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libGDX change my system from pixels to units

I want everyone to see the same things on their screen regardless of their screen size and aspect ratio so this is the code I am currently using. (also I am sending net data across with the coordinates of where the other players are on the screen)
int width = 1920, height = 1080;
public OrthographicCamera camera;
Viewport viewport;
//constructor
camera = new OrthographicCamera();
viewport = new ScalingViewport(Scaling.stretch, width, height, camera);
viewport.apply();
camera.position.set(camera.viewportWidth / 2, camera.viewportHeight / 2, 0);
camera.update();
public void resize(int width, int height) {
viewport.update(width, height);
camera.position.set(camera.viewportWidth / 2, camera.viewportHeight / 2, 0);
}
now for example I wanted 10 perfect squares going across the middle of the screen so I made then 192 pixels by 192 pixels so I could have 10 perfect squares going across the middle of the screen my system right now works perfect except for the fact that it is rendered internally 1920x1080 on all devices big and small. How would I convert my camera to units and get the size needed for 10 perfect squares to go across the screen? Is that even possible?
Here is my code to draw 10 squares across the screen
float size = 192;
for(int i = 0; i<10; i++){
walls.add(new Stuff(i*size,height/2-size/2,size,size,"middle",1,1,0,1));
}
How would I convert all this code to say units? Or is this an acceptable approach?

You are already using units, they just aren't very meaningful (and it certainly aren't pixels). If you want to use meaningful units (e.g. SI units), then the only thing you have to change in this code are the values. E.g. if the size of your stuff (wall?) is, say 2 meter, then use the value 2 instead of 192. And if you want your users screen to be, say 20 meters (10 walls e.g.) in width and 16:9 aspect ratio, then use that for the Viewport worldWidth and worldHeight.
float worldWidth = 20;
float worldHeight = worldWidth * 9f / 16f;
...
viewport = new StretchViewport(worldWidth, worldHeight, camera);
Make sure to understand that these "pixels" you are talking about only exist in your imagination. See also: http://blog.xoppa.com/pixels/.

You created your ScalingViewport with a width of 1920, so the width in world units will be 1920 on all screens, no matter what. Also, your scene will be distorted on any screen that is not 16:9, since you are stretching to fit whatever the screen is. (Because of the distortion, I personally would never use ScalingViewport with Scaling.stretch, aka StretchViewport.)
If you want your squares to look square on all screens with this type of viewport, you'll have to do some math to change their height (but their width should always be 192 if you want exactly ten to fit across the screen).
public void resize(int width, int height){
float viewportAspect = 1920f / 1080f;
float screenAspect = (float)width / (float)height; //Make sure you cast to floats
boxHeight = 192 * screenAspect / viewportAspect;
viewport.update(width, height, true);
}
The camera always shows the scene in world units, so there's no conversion to do.

What is the source of these pixel gaps in between identical vertices in OpenGL's Ortho? How can I eliminate them?

Despite passing equal (exactly equal) coordinates for 'adjacent' edges, I'm ending up with some strange lines between adjacent elements when scaling my grid of rendered tiles.
My tile grid rendering algorithm accepts scaled tiles, so that I can adjust the grid's visual size to match a chosen window size of the same aspect ratio, among other reasons. It seems to work correctly when scaled to exact integers, and a few non-integer values, but I get some inconsistent results for the others.
Some Screenshots:
The blue lines are the clear color showing through. The chosen texture has no transparent gaps in the tilesheet, as unused tiles are magenta and actual transparency is handled by the alpha layer. The neighboring tiles in the sheet have full opacity. Scaling is achieved by setting the scale to a normalized value obtained through a gamepad trigger between 1f and 2f, so I don't know what actual scale was applied when the shot was taken, with the exception of the max/min.
Attribute updates and entity drawing are synchronized between threads, so none of the values could have been applied mid-draw. This isn't transferred well through screenshots, but the lines don't flicker when the scale is sustained at that point, so it logically shouldn't be an issue with drawing between scale assignment (and thread locks prevent this).
Scaled to 1x:
Scaled to A, 1x < Ax < Bx :
Scaled to B, Ax < Bx < Cx :
Scaled to C, Bx < Cx < 2x :
Scaled to 2x:
Projection setup function
For setting up orthographic projection (changes only on screen size changes):
.......
float nw, nh;
nh = Display.getHeight();
nw = Display.getWidth();
GL11.glOrtho(0, nw, nh, 0, 1, -1);
orthocenter.setX(nw/2); //this is a Vector2, floats for X and Y, direct assignment.
orthocenter.setY(nh/2);
.......
For the purposes of the screenshot, nw is 512, nh is 384 (implicitly casted from int). These never change throughout the example above.
General GL drawing code
After cutting irrelevant attributes that didn't fix the problem when cut:
#Override
public void draw(float xOffset, float yOffset, float width, float height,
int glTex, float texX, float texY, float texWidth, float texHeight) {
GL11.glLoadIdentity();
GL11.glTranslatef(0.375f, 0.375f, 0f); //This is supposed to fix subpixel issues, but makes no difference here
GL11.glTranslatef(xOffset, yOffset, 0f);
if(glTex != lastTexture){
GL11.glBindTexture(GL11.GL_TEXTURE_2D, glTex);
lastTexture = glTex;
}
GL11.glBegin(GL11.GL_QUADS);
GL11.glTexCoord2f(texX,texY + texHeight);
GL11.glVertex2f(-height/2, -width/2);
GL11.glTexCoord2f(texX + texWidth,texY + texHeight);
GL11.glVertex2f(-height/2, width/2);
GL11.glTexCoord2f(texX + texWidth,texY);
GL11.glVertex2f(height/2, width/2);
GL11.glTexCoord2f(texX,texY);
GL11.glVertex2f(height/2, -width/2);
GL11.glEnd();
}
Grid drawing code (dropping the same parameters dropped from 'draw'):
//Externally there is tilesize, which contains tile pixel size, in this case 32x32
public void draw(Engine engine, Vector2 offset, Vector2 scale){
int xp, yp; //x and y position of individual tiles
for(int c = 0; c<width; c++){ //c as in column
xp = (int) (c*tilesize.a*scale.getX()); //set distance from chunk x to column x
for(int r = 0; r<height; r++){ //r as in row
if(tiles[r*width+c] <0) continue; //skip empty tiles ('air')
yp = (int) (r*tilesize.b*scale.getY()); //set distance from chunk y to column y
tileset.getFrame(tiles[r*width+c]).draw( //pull 'tile' frame from set, render.
engine, //drawing context
new Vector2(offset.getX() + xp, offset.getY() + yp), //location of tile
scale //scale of tiles
);
}
}
}
Between the tiles and the platform specific code, vectors' components are retrieved and passed along to the general drawing code as pasted earlier.
My analysis
Mathematically, each position is an exact multiple of the scale*tilesize in either the x or y direction, or both, which is then added to the offset of the grid's location. It is then passed as an offset to the drawing code, which translates that offset with glTranslatef, then draws a tile centered at that location through halving the dimensions then drawing each plus-minus pair.
This should mean that when tile 1 is drawn at, say, origin, it has an offset of 0. Opengl then is instructed to draw a quad, with the left edge at -halfwidth, right edge at +halfwidth, top edge at -halfheight, and bottom edge at +halfheight. It then is told to draw the neighbor, tile 2, with an offset of one width, so it translates from 0 to that width, then draws left edge at -halfwidth, which should coordinate-wise be exactly the same as tile1's right edge. By itself, this should work, and it does. When considering a constant scale, it breaks somehow.
When a scale is applied, it is a constant multiple across all width/height values, and mathematically shouldn't make anything change. However, it does make a difference, for what I think could be one of two reasons:
OpenGL is having issues with subpixel filling, ie filling left of a vertex doesn't fill the vertex's containing pixel space, and filling right of that same vertex also doesn't fill the vertex's containing pixel space.
I'm running into float accuracy problems, where somehow X+width/2 does not equal X+width - width/2 where width = tilewidth*scale, tilewidth is an integer, and X is a float.
I'm not really sure about how to tell which one is the problem, or how to remedy it other than to simply avoid non-integer scale values, which I'd like to be able to support. The only clue I think might apply to finding the solution is how the pattern of line gaps isn't really consistant (see how it skips tiles in some cases, only has vertical or horizontal but not both, etc). However, I don't know what this implies.

This looks like it's probably a floating point precision issue. The critical statement in your question is this:
Mathematically, each position is an exact multiple [..]
While that's mathematically true, you're dealing with limited floating point precision. Sequences of operations that should mathematically produce the same result can (and often do) produce slightly different results due to rounding errors during expression evaluation.
Specifically in your case, it looks like you're relying on identities of this form:
i * width + width/2 == (i + 1) * width - width/2
This is mathematically correct, but you can't expect to get exactly the same numbers when evaluating the values with limited floating point precision. Depending on how the small errors end up getting rounded to pixels, it can result in visual artifacts.
The only good way to avoid this is that you actually use the same values for coordinates that must be the same, instead of using calculations that mathematically produce the same results.
In the case of coordinates on a grid, you could calculate the coordinates for each grid line (tile boundary) once, and then use those values for all draw operations. Say if you have n tiles in the x-direction, you calculate all the x-values as:
x[i] = i * width;
and then when drawing tile i, use x[i] and x[i + 1] as the left and right x-coordinates.

Flipping graphics object without using G2D?

Say I have a shape that I have drawn through paintComponent(), fillRect, or drawOval,
How would I go about flipping it, so that the shape would not be going from the top left, but from the bottom upwards?

All I had to do was use the height of my panel, and subtract it from the height of the object.
for ex:
int height = (1000 - n);
where n is the height that you wish the shape to be.

Efficient 2D Tile based lighting system

What is the most efficient way to do lighting for a tile based engine in Java?
Would it be putting a black background behind the tiles and changing the tiles' alpha?
Or putting a black foreground and changing alpha of that? Or anything else?
This is an example of the kind of lighting I want:

There are many ways to achieve this. Take some time before making your final decision. I will briefly sum up some techiques you could choose to use and provide some code in the end.
Hard Lighting
If you want to create a hard-edge lighting effect (like your example image),
some approaches come to my mind:
Quick and dirty (as you suggested)
Use a black background
Set the tiles' alpha values according to their darkness value
A problem is, that you can neither make a tile brighter than it was before (highlights) nor change the color of the light. Both of these are aspects which usually make lighting in games look good.
A second set of tiles
Use a second set of (black/colored) tiles
Lay these over the main tiles
Set the new tiles' alpha value depending on how strong the new color should be there.
This approach has the same effect as the first one with the advantage, that you now may color the overlay tile in another color than black, which allows for both colored lights and doing highlights.
Example:
Even though it is easy, a problem is, that this is indeed a very inefficent way. (Two rendered tiles per tile, constant recoloring, many render operations etc.)
More Efficient Approaches (Hard and/or Soft Lighting)
When looking at your example, I imagine the light always comes from a specific source tile (character, torch, etc.)
For every type of light (big torch, small torch, character lighting) you
create an image that represents the specific lighting behaviour relative to the source tile (light mask). Maybe something like this for a torch (white being alpha):
For every tile which is a light source, you render this image at the position of the source as an overlay.
To add a bit of light color, you can use e.g. 10% opaque orange instead of full alpha.
Results
Adding soft light
Soft light is no big deal now, just use more detail in light mask compared to the tiles. By using only 15% alpha in the usually black region you can add a low sight effect when a tile is not lit:
You may even easily achieve more complex lighting forms (cones etc.) just by changing the mask image.
Multiple light sources
When combining multiple light sources, this approach leads to a problem:
Drawing two masks, which intersect each other, might cancel themselves out:
What we want to have is that they add their lights instead of subtracting them.
Avoiding the problem:
Invert all light masks (with alpha being dark areas, opaque being light ones)
Render all these light masks into a temporary image which has the same dimensions as the viewport
Invert and render the new image (as if it was the only light mask) over the whole scenery.
This would result in something similar to this:
Code for the mask invert method
Assuming you render all the tiles in a BufferedImage first,
I'll provide some guidance code which resembles the last shown method (only grayscale support).
Multiple light masks for e.g. a torch and a player can be combined like this:
public BufferedImage combineMasks(BufferedImage[] images)
{
// create the new image, canvas size is the max. of all image sizes
int w, h;
for (BufferedImage img : images)
{
w = img.getWidth() > w ? img.getWidth() : w;
h = img.getHeight() > h ? img.getHeight() : h;
}
BufferedImage combined = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
// paint all images, preserving the alpha channels
Graphics g = combined.getGraphics();
for (BufferedImage img : images)
g.drawImage(img, 0, 0, null);
return combined;
}
The final mask is created and applied with this method:
public void applyGrayscaleMaskToAlpha(BufferedImage image, BufferedImage mask)
{
int width = image.getWidth();
int height = image.getHeight();
int[] imagePixels = image.getRGB(0, 0, width, height, null, 0, width);
int[] maskPixels = mask.getRGB(0, 0, width, height, null, 0, width);
for (int i = 0; i < imagePixels.length; i++)
{
int color = imagePixels[i] & 0x00ffffff; // Mask preexisting alpha
// get alpha from color int
// be careful, an alpha mask works the other way round, so we have to subtract this from 255
int alpha = (maskPixels[i] >> 24) & 0xff;
imagePixels[i] = color | alpha;
}
image.setRGB(0, 0, width, height, imagePixels, 0, width);
}
As noted, this is a primitive example. Implementing color blending might be a bit more work.

Raytracing might be the simpliest approach.
you can store which tiles have been seen (used for automapping, used for 'remember your map while being blinded', maybe for the minimap etc.)
you show only what you see - maybe a monster of a wall or a hill is blocking your view, then raytracing stops at that point
distant 'glowing objects' or other light sources (torches lava) can be seen, even if your own light source doesn't reach very far.
the length of your ray gives will be used to check amount light (fading light)
maybe you have a special sensor (ESP, gold/food detection) which would be used to find objects that are not in your view? raytrace might help as well ^^
how is this done easy?
draw a line from your player to every point of the border of your map (using Bresehhams Algorithm http://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm
walk along that line (from your character to the end) until your view is blocked; at this point stop your search (or maybe do one last final iteration to see what did top you)
for each point on your line set the lighning (maybe 100% for distance 1, 70% for distance 2 and so on) and mark you map tile as visited
maybe you won't walk along the whole map, maybe it's enough if you set your raytrace for a 20x20 view?
NOTE: you really have to walk along the borders of viewport, its NOT required to trace every point.
i'm adding the line algorithm to simplify your work:
public static ArrayList<Point> getLine(Point start, Point target) {
ArrayList<Point> ret = new ArrayList<Point>();
int x0 = start.x;
int y0 = start.y;
int x1 = target.x;
int y1 = target.y;
int sx = 0;
int sy = 0;
int dx = Math.abs(x1-x0);
sx = x0<x1 ? 1 : -1;
int dy = -1*Math.abs(y1-y0);
sy = y0<y1 ? 1 : -1;
int err = dx+dy, e2; /* error value e_xy */
for(;;){ /* loop */
ret.add( new Point(x0,y0) );
if (x0==x1 && y0==y1) break;
e2 = 2*err;
if (e2 >= dy) { err += dy; x0 += sx; } /* e_xy+e_x > 0 */
if (e2 <= dx) { err += dx; y0 += sy; } /* e_xy+e_y < 0 */
}
return ret;
}
i did this whole lightning stuff some time ago, a* pathfindin feel free to ask further questions
Appendum:
maybe i might simply add the small algorithms for raytracing ^^
to get the North & South Border Point just use this snippet:
for (int x = 0; x <map.WIDTH; x++){
Point northBorderPoint = new Point(x,0);
Point southBorderPoint = new Point(x,map.HEIGHT);
rayTrace( getLine(player.getPos(), northBorderPoint), player.getLightRadius()) );
rayTrace( getLine(player.getPos(), southBorderPoint, player.getLightRadius()) );
}
and the raytrace works like this:
private static void rayTrace(ArrayList<Point> line, WorldMap map, int radius) {
//int radius = radius from light source
for (Point p: line){
boolean doContinue = true;
float d = distance(line.get(0), p);
//caclulate light linear 100%...0%
float amountLight = (radius - d) / radius;
if (amountLight < 0 ){
amountLight = 0;
}
map.setLight( p, amountLight );
if ( ! map.isViewBlocked(p) ){ //can be blockeb dy wall, or monster
doContinue = false;
break;
}
}
}

I've been into indie game development for about three years right now. The way I would do this is first of all by using OpenGL so you can get all the benefits of the graphical computing power of the GPU (hopefully you are already doing that). Suppose we start off with all tiles in a VBO, entirely lit. Now, there are several options of achieving what you want. Depending on how complex your lighting system is, you can choose a different approach.
If your light is going to be circular around the player, no matter the fact if obstacles would block the light in real life, you could choose for a lighting algorithm implemented in the vertex shader. In the vertex shader, you could compute the distance of the vertex to the player and apply some function that defines how bright things should be in function of the computed distance. Do not use alpha, but just multiply the color of the texture/tile by the lighting value.
If you want to use a custom lightmap (which is more likely), I would suggest to add an extra vertex attribute that specifies the brightness of the tile. Update the VBO if needed. Same approach goes here: multiply the pixel of the texture by the light value. If you are filling light recursively with the player position as starting point, then you would update the VBO every time the player moves.
If your lightmap depends on where the sunlight hits your level, you could combine two sort of lighting techniques. Create one vertex attribute for the sun brightness and another vertex attribute for the light emitted by light points (like a torch held by the player). Now you can combine those two values in the vertex shader. Suppose the your sun comes up and goes down like the day and night pattern. Let's say the sun brightness is sun, which is a value between 0 and 1. This value can be passed to the vertex shader as a uniform. The vertex attribute that represents the sun brightness is s and the one for light, emitted by light points is l. Then you could compute the total light for that tile like this:
tileBrightness = max(s * sun, l + flicker);
Where flicker (also a vertex shader uniform) is some kind of waving function that represents the little variants in the brightness of your light points.
This approach makes the scene dynamic without having to recreate continuously VBO's. I implemented this approach in a proof-of-concept project. It works great. You can check out what it looks like here: http://www.youtube.com/watch?v=jTcNitp_IIo. Note how the torchlight is flickering at 0:40 in the video. That is done by what I explained here.

Convert 2d game world coordinates to screen position

I have a system that generates chunks of 2d game map tiles. Chunks are 16x16 tiles, tiles are 25x25.
The chunks are given their own coordinates, like 0,0, 0,1, etc. The tiles determine their coordinates in the world based on which chunk they're in. I've verified that the chunks/tiles are all showing the proper x/y coordinates.
My problem is translating those into screen coordinates. In a previous question someone recommended using:
(worldX * tileWidth) % viewport_width
Each tile's x/y are run through this calculation and a screen x/y coordinate is returned.
This works for tiles that fit within the viewport, but it resets the screen x/y position calculation for anything off-screen.
In my map, I load chunks of tiles within a radius around the player so some of the inner tiles will be off-screen (until they move around, tile positions on the screen are moved).
I tried a test with a tile that would be off screen:
Tile's x coord: 41
41 * 25 = 1025
Game window: 1024
1025 % 1024 = 1
This means that tile x (which, if the screen 0,0 is at map 0,0, should be at x:1025, just off the right-hand side of the screen) is actually at x:1, appearing in the top-left.
I can't think of how to properly handle this - it seems to me like I need take the tileX * tileWidth to determine it's "initial screen position" and then somehow use an offset to determine how to make it appear on screen. But what offset?
Update: I already store an x/y offset value when the player moves, so I know how to move the map. I can use these values as the current offset, and if someone saves the game I can simply store those and re-use them. There's no equation necessary, I would just have to store the cumulative offsets.

The modulo (worldX*tileWidth % screenWidth) is what's causing it to reset. Modulo (%) gives you the remainder of an integer division operation; so, if worldX * tileWidth is greater than screenWidth, it will give you the remainder of (worldX * tileWidth) / screenWidth; if worldX * tileWidth is screenWidth+1, remainder is 1: it starts over at the beginning of the row.
If you eliminate the modulo, it will continue to draw tiles past the edge of the screen. If your drawing buffer is the same size as the screen, you'll need to add a check for tiles at the edge of the screen to make sure you only draw the tile portion that will be visible.
If you're trying to keep the player centered on the screen, you need to offset each tile by the player's offset from tile 0,0 in pixels, minus half the screen width:
offsetX = (playerWorldX * tileWidth) - (screenWidth / 2);
screenX = (worldX * tileWidth) - offsetX;

x = ((worldX*tileWidth) > screenWidth) ? worldX*tileWidth : (worldX*tileWidth)%screenWidth;
That should work. Though I recommend implementing something like an interface and letting each tile decide where they want to be rendered. Something like this
interface Renderable {
void Render(Graphics2D g)
..
}
class Tile implements Renderable{
int x,y
//other stuff
Render(Graphics2D g){
if (!inScreen()){
return;
}
//...
//render
}
boolean inScreen(){
//if the map moves with the player you need to define the boundaries of your current screenblock in terms of the global map coordinates
//What you can do is store this globally in a singleton somewhere or pass it to the constructor of each tile.
//currentBlock.x is then player.x - screenWidth/2
//currentBlock.width is then player.x + screenWidth/2;
//similar for y
if(this.x < currentBlock.x || this.x > currentBlock.Width)
return false;
if (this.y < currentBlock.y || this.y > currentBlock.height)
return false;
return true;
//If the map are in blocks (think zelda on snes where you go from one screenblock to another) you still need to define the boundaries
//currentBlock.x = (player.x / screenWidth) (integer division) *screenWidth;
//currentBlock.width = (player.x /screenWidth) (...) * screenWidth + screenWidth;
//same for y
//Then perform above tests
}