项目中遇到一个需求，需要将图片中的前景色改成深蓝色#202189，背景色为白色。

方法1: 在原图img1叠一个深蓝色的矩形

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QImage img2 = img1;
QPainter painter(&img2);
painter.setCompositionMode(QPainter::CompositionMode_SourceIn);
painter.fillRect(img2.rect(), QColor(“#202189”));
painter.end();

此方法仅对有alpha通道的图片有效。

可以用下面的方法检测是否使用了alpha通道。

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bool useAlpha = false;
{
  const uchar* pixelData = img1.bits();
  int bytes = img1.byteCount();
  for (const QRgb* pixel = reinterpret_cast<const QRgb*>(pixelData); bytes > 0; pixel++, bytes -= sizeof(QRgb)){
    if (qAlpha(*pixel) != UCHAR_MAX) {
      useAlpha = true;
      break;
    }
  }
}

方法2：修改每个像素点的值

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void recolor(QImage *image, const QColor &foreground, const QColor &background)
{
    if (image->format() != QImage::Format_ARGB32_Premultiplied) {
        *image = image->convertToFormat(QImage::Format_ARGB32_Premultiplied);
    }
    Q_ASSERT(image->format() == QImage::Format_ARGB32_Premultiplied);
    const float scaleRed = background.redF() - foreground.redF();
    const float scaleGreen = background.greenF() - foreground.greenF();
    const float scaleBlue = background.blueF() - foreground.blueF();
    for (int y=0; y<image->height(); y++) {
        QRgb *pixels = reinterpret_cast<QRgb*>(image->scanLine(y));
        for (int x=0; x<image->width(); x++) {
            const int lightness = qGray(pixels[x]);
            pixels[x] = qRgba(scaleRed * lightness + foreground.red(),
                           scaleGreen * lightness + foreground.green(),
                           scaleBlue * lightness + foreground.blue(),
                           qAlpha(pixels[x]));
        }
    }
}

使用recolor:

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QImage img1("D:/1.png");
recolor(&img1, QColor("#202189"), QColor("#FFFFFF"));

Reference