   ; Proposition (L # 340).
BUF3 EQU # 8700; Picker (Associate

                   ; Address byte = 0!)


        LD HL, BUF3

        EXX


        XOR A

        LD (23560), A


        LD HL, BUF3; form a

        LD DE, BUF3 +1; palette

        LD (HL), 0

        LD BC, 8

        LDIR

        LD A,% 1000

        LD (BUF3 +4), A

        LD (HL), A

        LD BC, 8

        LDIR

        LD (HL),% 10000

        LD BC, 8

        LDIR

        LD (HL),% 11000

        LD BC, # E7

        LDIR


        LD HL, # 9000; transferred

        LD DE, # 4000 picture

        LD BC, # 1B00; screen

        LDIR


        LD HL, # 5800, saved

        LD DE, BUF1; its attributes

        LD BC, # 300

        LDIR


        LD HL, BUF2; resets

        LD (HL), 0; working

        LD DE, BUF2 +1; buffer

        LD BC, # 2FF

        LDIR

SSAVER LD HL, BUF2 + # 320; fill

        LD (HL), -60; part

        LD DE, BUF2 + # 321; buffer

        LD BC, # 1F; number -60

                    , (From its value

        LDIR; depends on type of effect)


        LD HL, 0; original position

                     ; In ROM

; Update cycle in the buffer BUF2:

LOOP_1 LD DE, BUF2 + # 300

        LD B, # 20

LOOP_2 INC HL

        LD A, H

        AND # 1F

        LD H, A

        LD A, (HL); sl.bayt from ROM

        AND% 00111111

        ADD A,% 11000

        JR NC, LL

        LD A,% 11000
LL LD (DE), A

        INC DE

        DJNZ LOOP_2


        PUSH HL


        LD IX, BUF2

        LD DE, # 5800

        LD HL, BUF1

        LD BC, # 2FF
LOOP_4 LD A, (IX +32)

        ADD A, (IX +33)

        ADD A, (IX +34)

        ADD A, (IX +65)

        SRL A

        SRL A

        JR Z, LOOP_5

        DEC A
LOOP_5 INC IX

        INC HL

        INC DE

        LD (IX), A

; Changed byte value in the buffer
And now are changing acc. attribute on the screen:


        PUSH BC


        EXX

        LD L, A; address in the palette

        LD A, (HL); A = desired color

        EXX

        LD B, A

; For the current attribute set
And instead of black or INK PAPER
, The color that was taken from the palette;
, Thus black is like
, "Transparent":


        LD A, (HL); APR. Pictures

        AND% 00111000; PAPER

        JP NZ, TST_2; JP-for speed

        LD A, B; now, instead of

        OR (HL); black -

        LD (DE), A; another color

TST_2 LD A, (HL)

        AND% 00000111; INK

        JR NZ, TST_Q

        LD A, B

        AND% 00111000

        RRCA

        RRCA

        RRCA

        OR (HL)

        LD (DE), A

TST_Q POP BC

        DEC BC

        LD A, B

        OR C

        JP NZ, LOOP_4

        POP HL

; Check the keyboard:


        LD A, (23560)

        AND A

        JP Z, LOOP_1

        RET
2

   I should note that: firstly, it is easy, changing the 
program to declare a "transparent" is not black, and any other 
color (or even multiple colors).

Secondly, we can very precisely adjust color "flame" with 
palette (array BUF3). Each byte palette contains one of eight 
values that determines the color:% 000000% 001000 ... % 
111000. During the program the value of the buffer BUF2 
(Numbers from 0 to 255), which You can say, determine the 
"brightness" of the flame, encoded in corresponding attribute 
values in accordance with this palette. 

   Although this effect (as well as all
discussed in the article) is not synchronized by interrupts,
they should be allowed - this is
need to poll the keyboard.