Understanding Piano Roll Notation
One of the main ways you can give "feeling" to the performance of notes, that you have previously entered with the mouse and keyboard, is to adjust the exact as-performed timing of the notes. That is, you can adjust the as-performed attack and release times of the notes.
Composer's Piano Roll Notation helps you visualize the as-performed timing of notes:
The rectangles drawn on top of the notes show the actual performed timing of the notes. These rectangles are called "Piano Roll Notation" because they resemble the hole cutouts for notes in piano rolls of mechanical player pianos that were popular in the late 1800's and early 1900's. The left and right edges of the rectangles indicate the as-performed timing of the notes. Because normal spacing of notes in a score is not exactly proportional to actual note durations, the lengths of the rectangles are not totally precise. Nevertheless, the rectangles serve to clearly show you where notes start and stop.
The color of rectangles may be yellow (below) instead of white (above):
The color of the a piano roll rectangle is white or yellow to show whether or not the performance of the note will be edited or not. The color of the notation (gray or black) shows whether the notation will be edited or not.
Here's a way to help remember the meaning of the yellow versus white rectangle. The yellow rectangle stands out more prominently than the white, and this is a clue that if you change the location or duration of the note, the rectangle will change. When the left or right edge of the yellow rectangle changes, the as-heard timing of the note changes. Therefore, if the rectangle is yellow, a command such as D+ will change the as-performed duration of the note. Conversely, if the rectangle is white, then the rectangle "in the background" is not affected by a command such as D+. That is, if the rectangle is white, then D+ affects only the notated duration of the note, not the as-performed (as-heard) duration.