Kirby's Dreamland 3 psuedo-transparency

[mikejmoffitt]

A long while ago I thought it was interesting that Kirby's Dreamland 3 for SFC seemed to have semi-transparency in the foreground when I was playing it in SNES9x. Now, I have the cartridge, and I see that the transparency is there but it has some slight interesting artifacts.

The research I've been able to find points towards the SNES being in psuedo-hi-res mode, but not much detail is available beyond that. Can anyone shed some light on what exactly was done to achieve this blending method?

[lidnariq]

Nocash actually calls Kirby out several times in his full snes specs ( http://nocash.emubase.de/fullsnes.htm )

[Sik]

If it's hi-res mode then most likely it's just dithering at work. If I recall corectly two consecutive pixels are going to be blended in the output signal making it look like a single color. Add dithering into the mix and you can have some sort of fake translucency.

I'll see if I can check the game later though.

[tepples]

It's dithering, and it's super effective. A 512px horizontal resolution on the Super NES means pixels are output at thrice the NTSC subcarrier frequency, or 10.74 MHz. Displaying hi-res requires a monitor to have bandwidth up to half that, or 5.37 MHz. The RF output unit is supposed to have a low-pass filter at 4.2 MHz so that video doesn't bleed into audio.

[mikejmoffitt]

It's dithering, and it's super effective. A 512px horizontal resolution on the Super NES means pixels are output at thrice the NTSC subcarrier frequency, or 10.74 MHz. Displaying hi-res requires a monitor to have bandwidth up to half that, or 5.37 MHz. The RF output unit is supposed to have a low-pass filter at 4.2 MHz so that video doesn't bleed into audio.

I see, so the game effectively stores graphics with "tall pixels" to take advantage of this?

[TmEE]

It is not doing vertical increase, only horizontal. No interlace mode, just 512 pixel horizontal res.
Sprites are not affected by this.

[mikejmoffitt]

It is not doing vertical increase, only horizontal. No interlace mode, just 512 pixel horizontal res.
Sprites are not affected by this.

If the image is effectively 512x224, aren't the pixels of the stored tiles squished horizontally?

[tepples]

Yes, as one can see from screenshots of the unofficial Simba firmware for the SNES PowerPak. The pixel aspect ratio in this mode is 4:7.

[Ramsis]

Also note that a "screen" of e.g. 32x32 tiles will still be of that same size in Mode 5. You don't suddenly have 64x32 tiles available (as one might imagine), because standard 8x8 tiles will be displayed as 16x8 tiles (with seemingly "empty" tiles in between).

This is what I mean (old WIP screenshot from June 2012):



So you have to find some way to work around this limitation.

[Bregalad]

Yeah, the way you worked around it is very clever, I'd never thought about it ^^

[mikejmoffitt]

Aw, that's all a little straight forward; I was hoping for some fun wacky software trick

Why wouldn't more games make use of this mode?

[Ramsis]

Yeah, the way you worked around it is very clever, I'd never thought about it ^^

Me neither. When I asked ikari_01 (the Great) about how he'd implemented pseudo-hi-res mode in his sd2snes firmware, he told me: 'twas using scroll registers. He also mentioned it could've been done easier using two graphically different fonts. So I decided to try and implement something like that instead.

Why wouldn't more games make use of this mode?

But they did! Have a look at nocash's Hires software list (although Syvalion's missing in the list, as is Romancing SaGa 3, and Dark Law - Meaning of Death (text), amongst others that won't come to mind until later).

[tepples]

Apple II's 80-column text mode is interleaved to put characters at even horizontal position in one 40x24 tile nametable and characters at odd horizontal position in another.

Reasons why more games didn't use 512px backgrounds include ROM cost, VRAM use, loss of BG3, and the fact that sprites weren't likewise 512px.

[mikejmoffitt]

Yeah, the way you worked around it is very clever, I'd never thought about it ^^

Me neither. When I asked ikari_01 (the Great) about how he'd implemented pseudo-hi-res mode in his sd2snes firmware, he told me: 'twas using scroll registers. He also mentioned it could've been done easier using two graphically different fonts. So I decided to try and implement something like that instead.

Why wouldn't more games make use of this mode?

But they did! Have a look at nocash's Hires software list (although Syvalion's missing in the list, as is Romancing SaGa 3, and Dark Law - Meaning of Death (text), amongst others that won't come to mind until later).

I saw, but it is so few!