golang-image/draw/scale.go

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// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package draw
// TODO: should Scale and NewScaler also take an Op argument?
import (
"image"
"math"
)
// Scale scales the part of the source image defined by src and sr and writes
// to the part of the destination image defined by dst and dr.
//
// Of the interpolators provided by this package:
// - NearestNeighbor is fast but usually looks worst.
// - CatmullRom is slow but usually looks best.
// - ApproxBiLinear has reasonable speed and quality.
//
// The time taken depends on the size of dr. For kernel interpolators, the
// speed also depends on the size of sr, and so are often slower than
// non-kernel interpolators, especially when scaling down.
func Scale(dst Image, dr image.Rectangle, src image.Image, sr image.Rectangle, q Interpolator) {
q.NewScaler(int32(dr.Dx()), int32(dr.Dy()), int32(sr.Dx()), int32(sr.Dy())).Scale(dst, dr.Min, src, sr.Min)
}
// Scaler scales part of a source image, starting from sp, and writes to a
// destination image, starting from dp. The destination and source width and
// heights are pre-determined, as part of the Scaler.
//
// A Scaler is safe to use concurrently.
type Scaler interface {
Scale(dst Image, dp image.Point, src image.Image, sp image.Point)
}
// Interpolator creates scalers for a given destination and source width and
// heights.
type Interpolator interface {
NewScaler(dw, dh, sw, sh int32) Scaler
}
// Kernel is an interpolator that blends source pixels weighted by a symmetric
// kernel function.
type Kernel struct {
// Support is the kernel support and must be >= 0. At(t) is assumed to be
// zero when t >= Support.
Support float64
// At is the kernel function. It will only be called with t in the
// range [0, Support).
At func(t float64) float64
}
// NewScaler implements the Interpolator interface.
func (k *Kernel) NewScaler(dw, dh, sw, sh int32) Scaler {
return &kernelScaler{
dw: dw,
dh: dh,
sw: sw,
sh: sh,
horizontal: newDistrib(k, dw, sw),
vertical: newDistrib(k, dh, sh),
}
}
var (
// NearestNeighbor is the nearest neighbor interpolator. It is very fast,
// but usually gives very low quality results. When scaling up, the result
// will look 'blocky'.
NearestNeighbor = Interpolator(nnInterpolator{})
// ApproxBiLinear is a mixture of the nearest neighbor and bi-linear
// interpolators. It is fast, but usually gives medium quality results.
//
// It implements bi-linear interpolation when upscaling and a bi-linear
// blend of the 4 nearest neighbor pixels when downscaling. This yields
// nicer quality than nearest neighbor interpolation when upscaling, but
// the time taken is independent of the number of source pixels, unlike the
// bi-linear interpolator. When downscaling a large image, the performance
// difference can be significant.
ApproxBiLinear = Interpolator(ablInterpolator{})
// BiLinear is the tent kernel. It is slow, but usually gives high quality
// results.
BiLinear = &Kernel{1, func(t float64) float64 {
return 1 - t
}}
// CatmullRom is the Catmull-Rom kernel. It is very slow, but usually gives
// very high quality results.
//
// It is an instance of the more general cubic BC-spline kernel with parameters
// B=0 and C=0.5. See Mitchell and Netravali, "Reconstruction Filters in
// Computer Graphics", Computer Graphics, Vol. 22, No. 4, pp. 221-228.
CatmullRom = &Kernel{2, func(t float64) float64 {
if t < 1 {
return (1.5*t-2.5)*t*t + 1
}
return ((-0.5*t+2.5)*t-4)*t + 2
}}
// TODO: a Kaiser-Bessel kernel?
)
type nnInterpolator struct{}
func (nnInterpolator) NewScaler(dw, dh, sw, sh int32) Scaler { return &nnScaler{dw, dh, sw, sh} }
type nnScaler struct {
dw, dh, sw, sh int32
}
type ablInterpolator struct{}
func (ablInterpolator) NewScaler(dw, dh, sw, sh int32) Scaler { return &ablScaler{dw, dh, sw, sh} }
type ablScaler struct {
dw, dh, sw, sh int32
}
type kernelScaler struct {
dw, dh, sw, sh int32
horizontal, vertical distrib
}
// source is a range of contribs, their inverse total weight, and that ITW
// divided by 0xffff.
type source struct {
i, j int32
invTotalWeight float64
invTotalWeightFFFF float64
}
// contrib is the weight of a column or row.
type contrib struct {
coord int32
weight float64
}
// distrib measures how source pixels are distributed over destination pixels.
type distrib struct {
// sources are what contribs each column or row in the source image owns,
// and the total weight of those contribs.
sources []source
// contribs are the contributions indexed by sources[s].i and sources[s].j.
contribs []contrib
}
// newDistrib returns a distrib that distributes sw source columns (or rows)
// over dw destination columns (or rows).
func newDistrib(q *Kernel, dw, sw int32) distrib {
scale := float64(sw) / float64(dw)
halfWidth, kernelArgScale := q.Support, 1.0
if scale > 1 {
halfWidth *= scale
kernelArgScale = 1 / scale
}
// Make the sources slice, one source for each column or row, and temporarily
// appropriate its elements' fields so that invTotalWeight is the scaled
// co-ordinate of the source column or row, and i and j are the lower and
// upper bounds of the range of destination columns or rows affected by the
// source column or row.
n, sources := int32(0), make([]source, dw)
for x := range sources {
center := (float64(x)+0.5)*scale - 0.5
i := int32(math.Floor(center - halfWidth))
if i < 0 {
i = 0
}
j := int32(math.Ceil(center + halfWidth))
if j >= sw {
j = sw - 1
if j < i {
j = i
}
}
sources[x] = source{i: i, j: j, invTotalWeight: center}
n += j - i + 1
}
contribs := make([]contrib, 0, n)
for k, b := range sources {
totalWeight := 0.0
l := int32(len(contribs))
for coord := b.i; coord <= b.j; coord++ {
t := (b.invTotalWeight - float64(coord)) * kernelArgScale
if t < 0 {
t = -t
}
if t >= q.Support {
continue
}
weight := q.At(t)
if weight == 0 {
continue
}
totalWeight += weight
contribs = append(contribs, contrib{coord, weight})
}
totalWeight = 1 / totalWeight
sources[k] = source{
i: l,
j: int32(len(contribs)),
invTotalWeight: totalWeight,
invTotalWeightFFFF: totalWeight / 0xffff,
}
}
return distrib{sources, contribs}
}
func ftou(f float64) uint16 {
i := int32(0xffff*f + 0.5)
if i > 0xffff {
return 0xffff
} else if i > 0 {
return uint16(i)
}
return 0
}