I wrote a code using c++ and directx to make a small rectangle move quickly on the screen. To make the rectangle, I glued two triangles together and timed it to determine its position. The moving rectangle make long tail on the screen which I didn’t expecte.

So I filmed it with the camera at 120 fps, I found that several rectangles(more than 3) were being drawn at the same time. The rectangles fade in on front and fade out on back, making it look like the tail of a comet. This is not something I coded in and I don’t know where to go to fix it.

Screen captured on camera