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# Sprite "blocked" by another sprite (w/ pictures and code)

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3 replies to this topic

### #1neomaycry  Members

Posted 24 July 2013 - 06:42 AM

I am working on a space shooter game, using SFML and c++.  My game has an enemy that flies to the center of the screen and shoots a laser which rotates 360 degrees.  Everything about this process looks great except one fact: the laser needs to be "clipped" when it encounters a tile (so the player can hide behind the tile to avoid it). Take a look at the following image:

Also, here is the code I have written:

for(int i=0;i<numBlocks;i++)
for(int j=0;j<numEnemies;j++)
if(Collision::BoundingBoxTest(block[i].sprite, enemy[j].laser)
&& block[i].alive && enemy[j].alive && enemy[j].id == LASERBALL)
{
std::cout << "Laser collided with tile!" << std::endl;
enemy[j].laserCollidedWithTile = true;
enemy[j].laser.setScale(laserW * (block[i].sprite.getPosition().x /
enemy[j].sprite.getGlobalBounds().width), 1); //fix this
}


The collision itself is working fine.  The problem I am having is figuring out the math of how to scale the laser so it appears to be blocked by the tile.  I've also considered approaching this problem by resizing the sprite's textureRect.

If you have time, I would appreciate feedback regarding my approach and how to fix it.  Remember that the laser is a rotated object.

EDIT: 'laserW' is a global variable used for testing purposes.  It contains the width of the laser sprite, which is 1060px.

Edited by neomaycry, 24 July 2013 - 07:12 AM.

### #2Kaptein  Prime Members

Posted 24 July 2013 - 11:07 AM

the laser sprite is 1060 pixels wide? oh my

do you draw a line on it? then draw the sprite on the sheet?

there are multiple ways to solve this problem, such as just drawing a line from (tankX, tankY) to (objectX, objectY)

the line would be red with some LineWidth = 4.f; or so, if you are using .NET

EDIT: oh right, SFML and c++, i have never used SFML before.. so i can't help you there

but, can't you just draw a line? from A to B..

Depending on how far you want to go, lasers can be complex effects

In the old days, you could combine many small sprites that formed a line, and i'm sure you can do that today too with some alpha+shader magic

Not sure what your options are with SFML though.. sorry

Edited by Kaptein, 24 July 2013 - 11:13 AM.

### #3KnolanCross  Members

Posted 24 July 2013 - 11:21 AM

Take a look for something called Raycast test, it will find if your laser collides with something, then you can just test if the colision was on an object (in this case you will need to render it partially) or on an enemy.

You can also create an invisible dummy laser to execute the raycast test and find the size that the laser should have.

Currently working on a scene editor for ORX (http://orx-project.org), using kivy (http://kivy.org).

Posted 24 July 2013 - 12:14 PM

I wouldn't try scaling the sprite.  I would set the rect.  Also, the size of my laser sprite was only 1x13.  Yup, 1 pixel by 13 pixels.  Here's an example of how I would draw a laser beam originating at 300,500, that was 300 pixels long.  My collision detection algorithm would return the point of collision, and with that I could compute the length of my laser beam.

	int width = 13, length = 300;
sf::RectangleShape laser(sf::Vector2f(width,length));
sf::Uint8 r = 0, g = 255, b = 0;
sf::Uint8 pixels[] = {
r, g, b, 10,
r, g, b, 10,
r, g, b, 40,
r, g, b, 80,
r, g, b, 100,
r, g, b, 150,
r, g, b, 200,
r, g, b, 150,
r, g, b, 100,
r, g, b, 80,
r, g, b, 40,
r, g, b, 10,
r, g, b, 10
};

laser.setOrigin(width/2, length);
laser.setPosition(firePoint);
laser.setRotation(firAngle);
target->draw(laser); //target being a RenderTarget


The texture is repeated the entire length of the rectangle.  Unless your laser isn't symmetrical along the axis of fire, there's no need for the texture to be any taller than 1 pixel.

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