Finding the World Position of Mouse (2d)

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Alaror

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Joined: 9th May 2011

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Posted: 11th Sep 2011 23:37

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In my 2d, side view game my terrain is displayed using a matrix. I'm having difficulty figuring out the position of the mouse in the world. Because my world is not displayed using objects I can't make use of the PICK OBJECT command; I need a mathematically based way to find the exact location of the mouse, which is then used to determine what block the mouse is currently over.

The data I have to work with is the mouse's position on the screen (using MOUSEX) and the player's world position. I thought it would be enough to offset the MOUSEX position by half of the screen width (making 0 the exact center of the player, since they're located in the middle) and then add that to the player's world position, but evidently not. What's most bizarre is that there's no immediately apparent rhyme or reason for what the game is considering to be the end of one block and the start of another. In the screenshot I attached I've drawn lines that show approximately where the game says a block is starting and finishing for three grey blocks, marked by a red line. As you can see it varies quite a bit.

I'm open to suggestions for other, more accurate ways to find the mouse world position, or thoughts on ways to fix it using my current method.

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LastChance

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Posted: 12th Sep 2011 00:55

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Algorithm...

let say 1 block is 32 pixels wide and 32 pixels high.
if we assume that display resolution is 1024x768,
then we can draw:
1024/32 = 32 blocks per row
768/32 = 24 blocks per column

so we have to draw 24x32 = 768 blocks every time we redraw the screen.
we can store them in 1D array of type block:

+ Code Snippet


type Block

    img as integer
    collision as integer
    `other params

endtype

global dim Map(768) as Block

to draw the map we'll use the function draw_map()

+ Code Snippet


function draw_map()
   
   index = 0

    for x = 0 to 1024 step 32
    
        for y = 0 to 768 step 32

            paste image Map(index).img, x, y
            inc index
         
        next y

    next x

endfunction

to find which block mouse points to we use:

+ Code Snippet


function over_block()

    x = mousex()/32
    y = mousey()/32
    
   result = y*32 + x

endfunction result

use that in a loop:

+ Code Snippet


do

    cls
    mapIndex = over_block()
    print "You point block number: ";mapIndex

loop

with that index you can access the exact block in the Map array and modify it, check for collision etc.

P.S. I think that draw_map will raise out of bonds error. If this happen try to add 1 more element to the array.

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Webber

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Posted: 12th Sep 2011 01:09

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Finding the mouse positions would be a lot easier if you were using sprites instead.

Assuming that the height of the camera does not change during game play, the simplest solution would be to adjust the cameras height to make the world fit the size of the screen.

Example code of how to get the distance of the camera and the world to fit the screen resolution.

+ Code Snippet

Rem the World/Screen mouse positions program

sync on

Rem The object used to measure the size of the screen 
make object plain 1,640,480 `The same size of the screen resolution
pitch object down 1,90

Rem the object for testing the screen/world mouse positions
make object box 2,10,0.1,10 

Rem Positions and rotates the camera
rotate camera 0,90,0,0
position camera 0,0,399,0 `the distance away from the world

Rem the loop
do
MX = MouseX() : MY = MouseY() `Variables for the mouse positions
MMZ = MouseMoveZ() `Variable for the motion of mouse z

position object 2,MX-320,0,-MY+240 `Tests the position of the mouse

Rem Adjusts the distance of the camera and the world
if MMZ > 0 then move camera 0,1
if MMZ < 0 then move camera 0,-1
if upkey() = 1 then move camera 0,1
if downkey() = 1 then move camera 0,-1

Rem display the distance of the camera and the world
set cursor 0,0
print Camera position y(0)

sync
loop

Example of how to get the positions of the mouse in the world using the cameras x, and z positions.

Rem the World/Screen mouse positions program

sync on

Rem The object used to measure the size of the screen 
make object plain 1,640,480 `The same size of the screen resolution
pitch object down 1,90

Rem the object for testing the screen/world mouse positions
make object box 2,10,0.1,10

Rem Positions and rotates the camera
rotate camera 0,90,0,0
position camera 0,0,399,0

Rem the loop
do
MX = MouseX() : MY = MouseY() `Variables for the mouse positions
MMZ = MouseMoveZ() `Variable for the motion of mouse z

position object 2,MX-320+Camera position x(0),0,-MY+240+Camera position z(0) `Tests the position of the mouse

rem moves the camera with the arrowkeys
if upkey() = 1 then position camera 0,camera position x(0),camera position y(0),camera position z(0)+1
if downkey() = 1 then position camera 0,camera position x(0),camera position y(0),camera position z(0)-1
if rightkey() = 1 then position camera 0,camera position x(0)+1,camera position y(0),camera position z(0)
if leftkey() = 1 then position camera 0,camera position x(0)-1,camera position y(0),camera position z(0)

sync
loop

+ Code Snippet

Rem the World/Screen mouse positions program

sync on

Rem The object used to measure the size of the screen 
make object plain 1,640,480 `The same size of the screen resolution
pitch object down 1,90

Rem the object for testing the screen/world mouse positions
make object box 2,10,0.1,10 

Rem Positions and rotates the camera
rotate camera 0,90,0,0
position camera 0,0,399,0

Rem the loop
do
MX = MouseX() : MY = MouseY() `Variables for the mouse positions
MMZ = MouseMoveZ() `Variable for the motion of mouse z

position object 2,MX-320+Camera position x(0),0,-MY+240+Camera position z(0) `Tests the position of the mouse

rem moves the camera with the arrowkeys
if upkey() = 1 then position camera 0,camera position x(0),camera position y(0),camera position z(0)+1
if downkey() = 1 then position camera 0,camera position x(0),camera position y(0),camera position z(0)-1
if rightkey() = 1 then position camera 0,camera position x(0)+1,camera position y(0),camera position z(0)
if leftkey() = 1 then position camera 0,camera position x(0)-1,camera position y(0),camera position z(0)

sync
loop

Again, using sprites would be easier, and more importantly the program will run faster too.

I like games, and stuff.

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Alaror

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Posted: 12th Sep 2011 06:39

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Thanks for your responses. I ended up using this tutorial on the PICK SCREEN command: http://forum.thegamecreators.com/?m=forum_view&t=154753&b=22 There's some tweaking that needs to be done but I'm pretty much happy with the results.

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