Page 99 - Learn To Program With Scratch

running without Screen refreSh
Implementing the DrawFlower procedure with custom blocks brings up
another feature that can shorten the execution time of the drawing script . To
demonstrate, perform the following:
1 . Right-click the DrawFlower block under the More Blocks palette and
select edit from the pop-up menu . This should bring up the dialog from
Figure 4-11, except that the title will be Edit Block instead of New Block .
2 . Click the arrow next to Options, check the Run without screen refresh box
and click OK (see Figure 4-15) .
3 . Now, click the mouse on the Stage and see what happens . Instead of
seeing the individual rotating and stamping steps as the five flowers are
drawn, you should see them appear on the Stage almost at once . Here is
an explanation of what’s happening .
The DrawFlower procedure contains many blocks that change a sprite’s
appearance, including set color, set brightness, set size, and stamp .
After executing such a block, Scratch normally pauses for a while to refresh (that
is, redraw) the screen . This is why we were able to see the drawing progress
when the application ran before .
If you select the Run without screen refresh option, the blocks will run with-
out pausing to refresh the screen, allowing the procedure to run much faster .
The screen will refresh after Scratch executes the entire procedure .
In addition to speeding up a procedure, the Run without screen refresh
option helps to prevent the flickering that repeated redrawing can cause .

Passing Parameters to Custom Blocks
Let’s start by creating a custom block named Square, which draws a square
whose side length is 100 pixels, as shown in Figure 4-14.

Figure 4-14: A Square procedure that draws a
fixed-size square

Procedures 77

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