tutorial: make the GLSL Mandelbrot shader completely branchless.

test/tutorial/tut03.cpp

@@ -391,6 +391,8 @@ public:
                 "uniform mat4 in_ZoomSettings;"
                 "uniform sampler2D in_Texture;"
                 ""
+                "vec2 v05 = vec2(0.5, 0.5);"
+                ""
                 "float mylen(vec2 p)"
                 "{"
                 "    p *= in_TexelSize.zw;" /* Correct for aspect ratio */
@@ -424,8 +426,8 @@ public:
                       * so that we know the error between the pixel we get
                       * and the pixel we wanted. */
                 "    vec2 r = q - 0.5 * in_TexelSize.xy;"
-                "    float l = (abs(q.x - .5) < .5 && abs(q.y - .5) < .5)"
-                "             ? 1.0 / mylen(r - p) : 0.0;"
+                "    vec2 t = step(abs(q - v05), v05);"
+                "    float l = t.x * t.y / mylen(r - p);"
                      /* Step 4: return final texel coordinates and
                       * corresponding error value. */
                 "    return vec3(q * vec2(1.0, 0.25), l);"
@@ -441,8 +443,9 @@ public:
                 "    q = in_TexelSize.xy"
                 "         * (1.0 + 2.0 * floor(q * 0.5 * in_TexelSize.zw));"
                 "    vec2 r = q - -0.5 * in_TexelSize.xy;"
-                "    float l = (abs(q.x - .5) < .5 && abs(q.y - .5) < .5)"
-                "             ? 1.0 / mylen(r - p) : 0.0;"
+                ""
+                "    vec2 t = step(abs(q - v05), v05);"
+                "    float l = t.x * t.y / mylen(r - p);"
                 "    return vec3(q * vec2(1.0, 0.25) + vec2(0.0, 0.25), l);"
                 "}"
                 ""
@@ -456,8 +459,9 @@ public:
                 "    q = in_TexelSize.xy"
                 "         * (1.0 + 2.0 * floor(q * 0.5 * in_TexelSize.zw));"
                 "    vec2 r = q - vec2(0.5, -0.5) * in_TexelSize.xy;"
-                "    float l = (abs(q.x - .5) < .5 && abs(q.y - .5) < .5)"
-                "             ? 1.0 / mylen(r - p) : 0.0;"
+                ""
+                "    vec2 t = step(abs(q - v05), v05);"
+                "    float l = t.x * t.y / mylen(r - p);"
                 "    return vec3(q * vec2(1.0, 0.25) + vec2(0.0, 0.50), l);"
                 "}"
                 ""
@@ -471,8 +475,9 @@ public:
                 "    q = in_TexelSize.xy"
                 "         * (1.0 + 2.0 * floor(q * 0.5 * in_TexelSize.zw));"
                 "    vec2 r = q - vec2(-0.5, 0.5) * in_TexelSize.xy;"
-                "    float l = (abs(q.x - .5) < .5 && abs(q.y - .5) < .5)"
-                "             ? 1.0 / mylen(r - p) : 0.0;"
+                ""
+                "    vec2 t = step(abs(q - v05), v05);"
+                "    float l = t.x * t.y / mylen(r - p);"
                 "    return vec3(q * vec2(1.0, 0.25) + vec2(0.0, 0.75), l);"
                 "}"
                 ""
@@ -483,19 +488,24 @@ public:
 #else
                 "    vec2 coord = gl_TexCoord[0].xy;"
 #endif
-                /* Get a pixel from each slice */
+                     /* Get a pixel coordinate from each slice */
                 "    vec3 k0 = nearest0(coord);"
                 "    vec3 k1 = nearest1(coord);"
                 "    vec3 k2 = nearest2(coord);"
                 "    vec3 k3 = nearest3(coord);"
-                "    vec4 p0 = texture2D(in_Texture, k0.xy);"
-                "    vec4 p1 = texture2D(in_Texture, k1.xy);"
-                "    vec4 p2 = texture2D(in_Texture, k2.xy);"
-                "    vec4 p3 = texture2D(in_Texture, k3.xy);"
-"if (k0.z >= k1.z && k0.z >= k2.z && k0.z >= k3.z) gl_FragColor = p0;"
-"if (k1.z >= k0.z && k1.z >= k2.z && k1.z >= k3.z) gl_FragColor = p1;"
-"if (k2.z >= k0.z && k2.z >= k1.z && k2.z >= k3.z) gl_FragColor = p2;"
-"if (k3.z >= k0.z && k3.z >= k1.z && k3.z >= k2.z) gl_FragColor = p3;"
+                     /* Get the nearest pixel */
+                "    float t01 = step(k0.z, k1.z);"
+                "    k0 = mix(k0, k1, t01);"
+                "    float t23 = step(k2.z, k3.z);"
+                "    k2 = mix(k2, k3, t23);"
+                "    float t02 = step(k0.z, k2.z);"
+                "    k0 = mix(k0, k2, t02);"
+                "    gl_FragColor = texture2D(in_Texture, k0.xy);"
+                     /* Alternate version: some kind of linear interpolation */
+//                "    vec4 p0 = texture2D(in_Texture, k0.xy);"
+//                "    vec4 p1 = texture2D(in_Texture, k1.xy);"
+//                "    vec4 p2 = texture2D(in_Texture, k2.xy);"
+//                "    vec4 p3 = texture2D(in_Texture, k3.xy);"
 //                "    gl_FragColor = 1.0 / (k0.z + k1.z + k2.z + k3.z)"
 //                "          * (k0.z * p0 + k1.z * p1 + k2.z * p2 + k3.z * p3);"
                 "}"