How do I hide the bug image once it's hit by a bullet?

Question

I have almost solved my problem. I am able to shoot a bullet and hit a bug and it then draws a collision bitmap. but then it redraws a bug, I want to clear the bug bitmap after the collision happens.

public Form1()
{
    InitializeComponent();
}
public void Form1_Load(object sender, EventArgs e)
{
    timer1.Interval = 20;
    timer1.Start();
    timer2.Interval = 100;
    timer2.Start();
    this.SetStyle(ControlStyles.AllPaintingInWmPaint | ControlStyles.UserPaint | ControlStyles.OptimizedDoubleBuffer, true);
}
int x = 0, y = 0;
Bitmap bug_one = new Bitmap("bug_one.bmp", true);
Bitmap ship = new Bitmap("ship.bmp", true);
Bitmap bullet = new Bitmap("bullet.bmp", true);
Bitmap coll = new Bitmap("coll.bmp", true);
Bitmap black = new Bitmap("black.bmp", true);
private void Form1_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyCode == Keys.Left)
    {
        x-=5;
        if(x<=-350)
        {
            x = -350;
        }
    }
    if(e.KeyCode == Keys.Right)
    {
        x+=5;
        if(x>=375)
        {
            x = 375;
        }
    }
    if (e.KeyCode == Keys.Space)
    {
        if (y <= -530)
        {
            y = 0;
        }
    }
}
int count = 102;
public void Form1_Paint(object sender, PaintEventArgs e)
{
    e.Graphics.Clear(Color.Black);
    e.Graphics.DrawImage(ship, 350 + x, 530);
    e.Graphics.DrawImage(bullet, 375 + x, 520 + y);
    e.Graphics.DrawImage(bug_one, 350, 0);
    if (y <= -510 && x >= -15 && x <= 15 && count > 0)
    {
        e.Graphics.DrawImage(coll, 350, 0);
        count = 0;
    }
}
public void timer1_Tick(object sender, EventArgs e)
{
    y -= 5;
    Invalidate();
}
public void timer2_Tick(object sender, EventArgs e)
{
    count--;
    Invalidate();
}

---

This attempt almost works: when a bullet hits a bug it draws a collision image, but then all the bugs are cleared. I only want one bug to be cleared at a time.

    public void Form1_Paint(object sender, PaintEventArgs e)
    {
        e.Graphics.DrawImage(ship, x-25, 530);
        e.Graphics.DrawImage(bullet, x, y);
        if (collision)
        {
            return;
        }
        foreach (Bug bug in bugs)
        {
            e.Graphics.DrawImage(bug_one, bug.px, bug.py);
        }
        foreach (Bug bug in bugs)
        {
            if (x >= bug.px && y >= bug.py && x <= bug.px + bug.width && y <= bug.py + bug.height) 
            {
                e.Graphics.DrawImage(coll, bug.px, bug.py);
                collision = true;
            }
        }
    }

Answer 1

I assume that Form1_Paint is called repeatedly. Why not add some kind of flag indicating that a collision happened, and just avoid drawing the image?

public void Form1_Paint(object sender, PaintEventArgs e)
{
    e.Graphics.Clear(Color.Black);
    e.Graphics.DrawImage(ship, 350 + x, 530);
    if (collisionHapened) {
        return;
    }
    e.Graphics.DrawImage(bullet, 375 + x, 520 + y);
    e.Graphics.DrawImage(bug_one, 350, 0);
    if (y <= -510 && x >= -15 && x <= 15 && count > 0)
    {
        e.Graphics.DrawImage(coll, 350, 0);
        count = 0;
        collisionHappened = true;
    }
}

If you want to really clear the bitmap, which is a relatively slow operation, use MakeTransparent and then set the each of the bitmap's pixels' alpha to zero (see example).

On a side note, consider not relying on global state, and instead pass to each method its required parameters. If the behavior of a method fully relies on its arguments instead of on global state, it's significantly easier to understand why erroneous behavior happens.

Answer 2

        public void Form1_Paint(object sender, PaintEventArgs e)
    {
        e.Graphics.DrawImage(ship, x-25, 530);
        e.Graphics.DrawImage(bullet, x, y);
        foreach (Bug bug in bugs)
        {
            if (bug.collision==false)
            {
                e.Graphics.DrawImage(bug_one, bug.px, bug.py);
            }
        }
            foreach (Bug bug in bugs)
            {
                if (x >= bug.px && y >= bug.py && x <= bug.px + bug.width && y <= bug.py + bug.height)
                {
                    e.Graphics.DrawImage(coll, bug.px, bug.py);
                    bug.collision = true;
                }
        }
    }

I finally got my code to work properly it draws a bug then it gets hit by a bullet and then it draws a collision image and then disappears, just like I wanted it to do.