Issue #107: Conversion algorithm

Pdf4QtLib/sources/pdfimageconversion.cpp

@@ -0,0 +1,188 @@
+#include "pdfimageconversion.h"
+
+namespace pdf
+{
+
+PDFImageConversion::PDFImageConversion()
+{
+
+}
+
+void PDFImageConversion::setImage(QImage image)
+{
+    m_image = std::move(image);
+    m_convertedImage = QImage();
+    m_automaticThreshold = DEFAULT_THRESHOLD;
+}
+
+void PDFImageConversion::setConversionMethod(ConversionMethod method)
+{
+    m_conversionMethod = method;
+}
+
+void PDFImageConversion::setThreshold(int threshold)
+{
+    m_manualThreshold = threshold;
+}
+
+bool PDFImageConversion::convert()
+{
+    if (m_image.isNull())
+    {
+        return false;
+    }
+
+    QImage bitonal(m_image.width(), m_image.height(), QImage::Format_Mono);
+    bitonal.fill(0);
+
+    // Thresholding
+    int threshold = DEFAULT_THRESHOLD;
+
+    switch (m_conversionMethod)
+    {
+        case pdf::PDFImageConversion::ConversionMethod::Automatic:
+            m_automaticThreshold = calculateOtsu1DThreshold();
+            threshold = m_automaticThreshold;
+            break;
+
+        case pdf::PDFImageConversion::ConversionMethod::Manual:
+            threshold = m_manualThreshold;
+            break;
+
+        default:
+            Q_ASSERT(false);
+            break;
+    }
+
+    for (int y = 0; y < m_image.height(); ++y)
+    {
+        for (int x = 0; x < m_image.width(); ++x)
+        {
+            QColor pixelColor = m_image.pixelColor(x, y);
+            int pixelValue = pixelColor.lightness();
+            bool bit = (pixelValue >= threshold);
+            bitonal.setPixel(x, y, bit);
+        }
+    }
+
+    m_convertedImage = std::move(bitonal);
+    return true;
+}
+
+int PDFImageConversion::getThreshold() const
+{
+    switch (m_conversionMethod)
+    {
+        case pdf::PDFImageConversion::ConversionMethod::Automatic:
+            return m_automaticThreshold;
+
+        case pdf::PDFImageConversion::ConversionMethod::Manual:
+            return m_manualThreshold;
+
+        default:
+            Q_ASSERT(false);
+            break;
+    }
+
+    return DEFAULT_THRESHOLD;
+}
+
+QImage PDFImageConversion::getConvertedImage() const
+{
+    return m_convertedImage;
+}
+
+int PDFImageConversion::calculateOtsu1DThreshold() const
+{
+    if (m_image.isNull())
+    {
+        return 128;
+    }
+
+    // Histogram of lightness occurences
+    std::array<int, 256> histogram = { };
+
+    for (int x = 0; x < m_image.width(); ++x)
+    {
+        for (int y = 0; y < m_image.height(); ++y)
+        {
+            int lightness = m_image.pixelColor(x, y).lightness();
+            Q_ASSERT(lightness >= 0 && lightness <= 255);
+
+            int clampedLightness = qBound(0, lightness, 255);
+            histogram[clampedLightness] += 1;
+        }
+    }
+
+    float factor = 1.0f / float(m_image.width() * m_image.height());
+
+    std::array<float, 256> normalizedHistogram = { };
+    std::array<float, 256> cumulativeProbabilities = { };
+    std::array<float, 256> interClassVariance = { };
+
+    // Compute probabilities
+    for (size_t i = 0; i < histogram.size(); ++i)
+    {
+        normalizedHistogram[i] = histogram[i] * factor;
+        cumulativeProbabilities[i] = normalizedHistogram[i];
+
+        if (i > 0)
+        {
+            cumulativeProbabilities[i] += cumulativeProbabilities[i - 1];
+        }
+    }
+
+    // Calculate the inter-class variance for each threshold. Variables
+    // with the subscript 0 denote the background, while those with
+    // subscript 1 denote the foreground.
+    for (size_t i = 0; i < histogram.size(); ++i)
+    {
+        const float w0 = cumulativeProbabilities[i] - normalizedHistogram[i];
+        const float w1 = 1.0f - w0;
+
+        float u0 = 0.0f;
+        float u1 = 0.0f;
+
+        // Calculate mean intensity value of the background.
+        if (!qFuzzyIsNull(w0))
+        {
+            for (size_t j = 0; j < i; ++j)
+            {
+                u0 += j * normalizedHistogram[j];
+            }
+
+            u0 /= w0;
+        }
+
+        // Calculate mean intensity value of the foreground.
+        if (!qFuzzyIsNull(w1))
+        {
+            for (size_t j = i; j < histogram.size(); ++j)
+            {
+                u1 += j * normalizedHistogram[j];
+            }
+
+            u1 /= w1;
+        }
+
+        const float variance = w0 * w1 * std::powf(u0 - u1, 2);
+        interClassVariance[i] = variance;
+    }
+
+    // Find maximal value of the variance
+    size_t maxVarianceIndex = 0;
+    float maxVarianceValue = 0.0f;
+
+    for (size_t i = 0; i < interClassVariance.size(); ++i)
+    {
+        if (interClassVariance[i] > maxVarianceValue)
+        {
+            maxVarianceValue = interClassVariance[i];
+            maxVarianceIndex = i;
+        }
+    }
+
+    return int(maxVarianceIndex);
+}
+
+}   // namespace pdf

Pdf4QtLib/sources/pdfimageconversion.h

@@ -0,0 +1,97 @@
+//    Copyright (C) 2023 Jakub Melka
+//
+//    This file is part of PDF4QT.
+//
+//    PDF4QT is free software: you can redistribute it and/or modify
+//    it under the terms of the GNU Lesser General Public License as published by
+//    the Free Software Foundation, either version 3 of the License, or
+//    with the written consent of the copyright owner, any later version.
+//
+//    PDF4QT 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 for more details.
+//
+//    You should have received a copy of the GNU Lesser General Public License
+//    along with PDF4QT.  If not, see <https://www.gnu.org/licenses/>.
+
+#ifndef PDFIMAGECONVERSION_H
+#define PDFIMAGECONVERSION_H
+
+#include "pdfglobal.h"
+
+#include <QImage>
+
+namespace pdf
+{
+
+/// This class facilitates various image conversions,
+/// including transforming colored images into monochromatic (or bitonal) formats.
+class PDF4QTLIBSHARED_EXPORT PDFImageConversion
+{
+public:
+    PDFImageConversion();
+
+    enum class ConversionMethod
+    {
+        Automatic,  ///< The threshold is determined automatically using an algorithm.
+        Manual      ///< The threshold is manually provided by the user.
+    };
+
+    /// Sets the image to be converted using the specified conversion method.
+    /// This operation resets any previously converted image and the automatic threshold,
+    /// thereby erasing all prior image data.
+    /// \param image The image to be set for conversion.
+    void setImage(QImage image);
+
+    /// Sets the method for image conversion. Multiple methods are available
+    /// for selection. If the manual method is chosen, an appropriate threshold
+    /// must also be set by the user.
+    /// \param method The conversion method to be used.
+    void setConversionMethod(ConversionMethod method);
+
+    /// Sets the manual threshold value. When a non-manual (e.g., automatic) conversion
+    /// method is in use, this function will retain the manual threshold settings,
+    /// but the conversion will utilize an automatically calculated threshold for the image.
+    /// The manually set threshold is preserved and not overwritten. Therefore, if the
+    /// manual conversion method is later selected, the previously established manual
+    /// threshold will be applied.
+    /// \param threshold The manual threshold value to be set.
+    void setThreshold(int threshold);
+
+    /// This method converts the image into a bitonal (monochromatic) format. If
+    /// the automatic threshold calculation is enabled, it executes Otsu's 1D algorithm
+    /// to determine the threshold. When the manual conversion method is selected,
+    /// the automatic threshold calculation is bypassed, and the predefined manual threshold
+    /// value is utilized instead. This method returns true if the conversion is
+    /// successful, and false otherwise.
+    bool convert();
+
+    /// Returns the threshold used in image conversion. If the automatic conversion method is
+    /// selected, this function should be called only after executing the convert() method;
+    /// otherwise, it may return invalid data. The automatic threshold calculation is
+    /// performed within the convert() method.
+    /// \returns The threshold value used in image conversion.
+    int getThreshold() const;
+
+    /// Returns the converted image. This method should only be called after
+    /// the convert() method has been executed, and additionally, only if the
+    /// convert() method returns true. If these conditions are not met, the result
+    /// is undefined.
+    QImage getConvertedImage() const;
+
+private:
+    int calculateOtsu1DThreshold() const;
+
+    static constexpr int DEFAULT_THRESHOLD = 128;
+
+    QImage m_image;
+    QImage m_convertedImage;
+    ConversionMethod m_conversionMethod = ConversionMethod::Automatic;
+    int m_automaticThreshold = DEFAULT_THRESHOLD;
+    int m_manualThreshold = DEFAULT_THRESHOLD;
+};
+
+}   // namespace pdf
+
+#endif // PDFIMAGECONVERSION_H