Gemini is the modern systems desktop twin of GEM for Atari TOS. The intention is to adapt GEM to the X Window System, plain framebuffers, and possibly other kinds of video display.

The Gemini library implements parts of the application environment services (AES) and the virtual device interface (VDI) of GEM, to eventually become compatible with a small subset of TOS/libc. A carefully designed GEM application would then be possible to have for both the Atari ST and modern systems such as Linux.

The Gemini AES reference implementation draws objects by ray casting every pixel. This method is somewhat slow without optimisations, but there are several advantages:

• significantly simplified code, compared to bit blit and similar traditional methods;
• small and constant memory footprint regardless of video resolution;
• flicker-free video updates without double buffering are possible, because no intermediate drawing is necessary;
• write-only memory can be used for the video display;
• finally, drawing is pleasingly parallel, because all pixels are independent of one another, so modern multiprocessing hardware can be very effective.

The --draw option to the rsc tool demonstrates the ray casting method by producing a multipart TIFF image file, having drawings of each object tree in the given RSC file. The make tiff command generates test/tos206se.tiff and test/tos206uk.tiff. The make png commands splits those TIFF images into separate PNG image files using Image Magick.

Usage: rsc [options]... <RSC-file>

Displays Atari TOS GEM resource (RSC) file header, strings, images, icons,
objects, and other details, as text on standard output.

Options:

    -h, --help            display this help and exit
    --version             display version and exit

    --identify            exit sucessfully if the file is a valid RSC
    --diagnostic          display diagnostic warnings and errors
    --encoding <original|utf-8>
                          display text with original or UTF-8 encoding;
                          default is UTF-8
    --map                 display RSC map

    --draw                draw RSC forms and dialogues as images
    -o, --output <path>   save images as a multipart TIFF file

Usage: fnt [options]... <FNT-file>

Displays Atari TOS GEM font (FNT) file header and character set, as text
on standard output.

Options:

    -h, --help            display this help and exit
    --version             display version and exit
    --format <text|bdf>   display format type text (default) or bdf
    --identify            exit sucessfully if the file is a valid FNT
    --diagnostic          display diagnostic warnings and errors