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# How do I center a game character on the screen?

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### #1black_darkness  Members   -  Reputation: 280

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Posted 22 December 2012 - 11:12 AM

I would like my character to be the center of the screen at all times. I am using allegro and C++. Right now I am rendering the full 30x30 map at all times. But my end goal is to have only 9x9 rendering at once. I don't know if I should do this by changing the displays variables. (tried this but it just changed the windows size.) Or change the which part of the map is displayed. ( I tried this but the character wasnt centered and the map did change but character moved left, and the map followed. So I stayed in the same spot indefinitely. )

I achieved this result with this code.

            for(int col=0;col<height;col++) {                 for(int row=0;row<width;row++) {                     al_draw_bitmap_region(tileset, mv[col][row].sx * 16, mv[col][row].sy * 16, 16, 16, (row*16 + hero.wloc*16), col*16, 0);                 }             }


And it looks like this (Before and after 7 movements. Notice how the character isnt centered and the map isnt being drawn around him.)

Here is something I am curious about. Should I draw the whole map or only the part I am currently on?

            for(int col=0;col<height;col++) {                 for(int row=0;row<width;row++) {                     al_draw_bitmap_region(tileset, mv[col][row].sx * 16, mv[col][row].sy * 16, 16, 16, row*16, col*16, 0);                 }             }

For reference here is a screenshot of my character. Notice how he is in the bottom right corner.

Okay. I deleted the previous attempt at centering and this is how my character is displayed. Every time he moves the screen stays stationary. (notice how he is at bottom right of screen instead of centered.)

The following video is an example of how I want my character to behave.

A release of my game.

https://www.dropbox.com/s/pxnnfmvwgslkxcs/negfaron.rar

Source code

main.cpp - http://pastie.org/5565697

make_map.h - http://pastie.org/5565698

main.h - http://pastie.org/5565699

Display initialization

ALLEGRO_DISPLAY* display = NULL;

Edited by black_darkness, 22 December 2012 - 11:35 AM.

### #2Khatharr  Crossbones+   -  Reputation: 2919

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Posted 22 December 2012 - 12:08 PM

Draw only the onscreen tiles and a 1 tile margin around the screen edges to make smooth scrolling easier. In other words, if your display is 15x15 tiles then draw 17x17 tiles.

As for centering the map around the character;

Start by getting your map x/y (which tile are you on) to use as the point to center on.

Subtract half the screen width (in tiles) from x and half the height from y.

This should give you the x/y of the tile to be placed in the upper-left corner.

I haven't looked at your code but I think you could probably take it from there.

If you want to the screen to not scroll its edges past the map edge then you'll need to do some bounds checking. For instance, run the above calculations and if x or y is negative then set them to 0. If they're greater than the map dims minus the screen dims then set them to map dims minus screen dims, etc.

void hurrrrrrrr() {__asm sub [ebp+4],5;}

There are ten kinds of people in this world: those who understand binary and those who don't.

### #3black_darkness  Members   -  Reputation: 280

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Posted 22 December 2012 - 12:19 PM

Draw only the onscreen tiles and a 1 tile margin around the screen edges to make smooth scrolling easier. In other words, if your display is 15x15 tiles then draw 17x17 tiles.

As for centering the map around the character;

Start by getting your map x/y (which tile are you on) to use as the point to center on.

Subtract half the screen width (in tiles) from x and half the height from y.

This should give you the x/y of the tile to be placed in the upper-left corner.

I haven't looked at your code but I think you could probably take it from there.

If you want to the screen to not scroll its edges past the map edge then you'll need to do some bounds checking. For instance, run the above calculations and if x or y is negative then set them to 0. If they're greater than the map dims minus the screen dims then set them to map dims minus screen dims, etc.

Okay, I will try to always render the hero at the center of the map first. brb.

Edit: Alright the game is centered properly.  Thank you for the answer.

Edited by black_darkness, 22 December 2012 - 12:39 PM.

### #4BaneTrapper  Members   -  Reputation: 1141

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Posted 24 December 2012 - 11:32 AM

if your display is 15x15 tiles then draw 17x17 tiles.

Shouldn't he/she draw only 16x16 if 15x15 from 0x, 0y fill whole screen?!

Current projects:
The Wanderer, 2d turn based rpg style game

www.gamedev.net/topic/641117-check-up-the-wanderer/

### #5Khatharr  Crossbones+   -  Reputation: 2919

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Posted 24 December 2012 - 10:36 PM

The player may move in either direction. You can do one additional and then just throw the margin onto the side that you're moving toward when you start scrolling, but it's easier to just do it on both sides and tile-based engines are pretty lightweight in the rendering department.

I think the first time I did it I actually had a 1-tile margin that I would just move around to the other side of the screen, like using logs to roll a boulder on, lol. I'd scroll the tile sprites and then when a row/column passed out of the screen I'd move those tiles to the other side of the tileset. It got pretty crazy for a while until I refactored it and did it right.

Edit - I guess I should be more clear about what I'm talking about.

If you use a single-tile margin then when scrolling triggers to the left you should 'snap' the tile sprites such that the current extreme-right tile is on the rightmost edge and the 'margin' (offscreen) column is on the leftmost edge, then move all the tile sprites smoothly to the right for the length of one tile. Then you 'snap' again and repeat as needed. However, if you scroll to the right you have to snap oppositely - such that the leftmost visible tiles are placed in the leftmost edge of your tile sprites and the margin column is offscreen to the right, etc.

Using a double-width margin means that you can have a margin in all 4 directions. This way you can skip the first step (the snapping to one side or the other) and just start smooth motion of the sprites, then just reset the sprite positions and snap the tiles to center when you finish scrolling by 1 tile.

It sounds trivial, and it kind of is, but in practice it makes the process less complicated by removing that first snap.

Edited by Khatharr, 24 December 2012 - 10:51 PM.

void hurrrrrrrr() {__asm sub [ebp+4],5;}

There are ten kinds of people in this world: those who understand binary and those who don't.

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