Re: Apple II graphics file format

[Andy McFadden <fadden@fadden.com>]

Ophidian <zacwbond@vt.edu> wrote:
> All right, the source of my confusion was that $78 in the image I am 
> examining has a 2, which was 560x192 based on the table, but that is too 
> large to fit in 8192 bytes (actually, my file is 8184 bytes, for 
> whatever reason).  However, Ciderpress is giving me a 280x192.

The last 8 bytes are also a screen "hole", i.e. not displayed.

The DOS 3.3 'B' file format includes a two-byte address and two-byte length
embedded in the file itself.  This meant that storing 8192 bytes required
8200 bytes, or 33 256-byte sectors.  If you saved 8188, you only needed
32 sectors.  So it was fairly common to drop the last few bytes.

> I guess the real question I should be asking is--how did the Apple II 
> display hardware work to create color images?

There are some references around.  The short answer is that it's a hack.

The comp.sys.apple2 FAQ probably has some links; googling for "Apple II
video hardware" should turn up some stuff.

> I am porting an slow & buggy old Applesoft game off the Apple, and I am 
> debating how to deal with the graphics stored in bitmap files.  In 
> particular, I have a color image with text, but the text looks 
> color-tinted and funny; I would like to make the text all white while 
> preserving the color elsewhere.

Picture two adjacent pixels.  If one is on, you get one color.  If the
other is on, you get a different color.  If they're both on you get white.
So, to make things white, make sure everything is at least two pixels thick.

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