Fix off-by-one in compressing/decompressing texture coordinates

CHANGES.md

@@ -11,6 +11,7 @@ Change Log
 * Fixed billboard rotation when sized in meters. [#3979](https://github.com/AnalyticalGraphicsInc/cesium/issues/3979)
 * Fixed timeline touch events. [#4305](https://github.com/AnalyticalGraphicsInc/cesium/pull/4305)
 * Fixed a bug that could lead to incorrect terrain heights when using `HeightmapTerrainData` with an encoding in which actual heights were equal to the minimum representable height.
+* Fixed a bug in `AttributeCompression.compressTextureCoordinates` and `decompressTextureCoordinates` that could cause a small inaccuracy in the encoded texture coordinates.
 * Added `DebugCameraPrimitive` to visualize the view frustum of a camera.
 
 ### 1.25 - 2016-09-01

Source/Core/AttributeCompression.js

@@ -75,9 +75,9 @@ define([
      * @param {Cartesian3} vector The normalized vector to be compressed into 2 byte 'oct' encoding.
      * @param {Cartesian2} result The 2 byte oct-encoded unit length vector.
      * @returns {Cartesian2} The 2 byte oct-encoded unit length vector.
-     * 
+     *
      * @exception {DeveloperError} vector must be normalized.
-     * 
+     *
      * @see AttributeCompression.octEncodeInRange
      * @see AttributeCompression.octDecode
      */
@@ -124,14 +124,14 @@ define([
 
     /**
      * Decodes a unit-length vector in 2 byte 'oct' encoding to a normalized 3-component vector.
-     * 
+     *
      * @param {Number} x The x component of the oct-encoded unit length vector.
      * @param {Number} y The y component of the oct-encoded unit length vector.
      * @param {Cartesian3} result The decoded and normalized vector.
      * @returns {Cartesian3} The decoded and normalized vector.
-     * 
+     *
      * @exception {DeveloperError} x and y must be an unsigned normalized integer between 0 and 255.
-     * 
+     *
      * @see AttributeCompression.octDecodeInRange
      */
     AttributeCompression.octDecode = function(x, y, result) {
@@ -268,7 +268,7 @@ define([
     /**
      * Pack texture coordinates into a single float. The texture coordinates will only preserve 12 bits of precision.
      *
-     * @param {Cartesian2} textureCoordinates The texture coordinates to compress
+     * @param {Cartesian2} textureCoordinates The texture coordinates to compress.  Both coordinates must be in the range 0.0-1.0.
      * @returns {Number} The packed texture coordinates.
      *
      */
@@ -279,8 +279,9 @@ define([
         }
         //>>includeEnd('debug');
 
-        var x = textureCoordinates.x === 1.0 ? 4095.0 : (textureCoordinates.x * 4096.0) | 0;
-        var y = textureCoordinates.y === 1.0 ? 4095.0 : (textureCoordinates.y * 4096.0) | 0;
+        // Move x and y to the range 0-4095;
+        var x = (textureCoordinates.x * 4095.0) | 0;
+        var y = (textureCoordinates.y * 4095.0) | 0;
         return 4096.0 * x + y;
     };
 
@@ -303,8 +304,9 @@ define([
         //>>includeEnd('debug');
 
         var temp = compressed / 4096.0;
-        result.x = Math.floor(temp) / 4096.0;
-        result.y = temp - Math.floor(temp);
+        var xZeroTo4095 = Math.floor(temp);
+        result.x = xZeroTo4095 / 4095.0;
+        result.y = (compressed - xZeroTo4095 * 4096) / 4095;
         return result;
     };
 

Source/Shaders/Builtin/Functions/decompressTextureCoordinates.glsl

@@ -10,7 +10,8 @@
  vec2 czm_decompressTextureCoordinates(float encoded)
  {
     float temp = encoded / 4096.0;
-    float stx = floor(temp) / 4096.0;
-    float sty = temp - floor(temp);
+    float xZeroTo4095 = floor(temp);
+    float stx = xZeroTo4095 / 4095.0;
+    float sty = (encoded - xZeroTo4095 * 4096.0) / 4095.0;
     return vec2(stx, sty);
  }

Specs/Core/AttributeCompressionSpec.js

@@ -490,24 +490,49 @@ defineSuite([
 
     it('compresses texture coordinates', function() {
         var coords = new Cartesian2(0.5, 0.5);
-        expect(AttributeCompression.decompressTextureCoordinates(AttributeCompression.compressTextureCoordinates(coords), new Cartesian2())).toEqual(coords);
+        expect(AttributeCompression.decompressTextureCoordinates(AttributeCompression.compressTextureCoordinates(coords), new Cartesian2())).toEqualEpsilon(coords, 1.0 / 4096.0);
     });
-    
+
     it('compress texture coordinates throws without texture coordinates', function() {
         expect(function() {
             AttributeCompression.compressTextureCoordinates(undefined);
         }).toThrowDeveloperError();
     });
-    
+
     it('decompress texture coordinates throws without encoded texture coordinates', function() {
         expect(function() {
             AttributeCompression.decompressTextureCoordinates(undefined, new Cartesian2());
         }).toThrowDeveloperError();
     });
-    
+
     it('decompress texture coordinates throws without result', function() {
         expect(function() {
             AttributeCompression.decompressTextureCoordinates(0.0, undefined);
         }).toThrowDeveloperError();
     });
+
+    it('compresses/decompresses 1.0', function() {
+        var coords = new Cartesian2(1.0, 1.0);
+        expect(AttributeCompression.decompressTextureCoordinates(AttributeCompression.compressTextureCoordinates(coords), new Cartesian2())).toEqual(coords);
+    });
+
+    it('compresses/decompresses 0.0', function() {
+        var coords = new Cartesian2(1.0, 1.0);
+        expect(AttributeCompression.decompressTextureCoordinates(AttributeCompression.compressTextureCoordinates(coords), new Cartesian2())).toEqual(coords);
+    });
+
+    it('compresses/decompresses 0.5 / 1.0', function() {
+        var coords = new Cartesian2(0.5, 1.0);
+        expect(AttributeCompression.decompressTextureCoordinates(AttributeCompression.compressTextureCoordinates(coords), new Cartesian2())).toEqualEpsilon(coords, 1.0 / 4095.0);
+    });
+
+    it('compresses/decompresses 1.0 / 0.5', function() {
+        var coords = new Cartesian2(1.0, 0.5);
+        expect(AttributeCompression.decompressTextureCoordinates(AttributeCompression.compressTextureCoordinates(coords), new Cartesian2())).toEqualEpsilon(coords, 1.0 / 4095.0);
+    });
+
+    it('compresses/decompresses values very close but not equal to 1.0', function() {
+        var coords = new Cartesian2(0.99999999999999, 0.99999999999999);
+        expect(AttributeCompression.decompressTextureCoordinates(AttributeCompression.compressTextureCoordinates(coords), new Cartesian2())).toEqualEpsilon(coords, 1.0 / 4095.0);
+    });
 });

Specs/Core/TerrainEncodingSpec.js

@@ -183,7 +183,7 @@ defineSuite([
         expect(encoding.getStride()).toEqual(3);
         expect(buffer.length).toEqual(encoding.getStride());
 
-        expect(encoding.decodeTextureCoordinates(buffer, 0)).toEqualEpsilon(texCoords, CesiumMath.EPSILON14);
+        expect(encoding.decodeTextureCoordinates(buffer, 0)).toEqualEpsilon(texCoords, 1.0 / 4095.0);
     });
 
     it('encodes textureCoordinates with quantization and normals', function() {
@@ -198,7 +198,7 @@ defineSuite([
         expect(encoding.getStride()).toEqual(4);
         expect(buffer.length).toEqual(encoding.getStride());
 
-        expect(encoding.decodeTextureCoordinates(buffer, 0)).toEqualEpsilon(texCoords, CesiumMath.EPSILON14);
+        expect(encoding.decodeTextureCoordinates(buffer, 0)).toEqualEpsilon(texCoords, 1.0 / 4095.0);
     });
 
     it('encodes height with quantization and without normals', function() {
@@ -214,7 +214,7 @@ defineSuite([
         expect(encoding.getStride()).toEqual(3);
         expect(buffer.length).toEqual(encoding.getStride());
 
-        expect(encoding.decodeHeight(buffer, 0)).toEqualEpsilon(height, CesiumMath.EPSILON14);
+        expect(encoding.decodeHeight(buffer, 0)).toEqualEpsilon(height, 200.0 / 4095.0);
     });
 
     it('encodes height with quantization and normals', function() {
@@ -230,7 +230,7 @@ defineSuite([
         expect(encoding.getStride()).toEqual(4);
         expect(buffer.length).toEqual(encoding.getStride());
 
-        expect(encoding.decodeHeight(buffer, 0)).toEqualEpsilon(height, CesiumMath.EPSILON14);
+        expect(encoding.decodeHeight(buffer, 0)).toEqualEpsilon(height, 200.0 / 4095.0);
     });
 
     it('gets oct-encoded normal', function() {

Specs/Renderer/BuiltinFunctionsSpec.js

@@ -219,7 +219,7 @@ defineSuite([
     it('has czm_decompressTextureCoordinates', function() {
         var fs =
             'void main() { ' +
-            '  gl_FragColor = vec4(czm_decompressTextureCoordinates(8390656.0) == vec2(0.5, 0.5)); ' +
+            '  gl_FragColor = vec4(czm_decompressTextureCoordinates(8386559.0) == vec2(0.4998779, 0.4998779)); ' +
             '}';
         context.verifyDrawForSpecs(fs);
     });