Add a set of image filters

With this you can do variants of this:

```
{{ $img := resources.Get "images/misc/3-jenny.jpg" }}
{{ $img := $img.Resize "300x" }}
{{ $g1 := $img.Filter images.Grayscale }}
{{ $g2 := $img | images.Filter (images.Saturate 30) (images.GaussianBlur 3) }}
```

Fixes #6255

1 files changed, 214 insertions, 0 deletions

diff --git a/resources/images/resampling.go b/resources/images/resampling.go
new file mode 100644
index 000000000..0cb267684
--- /dev/null
+++ b/resources/images/resampling.go
@@ -0,0 +1,214 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package images
+
+import "math"
+
+// We moved from imaging to the gift package for image processing at some point.
+// That package had more, but also less resampling filters. So we add the missing
+// ones here. They are fairly exotic, but someone may use them, so keep them here
+// for now.
+//
+// The filters below are ported from https://github.com/disintegration/imaging/blob/9aab30e6aa535fe3337b489b76759ef97dfaf362/resize.go#L369
+// MIT License.
+
+var (
+	// Hermite cubic spline filter (BC-spline; B=0; C=0).
+	hermiteResampling = resamp{
+		name:    "Hermite",
+		support: 1.0,
+		kernel: func(x float32) float32 {
+			x = absf32(x)
+			if x < 1.0 {
+				return bcspline(x, 0.0, 0.0)
+			}
+			return 0
+		},
+	}
+
+	// Mitchell-Netravali cubic filter (BC-spline; B=1/3; C=1/3).
+	mitchellNetravaliResampling = resamp{
+		name:    "MitchellNetravali",
+		support: 2.0,
+		kernel: func(x float32) float32 {
+			x = absf32(x)
+			if x < 2.0 {
+				return bcspline(x, 1.0/3.0, 1.0/3.0)
+			}
+			return 0
+		},
+	}
+
+	// Catmull-Rom - sharp cubic filter (BC-spline; B=0; C=0.5).
+	catmullRomResampling = resamp{
+		name:    "CatmullRomResampling",
+		support: 2.0,
+		kernel: func(x float32) float32 {
+			x = absf32(x)
+			if x < 2.0 {
+				return bcspline(x, 0.0, 0.5)
+			}
+			return 0
+		},
+	}
+
+	// BSpline is a smooth cubic filter (BC-spline; B=1; C=0).
+	bSplineResampling = resamp{
+		name:    "BSplineResampling",
+		support: 2.0,
+		kernel: func(x float32) float32 {
+			x = absf32(x)
+			if x < 2.0 {
+				return bcspline(x, 1.0, 0.0)
+			}
+			return 0
+		},
+	}
+
+	// Gaussian blurring filter.
+	gaussianResampling = resamp{
+		name:    "GaussianResampling",
+		support: 2.0,
+		kernel: func(x float32) float32 {
+			x = absf32(x)
+			if x < 2.0 {
+				return float32(math.Exp(float64(-2 * x * x)))
+			}
+			return 0
+		},
+	}
+
+	//  Hann-windowed sinc filter (3 lobes).
+	hannResampling = resamp{
+		name:    "HannResampling",
+		support: 3.0,
+		kernel: func(x float32) float32 {
+			x = absf32(x)
+			if x < 3.0 {
+				return sinc(x) * float32(0.5+0.5*math.Cos(math.Pi*float64(x)/3.0))
+			}
+			return 0
+		},
+	}
+
+	hammingResampling = resamp{
+		name:    "HammingResampling",
+		support: 3.0,
+		kernel: func(x float32) float32 {
+			x = absf32(x)
+			if x < 3.0 {
+				return sinc(x) * float32(0.54+0.46*math.Cos(math.Pi*float64(x)/3.0))
+			}
+			return 0
+		},
+	}
+
+	// Blackman-windowed sinc filter (3 lobes).
+	blackmanResampling = resamp{
+		name:    "BlackmanResampling",
+		support: 3.0,
+		kernel: func(x float32) float32 {
+			x = absf32(x)
+			if x < 3.0 {
+				return sinc(x) * float32(0.42-0.5*math.Cos(math.Pi*float64(x)/3.0+math.Pi)+0.08*math.Cos(2.0*math.Pi*float64(x)/3.0))
+			}
+			return 0
+		},
+	}
+
+	bartlettResampling = resamp{
+		name:    "BartlettResampling",
+		support: 3.0,
+		kernel: func(x float32) float32 {
+			x = absf32(x)
+			if x < 3.0 {
+				return sinc(x) * (3.0 - x) / 3.0
+			}
+			return 0
+		},
+	}
+
+	// Welch-windowed sinc filter (parabolic window, 3 lobes).
+	welchResampling = resamp{
+		name:    "WelchResampling",
+		support: 3.0,
+		kernel: func(x float32) float32 {
+			x = absf32(x)
+			if x < 3.0 {
+				return sinc(x) * (1.0 - (x * x / 9.0))
+			}
+			return 0
+		},
+	}
+
+	// Cosine-windowed sinc filter (3 lobes).
+	cosineResampling = resamp{
+		name:    "CosineResampling",
+		support: 3.0,
+		kernel: func(x float32) float32 {
+			x = absf32(x)
+			if x < 3.0 {
+				return sinc(x) * float32(math.Cos((math.Pi/2.0)*(float64(x)/3.0)))
+			}
+			return 0
+		},
+	}
+)
+
+// The following code is borrowed from https://raw.githubusercontent.com/disintegration/gift/master/resize.go
+// MIT licensed.
+type resamp struct {
+	name    string
+	support float32
+	kernel  func(float32) float32
+}
+
+func (r resamp) String() string {
+	return r.name
+}
+
+func (r resamp) Support() float32 {
+	return r.support
+}
+
+func (r resamp) Kernel(x float32) float32 {
+	return r.kernel(x)
+}
+
+func bcspline(x, b, c float32) float32 {
+	if x < 0 {
+		x = -x
+	}
+	if x < 1 {
+		return ((12-9*b-6*c)*x*x*x + (-18+12*b+6*c)*x*x + (6 - 2*b)) / 6
+	}
+	if x < 2 {
+		return ((-b-6*c)*x*x*x + (6*b+30*c)*x*x + (-12*b-48*c)*x + (8*b + 24*c)) / 6
+	}
+	return 0
+}
+
+func absf32(x float32) float32 {
+	if x < 0 {
+		return -x
+	}
+	return x
+}
+
+func sinc(x float32) float32 {
+	if x == 0 {
+		return 1
+	}
+	return float32(math.Sin(math.Pi*float64(x)) / (math.Pi * float64(x)))
+}