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Support gamma correction of images with alpha channel Favour shrink over affine when reducing by integral factor
158 lines
5.1 KiB
C++
158 lines
5.1 KiB
C++
#include <vips/vips8>
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#include "common.h"
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#include "operations.h"
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using vips::VImage;
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namespace sharp {
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/*
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Alpha composite src over dst
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Assumes alpha channels are already premultiplied and will be unpremultiplied after
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*/
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VImage Composite(VImage src, VImage dst) {
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using sharp::HasAlpha;
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// Split src into non-alpha and alpha
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VImage srcWithoutAlpha = src.extract_band(0, VImage::option()->set("n", src.bands() - 1));
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VImage srcAlpha = src[src.bands() - 1] * (1.0 / 255.0);
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// Split dst into non-alpha and alpha channels
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VImage dstWithoutAlpha;
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VImage dstAlpha;
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if (HasAlpha(dst)) {
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// Non-alpha: extract all-but-last channel
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dstWithoutAlpha = dst.extract_band(0, VImage::option()->set("n", dst.bands() - 1));
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// Alpha: Extract last channel
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dstAlpha = dst[dst.bands() - 1] * (1.0 / 255.0);
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} else {
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// Non-alpha: Copy reference
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dstWithoutAlpha = dst;
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// Alpha: Use blank, opaque (0xFF) image
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dstAlpha = VImage::black(dst.width(), dst.height()).invert();
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}
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//
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// Compute normalized output alpha channel:
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//
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// References:
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// - http://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending
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// - https://github.com/jcupitt/ruby-vips/issues/28#issuecomment-9014826
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//
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// out_a = src_a + dst_a * (1 - src_a)
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// ^^^^^^^^^^^
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// t0
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VImage t0 = srcAlpha.linear(-1.0, 1.0);
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VImage outAlphaNormalized = srcAlpha + dstAlpha * t0;
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//
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// Compute output RGB channels:
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//
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// Wikipedia:
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// out_rgb = (src_rgb * src_a + dst_rgb * dst_a * (1 - src_a)) / out_a
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// ^^^^^^^^^^^
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// t0
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//
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// Omit division by `out_a` since `Compose` is supposed to output a
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// premultiplied RGBA image as reversal of premultiplication is handled
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// externally.
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//
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VImage outRGBPremultiplied = srcWithoutAlpha + dstWithoutAlpha * t0;
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// Combine RGB and alpha channel into output image:
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return outRGBPremultiplied.bandjoin(outAlphaNormalized * 255.0);
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}
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/*
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* Stretch luminance to cover full dynamic range.
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*/
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VImage Normalize(VImage image) {
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// Get original colourspace
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VipsInterpretation typeBeforeNormalize = image.interpretation();
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if (typeBeforeNormalize == VIPS_INTERPRETATION_RGB) {
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typeBeforeNormalize = VIPS_INTERPRETATION_sRGB;
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}
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// Convert to LAB colourspace
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VImage lab = image.colourspace(VIPS_INTERPRETATION_LAB);
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// Extract luminance
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VImage luminance = lab[0];
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// Find luminance range
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VImage stats = luminance.stats();
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double min = stats(0, 0)[0];
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double max = stats(1, 0)[0];
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if (min != max) {
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// Extract chroma
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VImage chroma = lab.extract_band(1, VImage::option()->set("n", 2));
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// Calculate multiplication factor and addition
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double f = 100.0 / (max - min);
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double a = -(min * f);
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// Scale luminance, join to chroma, convert back to original colourspace
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VImage normalized = luminance.linear(f, a).bandjoin(chroma).colourspace(typeBeforeNormalize);
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// Attach original alpha channel, if any
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if (HasAlpha(image)) {
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// Extract original alpha channel
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VImage alpha = image[image.bands() - 1];
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// Join alpha channel to normalised image
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return normalized.bandjoin(alpha);
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} else {
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return normalized;
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}
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}
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return image;
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}
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/*
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* Gamma encoding/decoding
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*/
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VImage Gamma(VImage image, double const exponent) {
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if (HasAlpha(image)) {
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// Separate alpha channel
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VImage imageWithoutAlpha = image.extract_band(0,
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VImage::option()->set("n", image.bands() - 1));
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VImage alpha = image[image.bands() - 1];
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return imageWithoutAlpha.gamma(VImage::option()->set("exponent", exponent)).bandjoin(alpha);
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} else {
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return image.gamma(VImage::option()->set("exponent", exponent));
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}
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}
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/*
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* Gaussian blur (use sigma <0 for fast blur)
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*/
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VImage Blur(VImage image, double const sigma) {
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if (sigma < 0.0) {
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// Fast, mild blur - averages neighbouring pixels
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VImage blur = VImage::new_matrixv(3, 3,
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1.0, 1.0, 1.0,
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1.0, 1.0, 1.0,
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1.0, 1.0, 1.0);
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blur.set("scale", 9.0);
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return image.conv(blur);
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} else {
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// Slower, accurate Gaussian blur
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return image.gaussblur(sigma);
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}
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}
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/*
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* Sharpen flat and jagged areas. Use radius of -1 for fast sharpen.
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*/
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VImage Sharpen(VImage image, int const radius, double const flat, double const jagged) {
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if (radius == -1) {
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// Fast, mild sharpen
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VImage sharpen = VImage::new_matrixv(3, 3,
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-1.0, -1.0, -1.0,
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-1.0, 32.0, -1.0,
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-1.0, -1.0, -1.0);
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sharpen.set("scale", 24.0);
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return image.conv(sharpen);
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} else {
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// Slow, accurate sharpen in LAB colour space, with control over flat vs jagged areas
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return image.sharpen(
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VImage::option()->set("radius", radius)->set("m1", flat)->set("m2", jagged)
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);
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}
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}
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} // namespace sharp
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