02Credits Blog

Day81 - Sprite Rendering and Gap Fix

Finish up spritesheet rendering and fix pixel gap between tiles

2019-05-15

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Todo

Today I finished up the sprite renderer which renders subsets of the TileSheet generated by TexturePacker as tiles, and made a quick fix for the pixel gap issue between tiles.

SheetManager

Continuing on from last time, I created a draw function on the SheetManager which takes a given Tiles enum value, position, and width and renders it based on the Spritesheet metadata. Not only does TexturePacker layout the sprites and allow specifying of a pivot location, but it also trims the sprites so as to lay them out more compactly in the sheet. As a result the rendering code is slightly more complicated since it has to pay attention to the trimmed offset on top of just the source rectangle.

I make things slightly more complicated on top of the above issues by specifying a single position and width for the tile location instead of a full rectangle. This simplifies other game logic, so the trade-off of slightly more complexity in the SheetManager makes good sense.

TexturePacker provides 4 useful bits of information in the JSON format: The SourceSize, Frame, SpriteSourceSize, and Pivot. These are somewhat confusingly named, so I think of the SourceSize as the original sprite dimensions, the Frame as the source rectangle, the SpriteSourceSize as the offset rectangle, and the Pivot as the center point of the Tile.

  public void DrawTile(Tiles tile, Vector2 position, float width, Color color, float layerDepth) {
    SheetElement spritePosition = SpritePositions[tile];
    var scale = width / spritePosition.SourceSize.Width;
    var sourceRectangle = new Rectangle(spritePosition.Frame.X, spritePosition.Frame.Y, spritePosition.Frame.Width, spritePosition.Frame.Height);
    var destinationWidth = spritePosition.SpriteSourceSize.Width * scale;
    var destinationHeight = spritePosition.SpriteSourceSize.Height * scale;
    var destinationRectangle = new Rectangle(
        (int)(spritePosition.SpriteSourceSize.X * scale + position.X - spritePosition.Pivot.X * spritePosition.SourceSize.Width * scale),
        (int)(spritePosition.SpriteSourceSize.Y * scale + position.Y - spritePosition.Pivot.Y * spritePosition.SourceSize.Height * scale),
        (int)destinationWidth,
        (int)destinationHeight);
    spriteBatch.Draw(Sheet, destinationRectangle, sourceRectangle, color, 0, Vector2.Zero, SpriteEffects.None, layerDepth);
}

Given this data, I compute a scaling factor based on the desired Width, create a rectangle using the frame for the source rectangle to draw from, compute the trimmed dimensions of the destination rectangle, and offset the position by the desired position location and pivot.

With the draw code out of the way, I can use the new function as a drop in replacement for the SpriteBatch draw function. This greatly simplifies the tile rendering logic by removing the need to calculate tile offsets and base position. It also lets me set an arbitrary pivot location, which may be useful in the future.

  private void DrawTile(Tiles tileToDraw, Vector3 worldPosition, float layerOffset = 0) {
    Vector2 screenPosition = WorldToScreen(worldPosition);
    float layerDepth = (worldPosition.Y - worldPosition.X + worldPosition.Z * 0.5f + layerOffset + 50.0f) / 100.0f;
    sheetManager.DrawTile(tileToDraw, screenPosition, TileWidth, Color.White, layerDepth);
}

Pixel Gap Fix

After stumbling around trying to come up with a fix for the pixel gap, I decided to use a hack for now. The pixel gap arises primarily because the pixel size of the tiles frequently is not a constant or simple fraction multiple of the rendered tile size. Monogame doesn't know how to greedily scale up textures without making them look weird, so there sometimes is a super tiny gap between tile locations.

This gap shows up as a weird zigzag between tiles, but is never more than half a pixel wide. This hints at a potential solution. I can fix the pixel gap problem by rendering the tiles to a texture with resolution that matches a constant multiple of the tile pixel width. Then I can render that texture to the screen by upscaling it instead of each tile individually. With proper sampler settings, this means that every pixel is accounted for and no gap appears.

Monogame has support for rendering to a texture by using a RenderTarget2D which is basically just an editable and target-able Texture2D. The GraphicsDevice has a RenderTarget2D which defaults to the screen target (represented by null), but this value can be overridden with a custom value at any time causing the system to render to the target instead of the screen.

The only trick was figuring out the size of the RenderTarget texture. I decided to use 10 times the tile width (32) plus half a tile for a gap on either side of the screen resulting in 352. Then the height of the target is just that width times the aspect ratio of the screen at a given point in time. I wrapped this logic in a ResizeRenderTarget function so that it could be called any time the window size changes.

  private void ResizeRenderTarget() {
    RenderTarget = new RenderTarget2D(
        GraphicsDevice,
        352, 352 * GraphicsDevice.PresentationParameters.BackBufferHeight / GraphicsDevice.PresentationParameters.BackBufferWidth,
        false, GraphicsDevice.PresentationParameters.BackBufferFormat,
        DepthFormat.Depth24);
}

Then the draw function needed changed to draw to the target for normal draw calls, and then swap back to the screen to render the target to the screen.

  protected override void Draw(GameTime gameTime) {
    GraphicsDevice.SetRenderTarget(RenderTarget);
    graphics.GraphicsDevice.Clear(colorManager.Lookup[0]);
    foreach (IDrawable drawable in drawables) {
        spriteBatch.Begin(samplerState: SamplerState.PointClamp, sortMode: SpriteSortMode.FrontToBack);
        drawable.Draw(gameTime);
        spriteBatch.End();
    }
    GraphicsDevice.SetRenderTarget(null);
    spriteBatch.Begin(samplerState: SamplerState.PointClamp, sortMode: SpriteSortMode.FrontToBack);
    spriteBatch.Draw(RenderTarget, new Rectangle(0, 0, graphics.PreferredBackBufferWidth, graphics.PreferredBackBufferHeight), Color.White);
    spriteBatch.End();
    base.Draw(gameTime);
}

Lastly I updated the ScreenSizeManager to refer to the RenderTarget instead of the backbuffer.

  public class ScreenSizeManager {
    private Game game;
    public int PixelWidth => game.RenderTarget.Width;
    public int PixelHeight => game.RenderTarget.Height;
    public ScreenSizeManager(Game game) {
        this.game = game;
    }
}

Thats it! With these changes, tiles render without ever showing a gap between them. Once I get higher resolution assets, I will revert this change since the high resolution versions won't have as visible of gaps, but for now this works great.

No Gaps

Thats it for today, with this work out of the way, I can get back to gameplay features. This also enables me to write a custom renderer for tiles in the future which allows for actual z value layering since all draw calls will be using one texture. For now though I will continue to use the SpriteBatch since it is conceptually simpler and requires no work to use, but its nice to have options.

Till tomorrow,
Keith