This chapter is from the book
This chapter is from the book
This chapter is from the book
10.5 Coordinates
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A computer screen is a rectangular area composed of pixels. A pixel is a tiny spot that can be given some color. The most common way of modeling a screen in a program is as a rectangle of pixels. Each pixel is identified by an x (horizontal) coordinate and a y (vertical) coordinate. The x coordinates start with 0, indicating the leftmost pixel, and increase (toward the right) to the rightmost pixel. The y coordinates start with 0, indicating the topmost pixel, and increase (toward the bottom) to the lowest pixel:
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XX
Please note that y coordinates “grow downward.” Mathematicians, in particular, find this odd, but screens (and windows) come in many sizes, and the top left point is about all that they have in common.
The number of pixels available depends on the screen and varies a lot (e.g., 600-by-1024, 1280-by-1024, 1920-by-1080, 2412-by-1080, and 2880-by-1920).
In the context of interacting with a computer using a screen, a window is a rectangular region of the screen devoted to some specific purpose and controlled by a program. A window is addressed exactly like a screen. Basically, we see a window as a small screen. For example, when we said
Simple_window win {tl,600,400,"Canvas"};
we requested a rectangular area 600 pixels wide and 400 pixels high that we can address as 0–599 (left to right) and 0–399 (top to bottom). The area of a window that you can draw on is commonly referred to as a canvas. The 600-by-400 area refers to “the inside” of the window, that is, the area inside the system-provided frame; it does not include the space the system uses for the title bar, quit button, etc.