snavig BPal blorbs not backwards compatible with Frotz

[dfabulich]

Such Blorb files are backward compatible: they contain all information in the original Blorb plus the BPal chunk.

To repro:

• Download https://eblong.com/infocom/gamefiles/zork0-r393-s890714.z6
• Download https://ifarchive.org/if-archive/infocom/media/blorb/ZorkZero.blb
• Download and compile Frotz https://gitlab.com/DavidGriffith/frotz including sfrotz
• Run Zork Zero: sfrotz zork0-r393-s890714.z6 ZorkZero.blb. It starts normally.
• Create a snavig blorb. snavig -a -s zork0-r393-s890714.z6 -c -o zork0-r393-s890714-graphics.blb ZorkZero.blb
• Run the snavig blorb in sfrotz. sfrotz zork0-r393-s890714-graphics.blb

Expected: snavig blorb files are supposed to be backwards compatible, so this blorb should work in Frotz.
Actual: Frotz crashes with an error. "Frotz Fatal Error: Non-paletted graphics cannot be adaptive"

My goal was to create a single-file blorb that would work well in Glk interpreters or in Frotz, and primarily link to that highly compatible blorb on IFDB, but that doesn't seem to work.

[cspiegel]

I think this is an implementation issue in Frotz (which also affects Windows Frotz: it doesn't abort but also doesn't use the adaptive palette). It's a result of compressing the images. The Frotz code effectively does this:

if (m_adaptiveMode && (color_type == PNG_COLOR_TYPE_PALETTE) && bit_depth <= 4) {
    // use adaptive palette;
} else if is_adaptive {
    abort("non-paletted graphics cannot be adaptive");
}

But this is checking both for paletted images and images 4 bits or less. The compression code in Snavig will sometimes generate 8-bit images, and this is causing Frotz to fail. The Blorb specification says this about the adaptive palette:

However, the following rules still apply from the PNG standard:

• Any bit depth of PNG is valid (1, 2, 4, or 8 bits per pixel).

So technically speaking the Blorbs generated are valid, but since the Infocom Blorbs have contained 4-bit images since the dawn of time, Frotz is apparently making the assumption that they must be 4 bits. The quick fix here is to not use -c, but I can also modify things to prevent compression by default with Blorbs that contain an adaptive palette chunk.

[cspiegel]

I found another Frotz issue with embedded stories in Blorbs. It's not visible in Zork Zero as the message gets overwritten, but you can see it in Arthur. Frotz says:

Warning: @draw_picture called but picture resources are missing (PC = 4fd) (will ignore further occurrences)

It's not fatal, so is just cosmetic, but it's an issue with Frotz assuming pictures will always come from an external Blorb file, and not from an embedded story.

I've added two new issues to Frotz's tracker, one for the adaptive palette, and one for this "missing resources" issue.

[dfabulich]

Cross-linking those issues here:

• https://gitlab.com/DavidGriffith/frotz/-/issues/287 Blorb files with embedded version 6 Z-code display spurious errors on graphics drawing
• https://gitlab.com/DavidGriffith/frotz/-/issues/288 Frotz assumes that adaptive palette images will be 4-bit PNGs

[cspiegel]

I've also filed a report against Windows Frotz at DavidKinder/Windows-Frotz#34.