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The problem is that null bytes cannot be passed through command line arguments as they are internally used as argument terminators. All other bytes seem to be fine. So a somewhat more space-efficient (typically) alternative to using base64 would be to escape the null bytes and then use printf to convert the data to original form: pngString="$(sed ...
You could use convert from ImageMagick: convert <input_image> -fill '#000' +opaque '#000' <output_image> What it does it changes all the colors not matching #000 to #000.
The "emulated consoles" are system-specific, as you understand with your reference to fbdev. Furthermore, they are generally integrated into the kernel, so trying to modify them directly would require modifying the kernel, which is certainly more 'overkill' than putting up with X. You could use fbdev directly, or write something based on SDL like Thomas ...
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