[Oberon] **** SPAM **** [ 10.791 ] Re: System-V - Frame Clipping

[Jörg]

Tomas

Instead of what is now in Display.Mod, this should have been my favorite, as it is short and doesn’t need SYSTEM.VAL().
    mask := -LSL({0..31}, x);

The following code is even shorter source code but 100% equivalent to the above:
    mask := -LSL(-{}, x);

Compare this to my first "mask := SYSTEM.VAL(SET, ASR(7FFFFFFF, 31-x));"!!! It seems I was under time pressure I didn’t think long enough... (

br
Jörg

Am 04.03.19, 18:52 schrieb "Oberon im Auftrag von Jörg Straube" <oberon-bounces at lists.inf.ethz.ch im Auftrag von joerg.straube at iaeth.ch>:

    Tomas
    
    The mask you were referring to was introduced by me.
    While porting Walker.Mod (aka Sisiphus) I detected that the original CopyPattern had the restriction of width<32.
    But I needed width=32 for the Walker pattern.
    The restriction width<32 was there, because ASR() copies over the MSB while shifting right (correct sign extension).
    
    If the MSB of a pattern is set, ASR copies that bit over while shifting, resulting in an unwanted line on the display.
    
    In ASR shifting by -x is equivalent to shifting by 32-x.
    To get rid of this unwanted line I use a mask. If I shift the pattern by -26 bits and MSB is set, the result is this
       111111pat
    To get rid of these unwanted six 1s, I have to create a mask that looks like this
        000000111111111111
    6 zeros and 26 ones.
    
    There are different ways to produce such a mask:
    ASR(7FFFFFFF, 31-x) or
    ASR({0..30}, 31-x) or
    -LSL({0..31}, x)
    
    Jörg
    
    > Am 04.03.2019 um 15:39 schrieb Tomas Kral <thomas.kral at email.cz>:
    > 
    > On Mon, 4 Mar 2019 11:28:31 +0100
    > Andreas Pirklbauer <andreas_pirklbauer at yahoo.com> wrote:
    > 
    >> mask := SYSTEM.VAL(SET, ASR(7FFFFFFFH, 31-x));
    > 
    > Hi Andreas,
    > 
    > Thank you, will toy with your line code.
    > 
    > The mask above is interesting, not sure I get the idea yet, as I still
    > use older version of D.CopyPattern();
    > 
    > Only thinking about complement 31-x, if could be coded as -x?
    > 
    > mask := SYSTEM.VAL(SET, ASR(7FFFFFFFH, -(x) ));
    > 
    > As display buffer is reversed like in the mirror, so
    > shift right is visually moving pixels left on the screen and vice versa.
    > 
    > 7FFFFFFFH = binary all ones, except MSB, are shifted left, x, 31
    > complement times.
    > 
    > As LSL, ASR, ROR, are allowed on SETs, I am thinking of an
    > equivalent.
    > 
    > mask := ASR(-{31}, -(x)); but I do fear is not the same thing.
    > 
    > Apologies for analysing too much :-)
    > 
    > Thank you.
    > 
    > -- 
    > Tomas Kral <thomas.kral at email.cz>
    > --
    > Oberon at lists.inf.ethz.ch mailing list for ETH Oberon and related systems
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