Covering half a shape

Challenge 8 of 12

The mouth starts life as a whole red circle. A rectangle is drawn over its top half to leave only the bottom curve showing, which turns it into a smile.

Work out what colour that rectangle has to be.

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Work out what colour that rectangle has to be.

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Lesson Layers

Your code runs from the top of everyFrame to the bottom, one line at a time, and each shape is painted over whatever is already there. That is the whole rule of layers: later covers earlier.

It sounds obvious, but it is a tool. Draw a big circle, then a smaller circle in the background colour on top of it, and you have made a ring — with no ring command anywhere. The same move gives you crescents, wedges and smiles, none of which Visuco has a command for.

The trick to see is this: a shape painted the same colour as what is behind it has not vanished. It is still there, still drawn, still counted — it simply cannot be told apart from its surroundings. That is why it looks like a hole.

Overlap joins things up as well as cutting them out. Several circles of the same colour, with no outlines between them, read as one blob rather than several.

It is also the first thing to check when a shape "does not appear". Very often it did appear, and something after it painted straight over the top.

In this topic you will stack shapes on purpose and work out, from the picture you are aiming at, what colour a shape has to be to carve a piece out of another.