Buried in the blog post introducing display support for U-Boot, there are some notes about creating splash screens for the boot loader on Nitrogen boards.
Using a splash screen isn’t specific to Nitrogen, though, and since it’s a really common question and because the file format restriction is pretty strange, it deserves its’ own commentary.
To reiterate from the prior post:
U-Boot only supports 8-bit palletized, uncompressed bitmap (.BMP) files.
In English, this means that the files should use the Microsoft-defined BMP file format with a maximum of 256 colors and no compression.
Various versions of graphics tools like Paintbrush, Gimp, Photoshop can all create files in this format, but the option selections may be buried in several levels of “Advanced” menu options.
Which is why we recommend the use of ImageMagick. Using ImageMagick, you can convert to the proper format using this command line:
convert inputfile.png -depth 8 -colors 256 -compress none output.bmp
All common file formats are supported as inputs and this tool will do the right thing.
You can verify that the file is correct by inspecting its size. If you’ve converted things properly, your output file should be roughly 1k larger than
WIDTH*HEIGHT in pixels.