VT03 Color Test

Written by freem; initially released on 2021/05/27

Introduction

VT03 Color Test is a small utility ROM allowing you to preview palette colors using VT03's "new color mode". Since the values aren't always intuitive, it's a good idea to have a tool that can show you what value you need for a specific color.

More information can be found on the NESdev Wiki's "VT03+ Enhanced Palette" page.

Requirements

VT03 supporting emulator (NintendulatorNRS, EmuVT, MAME) or hardware (untested, as I don't own any VT03 devices)

Files

Aside from this text file, there are three ROM files included in this package:

colortest-vt03_emuvt.bin
colortest-vt03_onebus.nes
colortest-vt03_waixing.nes

The difference(s) are explained in the "Technical Details" section.

The source code builds with asm6f, but should also work with regular asm6.

Some screenshots are also included, mainly for documentation's sake.

Interface

Background palettes are found near the bottom middle of the screen, while the Sprite palettes are on the right. Each palette preview includes solid color swatches, as well as a single 4bpp tile showing all 16 colors of the palette set.

The value labeled "Hex" is the two bytes required to define the color. "Lo" is written to $3F00-$3F7F, "Hi" is written to $3F80-$3FFF.

There are five items that can be modified:

"Pal Set" refers to the active set of palettes being edited. Values range from BG (Background) 1 to BG 4, followed by SPR (Sprites) 1 to SPR 4.
"Pal Index" refers to the specific color within the selected palette set. Values range from 0-F (0-15).
"Hue" modifies the Hue of the current color.
"Sat" modifies the Saturation of the current color.
"Lum" modifies the Luminance of the current color.

The values for Hue, Sat, and Lum range from 0-F (0-15).

Usage

Use the D-Pad to move the cursor (Up/Down) and change values (Left/Right). If you need to reset, press Select+Start.

Technical Details

This program was developed using emulators as the testing platform. It may or may not work on real hardware. I don't have any to test with.

As for the three ROM files...

"colortest-vt03_waixing.nes" was the first version developed. It abuses a "feature" of NES 2.0 Mapper 256. If you look closely, you'll notice that the "_waixing" ROM has CHR-ROM defined. In reality, OneBus cartridges have no CHR-ROM, as everything is mapped into a shared address space.

The NESdev Wiki's Mapper 256 page has this to say:

"ROM images converted from Waixing's WXN format will have both PRG and CHR ROM, yet use the VTxx-native bankswitch registers and/or register address mangling. These images cannot be run unchanged on physical hardware, and were intended to be run by Waixing's proprietary VTxx emulator found on several 32-bit plug-and-play consoles. For simplicity, mapper 256 is also used for such ROM images"...

This solution, while great for quick tests, doesn't really work well on EmuVT, or even real hardware.

Indeed, running "colortest-vt03_waixing.nes" on EmuVT will produce incorrect behavior, shown above.

To solve this problem, one must produce a ROM image that matches what the VT03 expects in terms of CHR and PRG-ROM locations.

From the VT03 datasheet:

"In one bus mode, the program initial address A24-A0 is 007FFFC, and the video initial address is 0000XXX."

This means the smallest possible OneBus ROM is 512 kilobytes.

By arranging the CHR-ROM to appear at the beginning of the file, and adding enough padding between the CHR data and the PRG banks, the ROM file can work properly on EmuVT.

colortest-vt03_onebus.nes is a NES 2.0 Mapper 256 image that is meant for use with NintendulatorNRS.

colortest-vt03_emuvt.bin is the above file without an iNES header. It can be run in NintendulatorNRS, EmuVT, and MAME.

Running the bin file in MAME requires loading it as an existing ROM. I have been using "vtsndtest" as a surrogate. Make the "vtsndtest" directory if it doesn't already exist in your MAME ROMs folder, then copy the .bin file there and rename it to rom.bin.

Though it's not immediately obvious, the demo uses 4bpp tiles. (They are found at the right edge of the background palette preview, and at the top of the sprite palette preview.)

Each 4bpp graphics page is twice the size of its 2bpp counterpart. Keep this in mind if you're going to do tests with the Waixing emu hack.

Graphics for this tool were created using YY-CHR's "4BPP 8X8" mode.

For more technical information on the graphics formats supported by the VTxx hardware series, check the NESdev Wiki "VT02+ CHR Pattern Data Layout" page.

And now, the real important information: How to set up the palette for 4BPP mode.

Low Byte

Address	`$x0-$x3`	`$x4-$x7`	`$x8-$xB`	`$xC-$xF`
`$3F0x`	BG1 0-3	BG2 0-3	BG3 0-3	BG4 0-3
`$3F1x`	SPR1 0-3	SPR2 0-3	SPR3 0-3	SPR4 0-3
`$3F2x`	BG1 4-7	BG2 4-7	BG3 4-7	BG4 4-7
`$3F3x`	SPR1 4-7	SPR2 4-7	SPR3 4-7	SPR4 4-7
`$3F4x`	BG1 8-11	BG2 8-11	BG3 8-11	BG4 8-11
`$3F5x`	SPR1 8-11	SPR2 8-11	SPR3 8-11	SPR4 8-11
`$3F6x`	BG1 12-15	BG2 12-15	BG3 12-15	BG4 12-15
`$3F7x`	SPR1 12-15	SPR2 12-15	SPR3 12-15	SPR4 12-15

High Byte

Address	`$x0-$x3`	`$x4-$x7`	`$x8-$xB`	`$xC-$xF`
`$3F8x`	BG1 0-3	BG2 0-3	BG3 0-3	BG4 0-3
`$3F9x`	SPR1 0-3	SPR2 0-3	SPR3 0-3	SPR4 0-3
`$3FAx`	BG1 4-7	BG2 4-7	BG3 4-7	BG4 4-7
`$3FBx`	SPR1 4-7	SPR2 4-7	SPR3 4-7	SPR4 4-7
`$3FCx`	BG1 8-11	BG2 8-11	BG3 8-11	BG4 8-11
`$3FDx`	SPR1 8-11	SPR2 8-11	SPR3 8-11	SPR4 8-11
`$3FEx`	BG1 12-15	BG2 12-15	BG3 12-15	BG4 12-15
`$3FFx`	SPR1 12-15	SPR2 12-15	SPR3 12-15	SPR4 12-15

In old color mode, you just write the regular NES color values to the Low Byte section ($3F00-$3F7F).

For new color mode...

The low byte

76543210
xx|||__|
  ||  |
  ||  +-- Hue
  ++----- Luminance (lower 2 bits)

The high byte

76543210
xx|__|||
    | ||
    | ++-- Luminance (upper 2 bits)
    +----- Saturation

freem / colortest-vt03