Drawing Lines
Before you draw a line on an image, you need to determine the points from and to which you want to draw.
You can think of an image as a block of pixels indexed from 0 on both the horizontal and vertical axes. The origin is the upper-left corner of the image.
In other words, a pixel with the coordinates 5, 8 is the sixth pixel along and the ninth pixel down, looking from left to right, top to bottom.
The imageline() function draws a line between one pixel coordinate and another. It requires an image resource, four integers representing the start and end coordinates of the line, and a color resource.
Listing 15.2 adds to the image created in Listing 15.1, drawing a line from corner to corner.
Listing 15.2. Drawing a Line with imageline() ...
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