How to handle mouse hovering for object [Allegro5/C++]

Question

I want to be able to register/de-register Objects to a list and check if the mouse is hovering over them to display tool-tips. However I'm stumbling at the beginning.

I'm going to use: al_get_pixel & al_unmap_rgba to retrieve the alpha of each pixel and decide if it's visible enough to count as a hit when hovering over it with the mouse.

The major problem I'm having is working out how best to store this individual "hitmap" & the reference to the object that generated it; As many different types objects (as struct's) of different sizes may need hitmaps generated. I was hoping I could do something similar to checking if the complex object 'extends' the base object but I don't see how I can achieve this in c++.

Ps; I know I could create an array the size of the screen for each object, but I'm doing this mainly with the purpose of maximizing efficiency. I'd make dynamic sized arrays but...

Answer 1

al_get_pixel will work, but it will be terribly slow, even if you lock all your bitmaps, unless you use something like a picking buffer. The basic idea is to render every interactive area on each object with a different color id. This means you need to draw your scene twice, once normally, and once with picking colors. Then when you need to read back a mouse position, you can use the picking buffer to read a single pixel and get its color id.

You might also want to try different methods of collision detection, such as bounding boxes, bounding circles, or other easily collision detectable shapes.

There is a third option, which is pixel perfect collision. It involves making 1bpp masks out of all your objects and then checking for collision between those.