MR image
About
A MR image is an image format solely used inside an IP.BIN file to show off a logo during the boot screen process of the Sega Dreamcast.
This image format was used in commercialized Dreamcast games utilizing Windows CE SDK where we see a "Powered by Microsoft Windows CE" logo. It is now commonly used in homebrew to distinguish the creator(s) of the application or to disassociate the application being produced by or under license from Sega. Since IP.BIN is restricted to a 32k file size and the MR image must be inserted into that file in order to be displayed during the boot process, there are a couple of guidelines the MR image must meet to be shown during the boot process.
MR Image must be:
- 320x90 or less
- Max 128 colors
- Less than 8192 bytes to fit in a IP.BIN
- The transparent color is #c0c0c0, or 192, 192, 192 in RGB
Tools
Selfboot-Inducer
SiZious's Windows Tool to create homebrew compilations. Using this application you can view and create MR images from other various other image formats.
MR GIMP Plugin
GIMP is a cross-platform image editor available for GNU/Linux, OS X, Windows.
Using this GIMP plugin("file-mr.py"), you can create view, edit, and create a MR image from any other image format that GIMP supports.
File Format
The file format is composed of three sections: Header, Palette, and the Image Data. All of the data written in the file is in little-endian format.
A MR file starts off with a 30 byte header.
Header
| Size | Contents |
|---|---|
| 2 bytes | "MR" |
| 4 bytes | Total file size |
| 4 bytes | Crap (fill with 0's) |
| 4 bytes | Image data offset in bytes (Header size + Palette size) |
| 4 bytes | Image width |
| 4 bytes | Image height |
| 4 bytes | Crap (fill with 0's) |
| 4 bytes | Amount of colors in palette |
Palette
The header is followed by the image's palette. The palette is composed of palette entries where each palette entry is 4 bytes long and is stored as BGRA. The alpha byte goes unused.
| Palette Entry | |||
|---|---|---|---|
| B | G | R | A |
In total, the byte size of your palette should be number of colors * 4.
Image Data
Lastly, the image data. The image data is compressed using a form of Run-Length Encoding (RLE). The pseudocode algorithms to encode/decode the image data are shown below.