This is simply because the color of the pixels along the edge of this triangle have been chosen to reflect the degree to which they fall within the triangle. Each pixel should be thought of, not as a point, but as a small area that must be filled with the most appropriate color. The further outside of the red line a pixel falls, the darker it gets. In this way additional structural information is being encoded into the scene in the form of subtle color changes, and it is this incorporation of additional information that lies at the heart of most antialiasing techniques. Indeed without additional information it is only ever possible to blur an image, but never possible to render it with any additional accuracy.
Additional accuracy requires additional information.
This is a basic and important point that we will be coming back to from time to time. In practice though, both the triangle and its surroundings are likely to be multicolored due to the application of textures. Unfortunately the pixels that lie along the edge can only take one color, and as in the previous example that color is calculated from the relative proportions of the two colors that fill each divided pixel. Placing the mouse pointer over the image will cause those pixels to be filled with the most suitable color. I have described antialiasing in this simplified form initially, because it leads neatly to the technique known as edge antialiasing .