Robert Dale Smith (of ControllerAdapter.com) has just released a brand-new online VMU icon editor.
http://dcvmuicons.net/maker/
• Import/Edit/Generate ICONDATA_VMS files 100% in the browser. (all front-end, no backend)
• Import and extract color icons from existing game save files (.VMS, .DCI, or .DCM)
• Import most image formats with automatic resize and color reduction (4-bit RGBA, 16-color palette)
• Custom 4-bit color picker with alpha channel. (transparency)
• Color and monochrome 32x32 pixel drawing canvases with color palette selector.
• Easy color to monochrome color palette toggle tool.
• Custom file description string. (16 characters)
• Dreamcast BIOS Menu 3D mode enabler. (press start in DC bios for 3d mode)
• Generates zip file containing VMS, VMI, preview.gif, color.bmp, and mono.bmp files.
• Stores generated icon history in local storage.
