Scratch that, it won't work

[Kalakian]

Scratch has many limitations, but I thought I could find my way around one of the major ones ...

After some success with the Arkanoid demo I created in Scratch with only one brick sprite being stamped in multiple places, I thought I'd go for a parallax scrolling starfield using only one sprite object for next week's practical session.

I started off by creating a Star sprite with two costumes, one with a single white pixel for drawing the star, and one with a single black pixel for erasing the star, and the stage set to black. I then added in some movement code so it would repeatedly scroll down the screen and wrap when it got to the bottom ... worked ok.

I changed it so I had two stars (one sprite), each with x, y, and speed variables so that they would do the same ... move down the screen and wrap, but stamping the sprite in the appropriate places ... still seemed fine. At this point it looked possible, but I would need to create 3 more variables for each star and the code was gonna get messy, so I decided to use lists.

With three lists, I could keep track of x and y positions as well as the speed the star would move at, by adding values to the lists at the start and then replacing these values. Once I had the code matching the 6 variable version, I executed it and saw a major slow down :(. I thought that was it, lists are too slow and was about to give up when I realised that displaying all 6 variables and 3 changing lists on the stage is bound to slow things down a bit ... removed them from the screen and things went back to a reasonable speed.

Great! Or so I thought ...

I then add in a numStars and loop variable so I could set the number of stars at the top, and execute everything inside a loop, setting the loop variable to 1 at the start and incrementing it inside the repeat loop so I had an index into the lists. If this worked then I would have a parallax scrolling starfield demo that would work with however many stars you wanted ... but Scratch had other ideas. Although my code does in fact work, Scratch is just not for letting me away with that amount of processing, and just slows to a crawl.

Ah well, back to the drawing board. Guess I'll be giving a different practical exercise on Monday then :(

Scratch project is here if anyone wants a look at it