How to Do Run-Length Encoding on a Bitmap Row

To do run-length encoding (RLE) on a bitmap row, read the pixels from left to right. Count how many consecutive pixels have the same colour, then record that count and the colour. Start a new count whenever the colour changes.

A bitmap stores an image as pixels. A run is a group of neighbouring pixels with the same value or colour. RLE stores each run instead of listing every pixel separately.

To keep track, start with the first pixel and set its run count to $1$. Compare each following pixel with the one before it. If the colours match, increase the count by $1$. If the colour changes, record the completed run and start a new run with a count of $1$. When you reach the end of the row, record the final run too. Otherwise, your encoded row will be incomplete.

Worked example

Suppose a row contains white, white, white, black, black, white, white. Let $\mathrm{W}$ mean white and $\mathrm{B}$ mean black. The row is:

$$\mathrm{W},\mathrm{W},\mathrm{W},\mathrm{B},\mathrm{B},\mathrm{W},\mathrm{W}$$

Reading from the left, there are $3$ white pixels, then $2$ black pixels, then $2$ white pixels. Using count, colour pairs, the encoded row is:

$$(3,\mathrm{W}),(2,\mathrm{B}),(2,\mathrm{W})$$

You can check your answer by decoding it: write $3$ white pixels, $2$ black pixels and $2$ white pixels. You should get the original row back.

Watch out

Do not combine the white pixels at the beginning and end into one run. They are separated by black pixels, so they must be recorded separately. Also check the format given in the question: some questions specify a starting colour and ask for run lengths only, while others require both the count and the colour. If you are encoding more than one row, finish one row before starting the next.

RLE works best when there are long runs of identical pixels. A row whose colour changes after almost every pixel may not become shorter.