Re: [stella] Interlace screengrab

Subject: Re: [stella] Interlace screengrab
From: Glenn Saunders <cybpunks2@xxxxxxxxxxxxx>
Date: Sat, 24 Aug 2002 21:53:06 -0700
At 10:38 PM 8/24/2002 -0400, you wrote:
Ok, I've finally seen what's being drawn.  Glenn, can you
explain what I'm seeing?  There've been a few revisions to
the demo, and I haven't been keeping track.

There are two kernels, an odd and an even line kernel. If you run the demo on a real VCS and toggle the select switch you'll see the system go from showing the odd kernel, the even kernel, and then the interlaced kernel.

When interlaced, the odd and even alternate, naturally. The even lines appear above the odd ones.
bk pf
Zone 1 E Blue Red
Zone 1 O Red White

When you select through the noninterlaced kernels, Zone 1 shows solid blue/red or solid red/white, then a mixture in interlace that blends into a magenta and pink on the PF.

Zone 2 E        Black   Black (i.e. blank)
Zone 2 O        Red     White

Zone 3 E        Blue    Red
Zone 3 O        Black   Black (i.e. blank)

When you select through the noninterlaced kernels, zone 2 and zone 3 will alternate being displayed (solid) and when interlaced, they will show up flickery with lines in between them.

So these top scanlines are a good testbed for interlace.

Zone 4 E        Red     White
Zone 4 O        Red     White

Zone 5 E        Blue    Red
Zone 5 O        Blue    Red

These two zones are the "control". They will appear the same whether or not interlace happens, but obviously when interlacing, each scanline will alternately flicker.

Zone 5 E        Red     White
Zone 5 O        Black   Black

Zone 6 E        Black   Black
Zone 6 O        Blue    Red

These two zones show what a single scanline looks like with interlace, i.e. 30hz flicker.

Zone 7 E        00+4
Zone 7 O        02+4

In this zone each kernel cycles through colors +4 at a time, offset by 2. This has the effect of one kernel being slightly brighter than the other when you toggle through the noninterlaced modes. But in interlaced mode they fill in eachother for a full smooth color gradient across all 128 colors in the same vertical zone that would normally only cover 64 lines.

In Zone8 I create a diagonal line using playfield by doing something like this:


e = even
o = odd

So on the individual kernels I am skipping a pixel.

In noninterlaced you'll see these gaps from the skips. In interlaced, the gaps get filled in with a somewhat flickery but high-vertical-res diagonal line.

In the screengrab I merged the fields so you don't get to see the flicker.

Archives (includes files) at
Unsub & more at

Current Thread