Starfield

{Introduction }

Having stars moving in the background of your scene is an effective way to show motion for a game that’s set in space. Stars filling the screen as moving projectiles can create an illusion of a player sprite speeding through space, even if it remains at the same location on the screen.

{Step 1}

In the ||game:Game|| Toolbox drawer, get an ||game:on game update|| block and drag it into the workspace.

game.onUpdate(function () {

})

{Step 2}

From ||sprites:Sprites||, find the ||variables:set projectile to|| block that has a ||sprites:projectile from side|| inside of it. Drag it into the ||game:on game update||. Change vx to 0 and vy to 100.

game.onUpdate(function () {
    let projectile = sprites.createProjectileFromSide(img`
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`, 0, 100)
})

{Step 3 }

Click on the grey box in ||sprites:projectile from side|| and create a single white pixel for the star.

game.onUpdate(function () {
    let projectile = sprites.createProjectileFromSide(img`
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`, 0, 100)
})

{Step 4 }

Grab a ||Math:pick random 0 to 10|| from ||Math:Math||. Place it in the ||sprites:vy|| slot of ||sprites:projectile|| right over the 100. In the ||Math:pick random 0 to 10||, change the 0 to 20, and the 10 to 30

game.onUpdate(function () {
    let projectile = sprites.createProjectileFromSide(img`
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`, 0, randint(20, 30))
})

{Step 5}

Go back to ||sprites:Sprites|| and find the ||sprites:set mySprite position to||. Place it after ||variables:set projectile to|| and change ||variables:mySprite|| to ||variables:projectile||.

game.onUpdate(function () {
    let projectile = sprites.createProjectileFromSide(img`
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`, 0, randint(20, 30))
    projectile.setPosition(0, 0)
})

{Step 6}

Grab another ||Math:pick random 0 to 10|| and put it in as the ||sprites:x|| value for ||sprites:set projectile position to||. Find ||scene:screen width|| in ||scene:Scene|| and replace the 10 in the ||Math:pick random 0 to 10|| with it.

game.onUpdate(function () {
    let projectile = sprites.createProjectileFromSide(img`
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`, 0, randint(20, 30))
    projectile.setPosition(randint(0, scene.screenWidth()), 0)
})

{Step 7 }

At this point, too many stars are being created. This is fixed by surrounding the blocks inside ||game:on game update|| with an ||logic:if then|| block.

game.onUpdate(function () {
    if (true) {
        let projectile = sprites.createProjectileFromSide(img`
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`, 0, randint(20, 30))
        projectile.setPosition(randint(0, scene.screenWidth()), 0)
    }
})

{Step 8}

Get a ||Math:0 % chance|| block from ||math:Math|| and replace the true condition in the ||logic: if then|| with it. Change the percentage from 0 to 25.

game.onUpdate(function () {
    if (Math.percentChance(25)) {
        let projectile = sprites.createProjectileFromSide(img`
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`, 0, randint(20, 30))
        projectile.setPosition(randint(0, scene.screenWidth()), 0)
    }
})

{Complete}

Congratulations, your starfield is complete! Build any game you wish on top of it, or continue to the next step for a challenge that will help optimize the code.

{Extra Challenge }

In reality, the stars in outer space are millions of light years away from anything else. So, they shouldn’t interact or collide with other objects out there.

Find ||sprites:set mySprite auto destroy|| in ||sprites:Sprites||. Place it after ||variables:set projectile to|| and change ||variables:mySprite|| to ||variables:projectile||. Change ||sprites:auto destroy|| to ||sprites:ghost||, and click on OFF to turn it ON.

This also has a large effect on the frame rate, as the game can skip processing any actions related to the stars overlapping other sprites in the game.

game.onUpdate(function () {
    if (Math.percentChance(25)) {
        let projectile = sprites.createProjectileFromSide(img`
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`, 0, randint(20, 30))
        projectile.setPosition(randint(0, scene.screenWidth()), 0)
        projectile.setFlag(SpriteFlag.Ghost, true)
    }
})