Writing a string to a framebuffer device?

Question

This is on a Raspberry Pi, Linux 3.13, bash 4.2

Connected is a SPI 320x240 Display which is displaying a background image (I used fbi to display the image, the cat /dev/fb1 > framebuffer.contents, then whenever I need the image I do cat framebuffer.contents > /dev/fb1 to save the looong startup time of fbi).

Now I'd like to "amend" my image by a (changing) one-line text (a few word, the time, ...). Therefore I am looking for a way to write a string, from bash, to a specific location within the framebuffer. Eg. a tool is needed that a) converts a string given to some sort of framebuffer-redabe graphic (ppm?), then b) inserts that resulting image into a given part of the framebuffer.

Any ideas? Or is my good old bash-and-unix-tools approach over it's head here and I need to look into .... ?

Answer 1

I'm not quite sure in which situations you need to do what you do. But maybe you could "misuse" splashutils/fbsplash for your needs? It's rather meant to display a framebuffer bootsplash but has some tools to tinker with a fb (testing tools for splashscreen development as an example). You can read about how to use it on the Gentoo wiki.

Aside from that maybe you can find some inspiration on a thread at Unix & Linux - How to use /dev/fb0 as a console from userspace, or output text to it.