How Do I Understand What Is The Amount Of Bit Needed For Each Coordinate Values?

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data

storage

With line graph line at some point (such as an endpoint) began to record the coordinates of black pixels point by point, such as:（3，6），（4，7），（5，7），（6，6），（7，5），（8，5），（9，5），（10，4），（10，3），（9，2）。
With this way to store line diagram,each black pixel to be stored two coordinates(x,y),he number required to store a coordinate values and the size of the image.
For example, if an image size is 512 * 512 pixels, each coordinate values are represented with nine binary.

Why answer is 9？？？