249 lines
8.0 KiB
C
249 lines
8.0 KiB
C
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#ifndef INCLUDED_IMF_RGBA_YCA_H
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#define INCLUDED_IMF_RGBA_YCA_H
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//////////////////////////////////////////////////////////////////////////////
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//
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// Copyright (c) 2004, Industrial Light & Magic, a division of Lucasfilm
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// Entertainment Company Ltd. Portions contributed and copyright held by
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// others as indicated. All rights reserved.
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//
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// Redistribution and use in source and binary forms, with or without
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// modification, are permitted provided that the following conditions are
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// met:
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//
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// * Redistributions of source code must retain the above
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// copyright notice, this list of conditions and the following
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// disclaimer.
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//
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// * Redistributions in binary form must reproduce the above
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// copyright notice, this list of conditions and the following
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// disclaimer in the documentation and/or other materials provided with
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// the distribution.
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//
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// * Neither the name of Industrial Light & Magic nor the names of
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// any other contributors to this software may be used to endorse or
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// promote products derived from this software without specific prior
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// written permission.
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//
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
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// IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
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// THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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// PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
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// CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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// EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
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// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
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// LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
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// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
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// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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//
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//////////////////////////////////////////////////////////////////////////////
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//-----------------------------------------------------------------------------
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//
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// Conversion between RGBA (red, green, blue alpha)
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// and YCA (luminance, subsampled chroma, alpha) data:
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//
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// Luminance, Y, is computed as a weighted sum of R, G, and B:
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//
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// Y = yw.x * R + yw.y * G + yw.z * B
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//
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// Function computeYw() computes a set of RGB-to-Y weights, yw,
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// from a set of primary and white point chromaticities.
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//
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// Chroma, C, consists of two components, RY and BY:
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//
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// RY = (R - Y) / Y
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// BY = (B - Y) / Y
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//
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// For efficiency, the x and y subsampling rates for chroma are
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// hardwired to 2, and the chroma subsampling and reconstruction
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// filters are fixed 27-pixel wide windowed sinc functions.
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//
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// Starting with an image that has RGBA data for all pixels,
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//
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// RGBA RGBA RGBA RGBA ... RGBA RGBA
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// RGBA RGBA RGBA RGBA ... RGBA RGBA
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// RGBA RGBA RGBA RGBA ... RGBA RGBA
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// RGBA RGBA RGBA RGBA ... RGBA RGBA
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// ...
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// RGBA RGBA RGBA RGBA ... RGBA RGBA
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// RGBA RGBA RGBA RGBA ... RGBA RGBA
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//
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// function RGBAtoYCA() converts the pixels to YCA format:
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//
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// YCA YCA YCA YCA ... YCA YCA
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// YCA YCA YCA YCA ... YCA YCA
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// YCA YCA YCA YCA ... YCA YCA
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// YCA YCA YCA YCA ... YCA YCA
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// ...
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// YCA YCA YCA YCA ... YCA YCA
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// YCA YCA YCA YCA ... YCA YCA
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//
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// Next, decimateChomaHoriz() eliminates the chroma values from
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// the odd-numbered pixels in every scan line:
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//
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// YCA YA YCA YA ... YCA YA
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// YCA YA YCA YA ... YCA YA
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// YCA YA YCA YA ... YCA YA
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// YCA YA YCA YA ... YCA YA
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// ...
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// YCA YA YCA YA ... YCA YA
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// YCA YA YCA YA ... YCA YA
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//
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// decimateChromaVert() eliminates all chroma values from the
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// odd-numbered scan lines:
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//
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// YCA YA YCA YA ... YCA YA
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// YA YA YA YA ... YA YA
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// YCA YA YCA YA ... YCA YA
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// YA YA YA YA ... YA YA
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// ...
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// YCA YA YCA YA ... YCA YA
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// YA YA YA YA ... YA YA
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//
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// Finally, roundYCA() reduces the precision of the luminance
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// and chroma values so that the pixel data shrink more when
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// they are saved in a compressed file.
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//
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// The output of roundYCA() can be converted back to a set
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// of RGBA pixel data that is visually very similar to the
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// original RGBA image, by calling reconstructChromaHoriz(),
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// reconstructChromaVert(), YCAtoRGBA(), and finally
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// fixSaturation().
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//
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//-----------------------------------------------------------------------------
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#include <ImfRgba.h>
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#include <ImfChromaticities.h>
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namespace Imf {
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namespace RgbaYca {
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//
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// Width of the chroma subsampling and reconstruction filters
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//
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static const int N = 27;
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static const int N2 = N / 2;
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//
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// Convert a set of primary chromaticities into a set of weighting
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// factors for computing a pixels's luminance, Y, from R, G and B
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//
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Imath::V3f computeYw (const Chromaticities &cr);
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//
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// Convert an array of n RGBA pixels, rgbaIn, to YCA (luminance/chroma/alpha):
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//
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// ycaOut[i].g = Y (rgbaIn[i]);
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// ycaOut[i].r = RY (rgbaIn[i]);
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// ycaOut[i].b = BY (rgbaIn[i]);
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// ycaOut[i].a = aIsValid? rgbaIn[i].a: 1
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//
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// yw is a set of RGB-to-Y weighting factors, as computed by computeYw().
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//
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void RGBAtoYCA (const Imath::V3f &yw,
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int n,
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bool aIsValid,
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const Rgba rgbaIn[/*n*/],
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Rgba ycaOut[/*n*/]);
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//
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// Perform horizontal low-pass filtering and subsampling of
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// the chroma channels of an array of n pixels. In order
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// to avoid indexing off the ends of the input array during
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// low-pass filtering, ycaIn must have N2 extra pixels at
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// both ends. Before calling decimateChromaHoriz(), the extra
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// pixels should be filled with copies of the first and last
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// "real" input pixel.
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//
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void decimateChromaHoriz (int n,
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const Rgba ycaIn[/*n+N-1*/],
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Rgba ycaOut[/*n*/]);
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//
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// Perform vertical chroma channel low-pass filtering and subsampling.
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// N scan lines of input pixels are combined into a single scan line
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// of output pixels.
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//
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void decimateChromaVert (int n,
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const Rgba * const ycaIn[N],
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Rgba ycaOut[/*n*/]);
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//
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// Round the luminance and chroma channels of an array of YCA
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// pixels that has already been filtered and subsampled.
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// The signifcands of the pixels' luminance and chroma values
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// are rounded to roundY and roundC bits respectively.
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//
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void roundYCA (int n,
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unsigned int roundY,
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unsigned int roundC,
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const Rgba ycaIn[/*n*/],
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Rgba ycaOut[/*n*/]);
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//
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// For a scan line that has valid chroma data only for every other pixel,
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// reconstruct the missing chroma values.
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//
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void reconstructChromaHoriz (int n,
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const Rgba ycaIn[/*n+N-1*/],
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Rgba ycaOut[/*n*/]);
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//
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// For a scan line that has only luminance and no valid chroma data,
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// reconstruct chroma from the surronding N scan lines.
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//
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void reconstructChromaVert (int n,
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const Rgba * const ycaIn[N],
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Rgba ycaOut[/*n*/]);
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//
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// Convert an array of n YCA (luminance/chroma/alpha) pixels to RGBA.
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// This function is the inverse of RGBAtoYCA().
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// yw is a set of RGB-to-Y weighting factors, as computed by computeYw().
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//
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void YCAtoRGBA (const Imath::V3f &yw,
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int n,
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const Rgba ycaIn[/*n*/],
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Rgba rgbaOut[/*n*/]);
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//
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// Eliminate super-saturated pixels:
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//
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// Converting an image from RGBA to YCA, low-pass filtering chroma,
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// and converting the result back to RGBA can produce pixels with
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// super-saturated colors, where one or two of the RGB components
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// become zero or negative. (The low-pass and reconstruction filters
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// introduce some amount of ringing into the chroma components.
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// This can lead to negative RGB values near high-contrast edges.)
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//
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// The fixSaturation() function finds super-saturated pixels and
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// corrects them by desaturating their colors while maintaining
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// their luminance. fixSaturation() takes three adjacent input
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// scan lines, rgbaIn[0], rgbaIn[1], rgbaIn[2], adjusts the
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// saturation of rgbaIn[1], and stores the result in rgbaOut.
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//
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void fixSaturation (const Imath::V3f &yw,
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int n,
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const Rgba * const rgbaIn[3],
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Rgba rgbaOut[/*n*/]);
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} // namespace RgbaYca
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} // namespace Imf
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#endif
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