sega/model2: tilemap fixes

[gm-matthew]

When emulating System 24 tilemaps (which Model 2 uses) MAME currently treats color 0 of any palette as transparent, but some Model 2 games use color 0 to encode tile-specific colors. Here is a screenshot of Behind Enemy Lines demonstrating the problem:

Here is what that screenshot is supposed to look like:

The Sega System 24 Hardware Notes specifies the following when ABSEL=0:

Zero value pixels from the topmost layer are considered transparent.
Zero value pixels from the bottommost layer show color 0 of the specified
palette.

If the topmost layer is blanked, it is considered transparent.
If the bottommost layer is blanked, it is filled with color 0 of palette 0.

On Model 2, tilemap B is usually the bottommost layer unless it has the priority flag set, in which case either tilemap A or the framebuffer is the bottommost layer depending on whether tilemap A also has the priority flag set. However, I have observed that the polygon layer is never displayed if it is empty even if both tilemaps A and B have priority (Virtual On title screen). It is unknown whether tilemap A would instead be chosen if tilemap B has priority set and tilemap A does not, making tilemap A the bottommost layer; I have not observed any games that depend on this.

The easiest solution I have found is to display color 0 from the tile-specific palette of tilemap B if all three would otherwise be transparent; this seems to fix all tilegen issues. My current solution works by drawing the layers in this order:

• tilemap B without priority as opaque
• tilemap B with priority as opaque
• tilemap A without priority
• frame buffer
• tilemap B with priority
• tilemap A with priority

The solution is not quite as neat as I'd like it to be so I'm submitting this commit as a draft for now; any ideas to make the solution more elegant are welcome.

[angelosa]

There are several games that depends on background pen being chosen by lowermost layer, the most notable is crkdown.
I also vaguely remember a bug of the like in Model 3, don't remember which game tho.

[mamehaze]

Yeah the map display has been broken on Crackdown forever in MAME, bits that are meant to be solid are showing as transparent.

[gm-matthew]

I've come up with a more elegant fix which uses the TILEMAP_DRAW_OPAQUE flag when calling tilemap_t::draw() and also adds the feature to draw_rect() .

I am satisfied with my current solution so I am declaring this commit as ready for review.

Regarding System 24 specifically, are there any other games besides Crack Down that are known to have similar tilemap bugs?

[angelosa]

qsww draws like this at startup on master tip, which apparently is a transparent pen issue

[angelosa]

LGTM