Fix texture coordinate offsets

arcade/draw/rect.py

@@ -56,18 +56,20 @@ def draw_texture_rect(
         ctx.disable(ctx.BLEND)
 
     atlas = atlas or ctx.default_atlas
+    program = ctx.sprite_program_single
 
     texture_id, _ = atlas.add(texture)
     if pixelated:
         atlas.texture.filter = gl.NEAREST, gl.NEAREST
+        program.set_uniform_safe("uv_offset_bias", 0.0)
     else:
         atlas.texture.filter = gl.LINEAR, gl.LINEAR
+        program.set_uniform_safe("uv_offset_bias", 1.0)
 
     atlas.texture.use(unit=0)
     atlas.use_uv_texture(unit=1)
 
     geometry = ctx.geometry_empty
-    program = ctx.sprite_program_single
     program["pos"] = rect.center_x, rect.center_y, 0
     program["color"] = color.normalized
     program["size"] = rect.width, rect.height

arcade/resources/system/shaders/gui/nine_patch_gs.glsl

@@ -67,12 +67,6 @@ void main() {
     vec2 uv0, uv1, uv2, uv3;
     vec2 atlas_size = vec2(textureSize(sprite_texture, 0));
     getSpriteUVs(uv_texture, int(texture_id), uv0, uv1, uv2, uv3);
-    // TODO: Do center pixel interpolation. Revert by 0.5 pixels for now
-    vec2 half_px = 0.5 / atlas_size;
-    uv0 -= half_px;
-    uv1 += vec2(half_px.x, -half_px.y);
-    uv2 += vec2(-half_px.x, half_px.y);
-    uv3 += half_px;
 
     // Local corner offsets in pixels
     float left = start.x;

arcade/resources/system/shaders/sprites/sprite_list_geometry_cull_geo.glsl

@@ -10,7 +10,13 @@ uniform WindowBlock {
     mat4 view;
 } window;
 
+// Texture atlas
+uniform sampler2D sprite_texture;
+// Texture containing UVs for the entire atlas
 uniform sampler2D uv_texture;
+// How much half-pixel offset to apply to the UVs.
+// 0.0 is no offset, 1.0 is half a pixel offset
+uniform float uv_offset_bias;
 
 in float v_angle[];
 in vec4 v_color[];
@@ -49,11 +55,26 @@ void main() {
     vec2 uv0, uv1, uv2, uv3;
     getSpriteUVs(uv_texture, int(v_texture[0]), uv0, uv1, uv2, uv3);
 
+    // Apply half pixel offset modified by bias.
+    // What bias to set depends on the texture filtering mode.
+    // Linear can have 1.0 bias while nearest should have 0.0 (unless scale is 1:1)
+    // uvs (
+    //     0.0, 0.0,
+    //     1.0, 0.0,
+    //     0.0, 1.0,
+    //     1.0, 1.0
+    // )
+    vec2 hp = 0.5 / textureSize(sprite_texture, 0) * uv_offset_bias;
+    uv0 += hp;
+    uv1 += vec2(-hp.x, hp.y);
+    uv2 += vec2(hp.x, -hp.y);
+    uv3 += -hp;
+
     // Set the out color for all vertices
     gs_color = v_color[0];
     // Upper left
     gl_Position = mvp * vec4(rot * vec2(-hsize.x, hsize.y) + center.xy, center.z, 1.0);
-    gs_uv =  uv0;
+    gs_uv = uv0;
     EmitVertex();
 
     // lower left

arcade/resources/system/shaders/sprites/sprite_list_geometry_fs.glsl

@@ -1,6 +1,8 @@
 #version 330
 
+// Texture atlas
 uniform sampler2D sprite_texture;
+// Global color set on the sprite list
 uniform vec4 spritelist_color;
 
 in vec2 gs_uv;
@@ -11,6 +13,7 @@ out vec4 f_color;
 void main() {
     vec4 basecolor = texture(sprite_texture, gs_uv);
     basecolor *= gs_color * spritelist_color;
+    // Alpha test
     if (basecolor.a == 0.0) {
         discard;
     }

arcade/resources/system/shaders/sprites/sprite_list_geometry_no_cull_geo.glsl

@@ -10,7 +10,13 @@ uniform WindowBlock {
     mat4 view;
 } window;
 
+// Texture atlas
+uniform sampler2D sprite_texture;
+// Texture containing UVs for the entire atlas
 uniform sampler2D uv_texture;
+// How much half-pixel offset to apply to the UVs.
+// 0.0 is no offset, 1.0 is half a pixel offset
+uniform float uv_offset_bias;
 
 in float v_angle[];
 in vec4 v_color[];
@@ -36,6 +42,21 @@ void main() {
     vec2 uv0, uv1, uv2, uv3;
     getSpriteUVs(uv_texture, int(v_texture[0]), uv0, uv1, uv2, uv3);
 
+    // Apply half pixel offset modified by bias.
+    // What bias to set depends on the texture filtering mode.
+    // Linear can have 1.0 bias while nearest should have 0.0 (unless scale is 1:1)
+    // uvs (
+    //     0.0, 0.0,
+    //     1.0, 0.0,
+    //     0.0, 1.0,
+    //     1.0, 1.0
+    // )
+    vec2 hp = 0.5 / textureSize(sprite_texture, 0) * uv_offset_bias;
+    uv0 += hp;
+    uv1 += vec2(-hp.x, hp.y);
+    uv2 += vec2(hp.x, -hp.y);
+    uv3 += -hp;
+
     // Set the out color for all vertices
     gs_color = v_color[0];
     // Upper left

arcade/sprite_list/sprite_list.py

@@ -1103,6 +1103,14 @@ def draw(
 
         self.program["spritelist_color"] = self._color
 
+        # Control center pixel interpolation:
+        # 0.0 = raw interpolation using texture corners
+        # 1.0 = center pixel interpolation
+        if self.ctx.NEAREST in atlas_texture.filter:
+            self.program.set_uniform_safe("uv_offset_bias", 0.0)
+        else:
+            self.program.set_uniform_safe("uv_offset_bias", 1.0)
+
         atlas_texture.use(0)
         atlas.use_uv_texture(1)
         if not self._geometry:

arcade/texture_atlas/region.py

@@ -89,26 +89,25 @@ def __init__(
             # Width and height
             _width = self.width / atlas.width
             _height = self.height / atlas.height
-            # Half pixel correction
-            hp_x, hp_y = 0.5 / atlas.width, 0.5 / atlas.height
+
             # Upper left corner. Note that are mapping the texture upside down and corners
             # are named as from the vertex position point of view.
             ul_x, ul_y = self.x / atlas.width, self.y / atlas.height
 
             # upper_left, upper_right, lower_left, lower_right
             self.texture_coordinates = (
                 # upper_left
-                ul_x + hp_x,
-                ul_y + hp_y,
+                ul_x,
+                ul_y,
                 # upper_right
-                ul_x + _width - hp_x,
-                ul_y + hp_y,
+                ul_x + _width,
+                ul_y,
                 # lower_left
-                ul_x + hp_x,
-                ul_y + _height - hp_y,
+                ul_x,
+                ul_y + _height,
                 # lower_right
-                ul_x + _width - hp_x,
-                ul_y + _height - hp_y,
+                ul_x + _width,
+                ul_y + _height,
             )
 
     def verify_image_size(self, image_data: ImageData):

tests/unit/atlas/test_region.py

@@ -7,34 +7,37 @@
 from arcade.texture_atlas.atlas_default import DefaultTextureAtlas
 
 
-def test_region_coordinates(ctx):
-    """Test region class."""
+def test_region_coordinates_simple(ctx):
+    """Basic region test."""
     atlas = DefaultTextureAtlas(size=(8, 8), border=0, auto_resize=False)
     region = AtlasRegion(atlas=atlas, x=0, y=0, width=8, height=8)
     assert region.x == 0
     assert region.y == 0
     assert region.width == 8
     assert region.height == 8
     # Simulate the half pixel location
-    a, b = 0.5 / 8, 1 - 0.5 / 8
+    a, b = 0, 1.0
     assert region.texture_coordinates == (
         a, a,
         b, a,
         a, b,
         b, b,
     )
-    # Above raw values:
-    # (
-    #     0.0625, 0.0625,
-    #     0.9375, 0.0625,
-    #     0.0625, 0.9375,
-    #     0.9375, 0.9375)
-    # )
+
+
+def test_region_coordinates_complex(ctx):
+    """A more complex region test."""
+    atlas = DefaultTextureAtlas(size=(16, 16), border=0, auto_resize=False)
+    region = AtlasRegion(atlas=atlas, x=1, y=2, width=8, height=6)
+    assert region.x == 1
+    assert region.y == 2
+    assert region.width == 8
+    assert region.height == 6
+    assert region.texture_coordinates == (0.0625, 0.125, 0.5625, 0.125, 0.0625, 0.5, 0.5625, 0.5)
 
 
 def test_verify_size(ctx):
     im_data = ImageData(PIL.Image.new("RGBA", (8, 8)))
-    texture = Texture(im_data)
     region = AtlasRegion(atlas=ctx.default_atlas, x=0, y=0, width=8, height=8)
 
     region.verify_image_size(im_data)