Game Maker Studio - Check Collision With Subimages

Question

I have an obj_roulette, which contains 4 subimages, with value 2-5 and image_number 0-3. The value result from roulette stored as var global.roulette.

Then, I make many obj_meteorite, which contains 4 subimages too, spawn from above with random x value and random image_number. Player can shoot them with left-mouse click.

This is what I want:

If image_number obj_roulette is 0, and player shoot obj_meteorite with image_number 0, score +10. If image_number obj_roulette is 0, and player shoot obj_meteorite with image_number 1, score -10.

I don't know how to check collision between mouse_x/mouse_y and object image_number, and how to match obj_roulette image_number and obj_meteorite image_number.

Is it using collision checking? If it yes, then maybe the example in this link can help: link 1

Please explain your answer. Thanks.

update

[Simple Diagram Image]

update2

Simple Diagram Image 2

Answer 1

Yes, it is collision checking. Though, there's a little misunderstanding about variables that I'll first try to clarify before proceeding to a solution to your problem.

What does what

First of all, image_number is a constant (YoYo Docs call it read-only variable) that gives you the number of sub-images of an object's current sprite (its sprite_index). If you want information about the actual frame to be drawn in the current step, you need to check the instance variable image_index.

That's the story: sprite_index is the sprite assigned to an object to be drawn; image_index is the current sub-image (or frame) of its animation to be drawn in the next Draw() call; finally, image_number is the total number of sub-image of sprite_index. When you change an object's sprite_index its image_number value will be automatically updated by the GM engine.

So, let's work with image_number. This variable is usually set to -1, which means its value will be updated (so that the animation cycles through) depending on image_speed value. I'm assuming your image_speeds are already set to 0, so that you can set the value of image_indexes without having them changing automatically over time.

The first script

Now, to check mouse clicks onto objects depending on current image_indexes, just add to the Step Event of obj_meteorite the following code:

// Check for an object being hit by the
// player: just a click, not continuously
if (mouse_check_button_pressed(mb_left))
{
    // Is the mouse above me while clicking?
    if (position_meeting(mouse_x,mouse_y,self))
    {
        // Was that the right object to hit?
        if (image_index==global.roulette)
            score += 10;    // Yeah
        else
            score -= 10;    // Nope
        // You may want to destroy the object
        instance_destroy();
    }
}

The function collision_point() belongs to the Advanced Collision Checking section in the YoYo Docs, as it's a more advanced way to check for instances whose collision masks include a particular point in the plane.

Optimization

This script is executed by every obj_meteorite every time the left mouse button is pressed. That means, if there are currently 7 instances of obj_meteorite then this script will be called and executed seven times. But we have one mouse cursor only, so we would just check what lies below it. We can achieve this by migrating such script into a new object called obj_controller, which will execute a slightly different script in its Step Event:

// Check for an object being hit by the
// player: just a click, not continuously
var my_inst;    // Local support variable
if (mouse_check_button_pressed(mb_left))
{
    // Is there an instance below the mouse? Get its ID if yes
    my_inst = instance_position(mouse_x,mouse_y,obj_meteorite);
    // Is the mouse above the instance while clicking?
    if (my_inst!=noone)
    {
        // Was that the right object to hit?
        if (my_inst.image_index==global.roulette)
            score += 10;    // Yeah
        else
            score -= 10;    // Nope
        // You may want to destroy the object
        with (my_inst) instance_destroy();
    }
}

This way, the script is executed once only per mouse click. I guess your game isn't that complex and design optimization is no serious concern, but I suggest from time to time to check if your code can be designed and written differently to lower function calls and enhance your program performance, as it is good programming habit and practice.