A simple basic shape test, which is very customisable:D In here, you can choose between a triangle, a square, and a line, all of which stretch to your mouse. You can also change the size, the colour, the brightness, the transparency and the saturation!
All by me, @-GreenComet-! You can use this in your games if you wish, but credit is necessary. Setting 'transparency' to 20-70 when the size is large looks quite cool:P
