The main feature is a test screen like broadcast in former days before 24 hour TV programs. It is used to test geometry and resolution of the monitor.
Also included are several test screens for testing moiree effects, the horizontal and vertical screen resolution, and colour convergence.
There is a big circle in the middle and smaller circles in each corner. They are round pixelwise, so they should be circles if the screen size ratio (width:height of visible area) is equal to the pixel ratio. It usually should be 4:3. So the best sizes for testing are 320x240, 640x480, 800x600 and up.
In each corner there is one box with vertical stripes. These are one pixel wide, with one pixel distance, so you get maximum signal frequency and can see how well your monitor and video card handle the dotclock.
The middle field has eight solid blocks with the eight colours, i.e all combinations of the red, green and blue signals turned on and off. Below it there are four fields with vertical stripes as in the corners, but in white and the three basic colours red, green and blue. Below it there is a bar with these four colours red, blue, green and white blending from full intensity (left) to zero intensity (right), i.e. black.
This is tested by painting a grid of red, green and blue + signs. They should be aligned properly where they touch. Usually they don't...
Vertical white stripes with width 1, 2, 3 and 4 pixels. See what the highest dotclock is the monitor can handle.
Horizontal stripes with width 1 and 2 pixels. See how well the scan lines are separated.
Three stars of black lines on white, with a width and space (at the sceen borders) of 1 and 1, 1 and 5, 2 and 10 respectively. Watch for colour changes, and once again you can see the maximum frequency your monitor can do.
Vertical bars in red, blue, green and white, with the width of 4, 3, 2, 1, 2, 3 and 4 pixels for bar and space. See whether there is a difference in resolution between the colours. And watch, again, for the alignment.
If there is interference between the monitor mask and a grid displayed on the monitor, a change of colours can be seen, and is sometimes very annoying. There are three tests present, each one comes in the four colour combinations black, red, green and blue on white background : vertical stripes, one dot wide, with one dot space; white dots on colour ground, spaced two and two (run testscreen with a really low resolution to see what I mean :-); a chessboard.
The rectange is moved using the mouse or the arrow keys.
The color of the rectange can be changed by pressing the primary mouse button (usually the left one) or <Space>, cycling through (black, red, green, blue, white) or by pressing a number between 0 and 4 or the first letter of the colour (b is blue).