use premultiplied alpha to avoid black fringes on edges

src/app.zig

@@ -334,6 +334,7 @@ pub fn frame(self: *App, plt: Platform.Frame) !void {
 
     const clear_color = Color.rgb(40, 40, 40);
     Gfx.setClearColor(clear_color);
+    Gfx.setDefaultSampler(.linear);
 
     const tile_size = Vec2.init(48, 48);
     var canvas = CanvasScaler{

src/platform/graphics.zig

@@ -96,9 +96,11 @@ pub fn init(gpa: std.mem.Allocator, logger: sg.Logger) !void {
             s[0] = .{
                 .blend = .{
                     .enabled = true,
-                    .src_factor_rgb = .SRC_ALPHA,
+
+                    .src_factor_rgb = .ONE,
                     .dst_factor_rgb = .ONE_MINUS_SRC_ALPHA,
                     .op_rgb = .ADD,
+
                     .src_factor_alpha = .ONE,
                     .dst_factor_alpha = .ONE_MINUS_SRC_ALPHA,
                     .op_alpha = .ADD,
@@ -646,13 +648,19 @@ pub fn deinitTexture(id: Texture.Id) void {
     }
 }
 
-pub fn setTexture(id: Texture.Id, texture_data: ImageData) void {
+const SetTextureOptions = struct {
+    data: ImageData,
+    premultiplied: bool = false
+};
+
+pub fn setTexture(id: Texture.Id, opts: SetTextureOptions) void {
     const self = &g_state;
     const texture = self.textures.get(id) orelse {
         log.warn("Attempt to set texture that doesn't exist: {}", .{id});
         return;
     };
 
+    const texture_data = opts.data;
     const pixel_count = texture_data.width*texture_data.height;
 
     var image_data: sg.ImageData = .{};
@@ -671,6 +679,9 @@ pub fn setTexture(id: Texture.Id, texture_data: ImageData) void {
         }
     }
 
+    // TODO: Apply premultiply, will need scratch buffer
+    assert(opts.premultiplied);
+
     var new_image_desc = sg.ImageDesc{
         .type = ._2D,
         .width = @intCast(texture_data.width),
@@ -874,7 +885,8 @@ pub fn deinitSprite(id: Sprite.Id) void {
 
 const SetSpriteOptions = struct {
     image: ImageData,
-    rect: ?Rect = null
+    rect: ?Rect = null,
+    premultiplied: bool = false
 };
 
 pub fn setSprite(id: Sprite.Id, opts: SetSpriteOptions) void {
@@ -925,6 +937,10 @@ pub fn setSprite(id: Sprite.Id, opts: SetSpriteOptions) void {
         };
     }
 
+    if (!opts.premultiplied) {
+        sprite_data.premultiplyAlpha();
+    }
+
     var old_size: ?Vec2 = null;
     if (sprite.data) |old_sprite_data| {
         old_size = Vec2.initFromInt(u32, old_sprite_data.width, old_sprite_data.height);
@@ -1026,6 +1042,8 @@ fn repackSpritesheetIfNeeded(id: Spritesheet.Id) void {
     repackSpritesheet(id);
 }
 
+// TODO: This function is a bit large, there are some easy things that could be moved to separate function.
+// e.g. extruding padding.
 fn rebuildSpritesheetTextureIfNeeded(id: Spritesheet.Id) void {
     const self = &g_state;
     const spritesheet = self.spritesheets.get(id) orelse return;
@@ -1072,9 +1090,43 @@ fn rebuildSpritesheetTextureIfNeeded(id: Spritesheet.Id) void {
             sprite_data.width,
             sprite_data.height
         );
+        if (sprite.padding > 0) {
+            for (0..sprite_data.width) |ox| {
+                const pixel = image_data.getPixel(x+ox, y);
+                for (0..sprite.padding) |i| {
+                    @memcpy(image_data.getPixel(x+ox, y-(i+1)), pixel);
+                }
             }
 
-    setTexture(spritesheet.texture, image_data);
+            for (0..sprite_data.width) |ox| {
+                const pixel = image_data.getPixel(x+ox, y+sprite_data.height-1);
+                for (0..sprite.padding) |i| {
+                    @memcpy(image_data.getPixel(x+ox, y+sprite_data.height+i), pixel);
+                }
+            }
+
+            for (0..sprite_data.height) |oy| {
+                const pixel = image_data.getPixel(x, y+oy);
+                for (0..sprite.padding) |i| {
+                    @memcpy(image_data.getPixel(x-(i+1), y+oy), pixel);
+                }
+            }
+
+            for (0..sprite_data.height) |oy| {
+                const pixel = image_data.getPixel(x+sprite_data.width-1, y+oy);
+                for (0..sprite.padding) |i| {
+                    @memcpy(image_data.getPixel(x+sprite_data.width+i, y+oy), pixel);
+                }
+            }
+
+            // TODO: Add padding to corners.
+        }
+    }
+
+    setTexture(spritesheet.texture, .{
+        .data = image_data,
+        .premultiplied = true
+    });
 
     spritesheet.needs_texture_rebuild = false;
 } 
@@ -1125,17 +1177,11 @@ pub fn getSpriteUVRect(id: Sprite.Id) ?Rect {
 
     assert(sprite.position != null);
 
-
     const sprite_size = Vec2.initFromInt(u32, sprite_data.width, sprite_data.height);
-    var uv_rect = Rect{
+    return Rect{
         .pos = sprite.position.?.divide(spritesheet.size),
         .size = sprite_size.divide(spritesheet.size),
     };
-
-    const texel_size = Vec2.init(1, 1).divide(spritesheet.size);
-    uv_rect = uv_rect.shrink(texel_size.divideScalar(8));
-
-    return uv_rect;
 }
 
 pub fn drawSpriteQuad(id: Sprite.Id, points: Quad, color: Color) void {
@@ -1283,7 +1329,7 @@ pub const ImageData = struct {
         }
     }
 
-    pub fn getPixel(self: ImageData, x: u32, y: u32) []u8 {
+    pub fn getPixel(self: ImageData, x: usize, y: usize) []u8 {
         const pixel_index = y * self.width + x;
         if (self.pixels) |pixels| {
             return switch (pixels) {
@@ -1330,6 +1376,23 @@ pub const ImageData = struct {
         self.setRect(x, y, width, height, pixel);
     }
 
+    pub fn premultiplyAlpha(self: ImageData) void {
+        const pixels = self.pixels orelse return;
+        if (pixels != .rgba8) return;
+        const rgba8_pixels = pixels.rgba8;
+
+        for (0..self.height) |y| {
+            for (0..self.width) |x| {
+                const pixel_index = y * self.width + x;
+                const rgba8_pixel = rgba8_pixels[(4*pixel_index)..][0..4];
+
+                inline for (0..3) |i| {
+                    rgba8_pixel[i] = @intCast(@as(u16, rgba8_pixel[i]) * rgba8_pixel[3] / 255);
+                }
+            }
+        }
+    }
+
     pub fn copyFrom(
         self: ImageData,
         source: ImageData,