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Forums > C64 Coding > Picture without badlines.
2018-02-08 11:15
HughJass

Registered: Jan 2018
Posts: 41
Picture without badlines.

In the last part of Time Machine by Booze Design HCL says that the picture looks very colorful despite not having any badlines. How is that possible to have pictures without badlines ? Lack of badlines causes FLD effect, right ?
2018-02-08 13:07
Trash

Registered: Jan 2002
Posts: 101
In bitmapmode you can stretch the last badline read by the VIC without distorting the bitmap. This is done by doing a "fld" at the right cycle, the downside is that that cycle is used for reading sprite 0 resulting in that you can only have seven sprites in the border and two colors (or three + d021 in multicolor) per column on the bitmap.

There is an early example of this effect made by a swedish group (I think it was Triad or Horizon) containing a seven sprite scroller over the Tetris-image. someone else might remember what it is called.
2018-02-08 13:13
AmiDog

Registered: Mar 2003
Posts: 94
I was actually thinking about this myself a while ago for totally different reasons. I think one badline/frame is the minimum to show a bitmap. After the first badline, it should be possible to change the yscroll just as for FLI, but instead to make sure there never is a match and thus no more badlines, right? Would result in the same colors for the entire picture I guess. I wonder if that first badline could be avoided too somehow... (Not for the first frame since the color data needs to be fetched alteast once, but for the following frames.)
2018-02-08 13:28
Trash

Registered: Jan 2002
Posts: 101
Quote: I was actually thinking about this myself a while ago for totally different reasons. I think one badline/frame is the minimum to show a bitmap. After the first badline, it should be possible to change the yscroll just as for FLI, but instead to make sure there never is a match and thus no more badlines, right? Would result in the same colors for the entire picture I guess. I wonder if that first badline could be avoided too somehow... (Not for the first frame since the color data needs to be fetched alteast once, but for the following frames.)

Like this one: Just an Illusion
2018-02-08 13:39
HughJass

Registered: Jan 2018
Posts: 41
That's cool. I Guess then you can display fullscreen grayscale pics with lots of free cycles.
2018-02-08 13:56
HughJass

Registered: Jan 2018
Posts: 41
Semi related question... Let's say normal badline starts at line 1, then 9. If I cause a badline at line 4 doing FLI, will the next badline still happen at line 9, or does the counter restart and it will happen at 12 ?
2018-02-08 14:11
lft

Registered: Jul 2007
Posts: 323
There are two different things in play. There's a counter (called RC in the vic article) going from 0 to 7 during each character row. Then there's the current Y-position (in d012).

Badlines occur when YSCROLL (low bits of d011) matches d012.

RC is reset if there's a badline condition on cycle 14. When doing FLI, you change YSCROLL to create a badline condition just after that, so the counter doesn't reset. But if you then leave YSCROLL at the new value, it's gonna match d012 eight lines later, i.e. at line 12 in your example.



VIC timing chart
2018-02-08 15:29
HughJass

Registered: Jan 2018
Posts: 41
So if I cause badline at exactly cycle 14 (the last sta $d011 cycle is at cycle 14) then I'll cause a badline and reset RC at the same time, correct ? Will I read the next text line from pixel 0 or the same text line from 0 ?
2018-02-08 15:45
Oswald

Registered: Apr 2002
Posts: 4175
basicly you are repeating the first screen row.

similar topic:

https://csdb.dk/forums/index.php?roomid=11&topicid=119133&first..


also the bitmap is not restricted to 4 colors, only each char column is restricted to 4 colors, because of displaying the same row of color attributes in all rows.
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