histogramwidget.cpp Example File

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 #include "histogramwidget.h"
 #include <QPainter>

 HistogramWidget::HistogramWidget(QWidget *parent)
     : QWidget(parent)
     , m_levels(128)
 {
 }

 void HistogramWidget::processFrame(QVideoFrame frame)
 {
     m_histogram.clear();

     do {
         if (!m_levels)
             break;

         if (!frame.map(QAbstractVideoBuffer::ReadOnly))
             break;

         m_histogram.resize(m_levels);

         if (frame.pixelFormat() == QVideoFrame::Format_YUV420P) {
             // Process YUV420P data
             uchar *b = frame.bits();
             for (int y = 0; y < frame.height(); y++) {
                 uchar *lastPixel = b + frame.width();
                 for (uchar *curPixel = b; curPixel < lastPixel; curPixel++)
                     m_histogram[(*curPixel * m_levels) / 256] += 1.0;
                 b += frame.bytesPerLine();
             }
         } else {
             QImage::Format imageFormat = QVideoFrame::imageFormatFromPixelFormat(frame.pixelFormat());
             if (imageFormat != QImage::Format_Invalid) {
                 // Process RGB data
                 QImage image(frame.bits(), frame.width(), frame.height(), imageFormat);
                 image = image.convertToFormat(QImage::Format_RGB32);

                 const QRgb* b = (const QRgb*)image.bits();
                 for (int y = 0; y < image.height(); y++) {
                     const QRgb *lastPixel = b + frame.width();
                     for (const QRgb *curPixel = b; curPixel < lastPixel; curPixel++)
                         m_histogram[(qGray(*curPixel) * m_levels) / 256] += 1.0;
                     b = (const QRgb*)((uchar*)b + image.bytesPerLine());
                 }
             }
         }

         // find maximum value
         qreal maxValue = 0.0;
         for (int i = 0; i < m_histogram.size(); i++) {
             if (m_histogram[i] > maxValue)
                 maxValue = m_histogram[i];
         }

         if (maxValue > 0.0) {
             for (int i = 0; i < m_histogram.size(); i++)
                 m_histogram[i] /= maxValue;
         }

         frame.unmap();
     } while (false);

     update();
 }

 void HistogramWidget::paintEvent(QPaintEvent *event)
 {
     Q_UNUSED(event);

     QPainter painter(this);

     if (m_histogram.isEmpty()) {
         painter.fillRect(0, 0, width(), height(), QColor::fromRgb(0, 0, 0));
         return;
     }

     qreal barWidth = width() / (qreal)m_histogram.size();

     for (int i = 0; i < m_histogram.size(); i++) {
         qreal h = m_histogram[i] * height();
         // draw level
         painter.fillRect(barWidth * i, height() - h, barWidth * (i + 1), height(), Qt::red);
         // clear the rest of the control
         painter.fillRect(barWidth * i, 0, barWidth * (i + 1), height() - h, Qt::black);
     }
 }