Amiga Computing - Commodore Is Awesome

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THIS MONTH IN ER SKIDMARKS MANCHESTER UNITED: THE DOUBLE Reviewed! A I 0 0 0 0 3 1 2 A S P S 1 1 LEN Revie I YOB LOSE ft A% - SUPER STREET FIGHTER 2 Previewed! 1 1 e 13 April. For a few weeks. iaIa v
ler code, even when reasonably 0 Vs well a documented, well known fact is rarely that assem- easy to understand unless you have a good idea of what the code is supposed to be doing in tie first place. Needless to say, this makes - ',luage coders look far more difficult than a- ne Db of making an Amiga display 'smooth y ;croll' and since it seemed to me this would or :e an area many of you would be interested f e in, I've chosen to devote some time to pust ta this subject. In fact, over the next two instalments I'll hl no modifying and extending the 680x0 e l code provided last month in order to pro- ty duce a demo that vertically smooth scrolls an ra Intuition screen. Before explaining how this ir type of smooth scrolling is actually done, ce however, let me kill off one false trail. k. Those of you who have the Amiga's graphics s A library documentation may have seen ut that there is a library routine called ScrolIVPort() sy that can be used to produce display ep scrolling effects. The plain truth is that d i while, in theory, this routine could conceivably bc be used to produce smooth scrolls, y a the results obtained by using this function al are lust not good enough. The autodocs themselves mention that sc the ScrolIVPort() function is slow and can sa produce visible 'hashing' of the display, So, if es ScrolIVPort() can't be used, is there an alternative me course ot action available to us? The bi answer here is very definitely yes, but in ln order to appreciate it it's necessary to be clear y p in your mind how Amiga displays are generated. Io ai All displays are created by allocating blocks n of memory called 'bitplanes,' in which -t each 'bit' represents a pixel position on i the display. Normal displays will contain a s number of separate bitplanes and by taking t the appropriate pixel bit from each bitplane, the Amiga's display hard- h ware is able to generate a colour register number. e F Values stored in each colour register 177Z 7 1 , • P 9 r r , ri .- -• r F 9 F9 PI r4 F9 P9 , 17 1 1 7 4 7 4 ; r 7 r . ii.14 •7717 + , . P 7. .7 T7 . 14 17 41 4 7 . 4 17 1I! L. L11 : - T 747 7 4 1 1'.117-T417 ; 17 TT17717 -T• . i.i.i 1 • WWIP1 • • - 1717 • gr; • 171;17 • , • 17 TrT•174117 . . . 7 117 . Te • , 17 + . 1 17417 1 7 . 71+ ; ..1 . 1 1. I. I. L 1 i I. 141: i I! i ill,Ahll! 1, J t L . 6 J 1, I. 1. .1 - J t t 1, 14 7 1P F . 7 - • • F1 P 77 . 1 P 9 P P P P P 4 iT r .i. 1i 1+4..4 l; q T • . . l1 41 +411 . . 7 4. j.. . 3 7 P P . 4 7 1 7. • • .• P + i P P P P P 9 P P 4 , P 4i P P P P P F 9 P 1 F P F - 7 P I ; ; 7 . Li . V . P 441+++ 14444++++++++++444++ . P P? ? P 1 • • • • • - I 9 P 7 4. 1iP P P F I1; 1 ; P 4 l r i V . I, ill. P ++++ 4• eq17 P P P , • • l P P P i, . P + P 9 P + 4+++++++++++++++++++++4444+++++J I 11:47 . r t r • P9 e r r , P r y • r r o • I r p • r p r r r r y r • • F P P P P P • • 4P • P • • • I • • • • • • • - • • • • • • . 4417 g1 ± . 1 ! 71 tP P P I P ; P . 1494++ 1 1 P P P 17 rt:71777 , s ; ? P P 9 : 117,7 1P . 1 I P P 9 A1 7' T 17 1 z L 1 P 9 P ' q7.T;T;T; 47T; Z t : . 9 t++++ 4l 1 4 + 4 4 4 4 4 4 + P P a- 1t r 7 a a? i . f , t LJL L L a al & JAI ! i ; dT 7 I+ Iilli . e • • • - • • , . ill i i 1 1 . 1 p 4 &17 r. diai IT-17;177 •, 7 , L . 7 AALAILILLILLEILLILILILL: • -•11-1111.1.: A L L L I S A : - 1 1 1 1 : - 1 1 1 1 A11747 ; 4i 7 • 1 : L1a. ir d , T+ 7 r 41, r J t e - . 1 r1 1 - 1 1 1 : * 7. 7 ; • + 1At 177:11:7 i s it + t • 1774,717 MI 7 1 • . 4- did a ; . a7 + +++4 11;g7TT • g7 17,17 : r ! 1 7x AZT;Taia; l . + + l r d,a; . 7 7171717-177;47-T-7;q547.7171117WW177.177111;711174171717171711747117177;g74;;E; l 1 nothing but an old otp. 05moothq! ,*;„6, determine the actual colours seen on the screen, and it's because a screen's colour register number range depends on the number of bitplanes being used that the colours available with different screen types varies. A one bitplane screen can only have two colours (corresponding to any single bit in the bitplane, being either a 0 or a 1). two bitplane screens can have four colours (each bit from each bitplane is combined to produce one of four values 00, 01, 10, or 11) and so on. MEMORY Although display bitplanes have to be stored in chip memory (because they need to be accessible to the custom chips) they do not have fixed positions as such. In fact, when a screen is opened the bitplanes will be allocated in any convenient area of chip memory available. Needless to say this means the graphics system needs to have some way of identifying the position of these bitplanes and the structure used is called a BitMap_ This structure can be found in the graphics/06 include file and you'll notice from the description below that it contains space for Smooth I S one of the 177051' LISP& ttirk5 1317 d501fing 58111111 fimiga (oder ran learn about th15 month Paul thiErdd 5tdrt5 Pllfildinifig how it 1.5 Some simple smooth scroll rputinos in pction Amiga Computing MAY 1995 TUTORIAL Part 10 done u5kg a55embN language up to eight bitplane pointers: STUCTUIE DitAR.D VOID bo_ D yte sPe rtoo VOID DTI bo_ Fie gs etvE be _ le pth VOID blue d PERT De _ PLa ne o, Doe the OitpLeoes LABEL bo_ SU EOF ;R iote rs Co Because the BilMap's bitplane pointers define the memory locations used to produce the display that appears on your monitor, you might expect that, by arranging for a display's bitplane pointers to be increased by an amount which corresponds to the pixel-width of the screen, it would be possible to shift the display memory downwards by one line. Similarly, by decreasing those pointers by the same amount the display might be expected to shift upwards one line. Although very close to the truth, this doesn't work because a part of the story is still missing. While the BitMap structure certainly defines the initial display memory being used, the Amiga's graphics coprocessor (the 'Copper') which handles the display generation doesn't actually collect its bitplane information from this source Instead it uses copies of the bitplane pointers that have been embedded into a sehes of instructions called a 'copper list'. Once Intuition has opened a screen and generated these copper instructions, the bitplane pointers held in the BitMap struc• hire's are essentially redundant as far as the display generation process is concerned. The important bitplane pointer 91
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Amiga Computing - Commodore Is Awesome

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