TextureTools: initial rude implementation of distance field computation.

Not yet ported anywhere else than GL >= 3.3 with some GL 4.3 features.
Still, this closes #3.

src/TextureTools/CMakeLists.txt

@@ -1,8 +1,15 @@
+corrade_add_resource(MagnumTextureTools_RCS MagnumTextureTools
+    DistanceFieldShader.vert DistanceFieldShader.frag
+    ../Shaders/compatibility.glsl ALIAS compatibility.glsl)
+
 set(MagnumTextureTools_SRCS
-    Atlas.cpp)
+    Atlas.cpp
+    DistanceField.cpp
+    ${MagnumTextureTools_RCS})
 
 set(MagnumTextureTools_HEADERS
     Atlas.h
+    DistanceField.h
 
     magnumTextureToolsVisibility.h)
 

src/TextureTools/DistanceField.cpp

@@ -0,0 +1,97 @@
+/*
+    Copyright © 2010, 2011, 2012 Vladimír Vondruš <mosra@centrum.cz>
+
+    This file is part of Magnum.
+
+    Magnum is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Lesser General Public License version 3
+    only, as published by the Free Software Foundation.
+
+    Magnum is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+    GNU Lesser General Public License version 3 for more details.
+*/
+
+#include "TextureTools/DistanceField.h"
+
+#include <Utility/Resource.h>
+#include "Math/Geometry/Rectangle.h"
+#include "AbstractShaderProgram.h"
+#include "Extensions.h"
+#include "Framebuffer.h"
+#include "Mesh.h"
+#include "Shader.h"
+#include "Texture.h"
+
+namespace Magnum { namespace TextureTools {
+
+namespace {
+
+class DistanceFieldShader: public AbstractShaderProgram {
+    public:
+        enum: Int {
+            TextureLayer = 8
+        };
+
+        explicit DistanceFieldShader();
+
+        inline DistanceFieldShader* setRadius(Int radius) {
+            setUniform(radiusUniform, radius);
+            return this;
+        }
+
+        inline DistanceFieldShader* setScaling(Vector2 scaling) {
+            setUniform(scalingUniform, scaling);
+            return this;
+        }
+
+    private:
+        static const Int radiusUniform = 0,
+            scalingUniform = 1;
+};
+
+DistanceFieldShader::DistanceFieldShader() {
+    MAGNUM_ASSERT_VERSION_SUPPORTED(Version::GL330);
+    MAGNUM_ASSERT_EXTENSION_SUPPORTED(Extensions::GL::ARB::explicit_attrib_location);
+    MAGNUM_ASSERT_EXTENSION_SUPPORTED(Extensions::GL::ARB::explicit_uniform_location);
+    MAGNUM_ASSERT_EXTENSION_SUPPORTED(Extensions::GL::ARB::shading_language_420pack);
+
+    /** @todo compatibility! */
+
+    Corrade::Utility::Resource rs("MagnumTextureTools");
+    attachShader(Shader::fromData(Version::GL330, Shader::Type::Vertex, rs.get("DistanceFieldShader.vert")));
+
+    Shader fragmentShader(Version::GL330, Shader::Type::Fragment);
+    fragmentShader.addSource(rs.get("compatibility.glsl"));
+    fragmentShader.addSource(rs.get("DistanceFieldShader.frag"));
+    attachShader(fragmentShader);
+
+    link();
+}
+
+}
+
+void distanceField(Texture2D* input, Texture2D* output, const Rectanglei& rectangle, const Int radius) {
+    MAGNUM_ASSERT_EXTENSION_SUPPORTED(Extensions::GL::EXT::framebuffer_object);
+
+    /** @todo Disable depth test and then enable it back (if was previously) */
+
+    Framebuffer framebuffer(rectangle);
+    framebuffer.attachTexture2D(Framebuffer::ColorAttachment(0), output, 0);
+    framebuffer.bind(Framebuffer::Target::Draw);
+
+    DistanceFieldShader shader;
+    shader.setRadius(radius)
+        ->setScaling(Vector2(input->imageSize(0))/rectangle.size())
+        ->use();
+
+    input->bind(DistanceFieldShader::TextureLayer);
+
+    Mesh mesh;
+    mesh.setPrimitive(Mesh::Primitive::Triangles)
+        ->setVertexCount(3)
+        ->draw();
+}
+
+}}

src/TextureTools/DistanceField.h

@@ -0,0 +1,65 @@
+#ifndef Magnum_TextureTools_DistanceField_h
+#define Magnum_TextureTools_DistanceField_h
+/*
+    Copyright © 2010, 2011, 2012 Vladimír Vondruš <mosra@centrum.cz>
+
+    This file is part of Magnum.
+
+    Magnum is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Lesser General Public License version 3
+    only, as published by the Free Software Foundation.
+
+    Magnum is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+    GNU Lesser General Public License version 3 for more details.
+*/
+
+/** @file
+ * @brief Function Magnum::TextureTools::distanceField()
+ */
+
+#include "Magnum.h"
+
+#include "TextureTools/magnumTextureToolsVisibility.h"
+
+namespace Magnum { namespace TextureTools {
+
+/**
+@brief Create signed distance field
+@param input        Input texture
+@param output       Output texture
+@param rectangle    Rectangle in output texture where to render
+@param radius       Max lookup radius in input texture
+
+Converts binary image (stored in red channel of @p input) to signed distance
+field (stored in red channel in @p rectangle of @p output). The purpose of this
+function is to convert high-resolution binary image (such as vector artwork or
+font glyphs) to low-resolution grayscale image. The image will then occupy much
+less memory and can be scaled without aliasing issues. Additionally it provides
+foundation for features like outlining, glow or drop shadow essentialy for free.
+
+For each pixel inside @p rectangle the algorithm looks at corresponding pixel in
+@p input and tries to find nearest pixel of opposite color in area given by
+@p radius. Signed distance between the points is then saved as value of given
+pixel in @p output. Value of `0` means that the pixel was originally colored
+white and nearest black pixel is farther than @p radius, value of `1` means that
+the pixel was originally black and nearest white pixel is farther than
+@p radius. Values around `0.5` are around edges.
+
+The resulting texture can be used with bilinear filtering. It can be converted
+back to binary form in shader using e.g. GLSL `smoothstep()` function with step
+around `0.5` to create antialiased edges. Or you can exploit the distance field
+features to create many other effects.
+
+Based on: *Chris Green - Improved Alpha-Tested Magnification for Vector Textures
+and Special Effects, SIGGRAPH 2007,
+http://www.valvesoftware.com/publications/2007/SIGGRAPH2007_AlphaTestedMagnification.pdf*
+
+@attention This is GPU-only implementation, so it expects active context.
+*/
+void MAGNUM_TEXTURETOOLS_EXPORT distanceField(Texture2D* input, Texture2D* output, const Rectanglei& rectangle, const Int radius);
+
+}}
+
+#endif

src/TextureTools/DistanceFieldShader.frag

@@ -0,0 +1,59 @@
+layout(location = 0) uniform int radius;
+layout(location = 1) uniform vec2 scaling;
+layout(binding = 8) uniform sampler2D texture;
+
+layout(pixel_center_integer) in vec4 gl_FragCoord;
+
+out float value;
+
+ivec2 rotate(const ivec2 vec) {
+    return ivec2(-vec.y, vec.x);
+}
+
+bool hasValue(const ivec2 position, const ivec2 offset) {
+    return texelFetch(texture, position+offset, 0).r > 0.5;
+}
+
+void main() {
+    const ivec2 position = ivec2(gl_FragCoord.xy*scaling);
+
+    /* If pixel at the position is inside (1), we are looking for nearest pixel
+       outside and the value will be positive (> 0.5). If it is outside (0), we
+       are looking for nearest pixel inside and the value will be negative
+       (< 0.5). */
+    const bool isInside = hasValue(position, ivec2(0, 0));
+    const float sign = isInside ? 1.0 : -1.0;
+
+    /* Minimal found distance is just out of the radius (i.e. infinity) */
+    float minDistanceSquared = float((radius+1)*(radius+1));
+
+    /* Go in circles around the point and find nearest value */
+    int radiusLimit = radius;
+    for(int i = 1; i <= radiusLimit; ++i) {
+        for(int j = 0, jmax = i*2; j != jmax; ++j) {
+            const ivec2 offset = {-i+j, i};
+
+            /* If any of the four values is opposite of what is on the pixel,
+               we found nearest value */
+            if(hasValue(position, offset) == !isInside ||
+               hasValue(position, rotate(offset)) == !isInside ||
+               hasValue(position, rotate(rotate(offset))) == !isInside ||
+               hasValue(position, rotate(rotate(rotate(offset)))) == !isInside) {
+                const float distanceSquared = dot(vec2(offset), vec2(offset));
+
+                /* Set smaller distance, if found, or continue with lookup for
+                   smaller */
+                if(minDistanceSquared < distanceSquared) continue;
+                else minDistanceSquared = distanceSquared;
+
+                /* Set radius limit to max radius which can contain smaller
+                   value, e.g. for distance 3.5 we can find smaller value even
+                   in radius 3 */
+                radiusLimit = min(radius, int(floor(length(vec2(offset)))));
+            }
+        }
+    }
+
+    /* Final signed distance, normalized from [-radius-1, radius+1] to [0, 1] */
+    value = sign*sqrt(minDistanceSquared)/float(radius*2+2)+0.5;
+}

src/TextureTools/DistanceFieldShader.vert

@@ -0,0 +1,4 @@
+void main() {
+    gl_Position = vec4((gl_VertexID == 2) ?  3.0 : -1.0,
+                       (gl_VertexID == 1) ? -3.0 :  1.0, 0.0, 1.0);
+}