The same programme is executed in parallel for each pixel. The only difference is that each pixel knows its own unique horizontal offset (the x variable) and its own unique vertical offset (the y variable) on the canvas.
Commands and data must be separated by a space or a line break. If you enter a number or a command that returns a number, that number is placed on the stack. The maximum stack depth is 16. Some commands perform an operation on several numbers at the top of the stack.
In Forth Demotool (unlike traditional Forth), all stack cells and memory locations are floating point numbers. Input commands are unique to Forth Demotool.
When, after the programme has finished running, there is 1 number remaining on the stack, it is interpreted as the brightness of the red component of the pixel (from 0 to 1, values less than 0 are displayed as black, and values greater than 1 are displayed as 1). If there are 2 numbers on the stack, these are the red and green components. 3 numbers include the blue component as well. 4 numbers also include the transparency value.