Screenshots manager for MegaSD
If you can't compile the source, or just want to use the tool, you can grab a precompiled build and sample library from the Built directory
- First, prepare a 64x40 screenshot for each game you want to add
- Go to the very last list entry, marked with an *, and write the game name in the first column. A new row will be created for your game.
- Fill in the year and genre.
- Press the button in the "Screenshot" column and select the PNG file you want to appear in the MegaSD menu
- Press the button in the "Hashes" column. In this screen, select "Add Hash", and then point to the cd .cue file or rom (.md,.bin,.32x...) you want to associate with this game. A game can have more than 1 hash attached for different versions or region variations, and they will all share the same screenshot.
- Repeat these steps for all games you want to add.
- Once finished adding games, press "Convert Imgs" button. This will convert your png files to console tilemaps. It will only convert the new files, so if you change the image after generating the tile cache, just delete it from the cache and then regenerate images.
- Once the database is built, it can be used to generate the game images, or to distribute it.
- For distribution, you must distribute the db.xml file, the executable, and the TileCache directory. The original images aren't needed, as they have been converted to tile data previously.
- Once you have built your DB, or have acquired a pre-built one, you can scan your SD card for known games, so their screenshots are stored in a file that MegaSD can use to show them on screen.
- Just press the "Scan Roms" and point to your SD card root, or any directory with roms.
- When finished, a "Games.dbs" file will be created in each directory with games having screenshots. This file is specific to your directory structure, so sharing it might not work if other user games aren't named the same way.
As commented above, png files used must be 64x40 pixels in size, but there are more restrictions. Because of the limited Megadrive/Genesis palette, only 3 16-color palettes can be used (1st palette of the 4 available is used by the menu) so you must build your images with as little colors as possible.
Setting them to 15 colors in your image edition software will work, as it will only use 1 palette (1 color is reserved for transparent). If you have more than 15 colors, then the tilemap conversion code will attempt to reduce the colors by first discarding color information not used by the console, then merging similar colors... but if it can't make all them fit in 3 palettes, an error will show in the program console screen when this image is processed with "Convert Imgs" button, and you'll need to further reduce the colors.
If you want to build you own tool from scratch, here is the internal working of the files created on each directory after scanning:
The file is divided in 3 sections:
Each 4 byte entry of this table is the 32-bit checksum of the filename for a ROM (for rom games) or directory (for cd games). This checksum is computed over the first 56 bytes of the file or directory name, converted to uppercase. The algorithm to use is the one in the provided in the Crc32 class, that is the same than the firmware uses to compute the crc on its side when scanning directories. Just compute it with Crc32.update_crc(0xFFFFFFFF, namecnv, 56).
It's stored in little endian format. This table must be sorted by checksum value, so the firmware can perform a fast binary search on it
Each 2 bytes in this table store a little endian 16-bit screenshot index for the next section. This table is indexed by the position of the crc in the previous table, so keep in mind this when sorting the CRCs. When moving a CRC value to a different position while sorting, the corresponding screenshot value must also be moved to match the new position. Several games can have the same screenshot ID if they are just rom version or regional variations with the same screenshot, saving space
Each 2048 entry on this table is further divided in 4 sub parts:
- Screenshot tiles: (offset 0) 1280 bytes: Tiles are stored in VDP order as is, ready to be transferred to the VDP. 8x5 tiles, 8x8 pixels each, 4 bit per pixel.
- Tile palette indices: (offset 1280) 40 (8 * 5): Palette index the tile uses (1 byte per tile), allowed values are 0,1 or 2.
- Palettes: (offset 1320) 96 bytes (3 palettes, 16 colors, 16 bits per color): Color palettes used in the tile. Each entry is a 16-bit, little endian, palette color in Megadrive/Genesis format: 0000BBB0GGG0RRR0
- Extra info: (offset 1792):
- Offset 0: Extra field version. Currently 1. Setting it to 0 disables year or genre showing in menu
- Offset 1: Genre index.
- Offset 2 and 3: Year, 16 bit, little endian