Why? For building huge builds just install other tools. There are all sorts of minimaps, and so. Beside, I tried to code such funny thingy, for my own amusment, and I will tell You one thing. It will eat Your CPU. Really. On distance 64 blocks (far render ?), It have to read each block in a minimal square of 64X128X128= 1 048 576 blocks. And when we want to add color masks You have to do a raycasting thingy in many directions. And all that on each tick, or on each action. Well that will actually eat lot of RAM, and CPU. Solution is to do a smaller distance (like 8 blocks), but then It will be not so good for building huge...
Other solution is to get x-ray mod (think that it was called something diffrente, but google it), and try build with that with a huge lag 
May be tool to detect number of solid blocks in a straight line would be enough for this task?
Or you can use a similar one to X ray : Cavefinder, which shows you air pockets underground.
If IC² ever code something like that it would be funny, because it still won't show ores surrounded by stone.
or, you know, an x-ray texture pack?
just for making the underground stuff, no need to overcheat.
No need to cheat at all.
How about some helmet (may be Quantum one or a new special Sonic Helmet) will show player direction of each sound in a visor area by simple ping indicator in HUD. This will help to locate "strange sounds" generated by Minecraft in the nearby dark caverns.
Why do we need X-Ray-Goggles, when we have TNT-Piston-Cheaty-X-Ray-Machines in vanilla Minecraft?
will show player direction of each sound in a visor area by simple ping indicator in HUD. This will help to locate "strange sounds" generated by Minecraft in the nearby dark caverns.
I guess this is a good, funny and non-OP way to do it.
Why do we need X-Ray-Goggles, when we have TNT-Piston-Cheaty-X-Ray-Machines in vanilla Minecraft?
Exploiting is kinda like cheating. (I know, it's strange from someone that had used so much CASUC or others mods-interaction exploits, but doing it with MC-vanilla ... I guess CASUC from example, even if OP, were in parts known from Alblaka/Elooram ... but what you used wasn't really planned by Notch ^^)
