Drawn once, and it sits still

Challenge 16 of 16

The numbers a call hands over can come from variables, so the whole scene is decided by two lines at the top. Change one and everything built from it moves — but it still only moves when you edit it.

Set the two numbers.

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Set the two numbers.

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Lesson Building a Scene

Last topic ended with nine lines to draw three houses. Every house needed a wall, a roof and two windows, and you wrote all four calls out every time.

A function body can call other functions. So write the house once:

function drawHouse(x, y, size) {
drawWall(x, y, size);
drawRoof(x, y, size);
drawWindow(x - size / 4, y - size / 2, size / 4);
drawWindow(x + size / 4, y - size / 2, size / 4);
}

and now one line of yours draws four shapes:

drawHouse(200, 330, 160);

Three houses is three lines. drawHouse knows how a house is put together; everyFrame only says where the houses go. That split is the whole idea of this topic.

Look at what the inner calls hand over. They are not plain numbers — they are worked out from the inputs drawHouse was given. A window at x - size / 4 sits a quarter of the way left of whatever x the house was told, and it is size / 4 across, so it moves and grows with its house. Write a plain number there instead and that window stays in one place and one size no matter which house it belongs to.

The calls still run top to bottom, and now that matters more than it did. Whatever is painted first can be painted over. Call drawWindow before drawWall and the wall covers the window completely — the window is still drawn, it is just underneath. A whole scene works the same way: the sky goes first, then the ground, then the things standing on it.

Some functions need nothing at all in their brackets. drawGround always draws the same grass, so there is nothing to tell it, and the brackets stay empty in both the definition and the call.

A function does not belong to whoever called it first. drawWindow can be called by drawHouse and by drawShop, and the colour of every window in the picture is written down in exactly one place. Change that one line and they all change.

Calls can go deeper than one level. drawStreet calls drawHouse three times, and each of those calls four more, so a single line in everyFrame ends up drawing twelve shapes. The number you hand drawStreet reaches all the way down to the windows.

Everything here is drawn once and then sits still. Next topic the numbers in the brackets start changing every frame, and the whole scene comes alive.