Merge bitcoin-core/gui#71: Fix visual quality of text in QR image

6954156 qt: Fix visual quality of text in QR image (Hennadii Stepanov)
8071c75 qt, refactor: Limit scope of QPainter object (Hennadii Stepanov)

Pull request description:

  Master (197450f):
  ![DeepinScreenshot_select-area_20200824001800](https://user-images.githubusercontent.com/32963518/90988962-96283680-e59f-11ea-8e20-42e9b23033f5.png)

  This PR (6954156):
  - macOS 10.15.6
  ![Screenshot from 2020-09-07 15-40-30](https://user-images.githubusercontent.com/32963518/92390251-2c716600-f123-11ea-96f0-0e9d35810c76.png)

  - Linux Mint 20
  ![Screenshot from 2020-09-07 15-48-13](https://user-images.githubusercontent.com/32963518/92390272-36936480-f123-11ea-8fee-4de23bb40ed9.png)

  Fix ElementsProject#54
  Fix bitcoin/bitcoin#19103

  ---

  The first commit is easy to review with [`git diff --word-diff`](bitcoin-core/gui@8071c75?w=1).

ACKs for top commit:
  jonasschnelli:
    Tested ACK 6954156 - tested on macOS 10.15. Fixes the problem.

Tree-SHA512: 6ecb3397d2a5094c5f00ee05fc09520751568404e000a8691b6de7e57f38c2d5da628694e5e45a2b4cc302a846bbc00014c40820233eb026d3ebd4f68c2c9913

src/qt/qrimagewidget.cpp

@@ -9,6 +9,7 @@
 #include <QApplication>
 #include <QClipboard>
 #include <QDrag>
+#include <QFontDatabase>
 #include <QMenu>
 #include <QMimeData>
 #include <QMouseEvent>
@@ -64,26 +65,28 @@ bool QRImageWidget::setQR(const QString& data, const QString& text)
     }
     QRcode_free(code);
 
-    QImage qrAddrImage = QImage(QR_IMAGE_SIZE, QR_IMAGE_SIZE + (text.isEmpty() ? 0 : 20), QImage::Format_RGB32);
+    const int qr_image_size = QR_IMAGE_SIZE + (text.isEmpty() ? 0 : 2 * QR_IMAGE_MARGIN);
+    QImage qrAddrImage(qr_image_size, qr_image_size, QImage::Format_RGB32);
     qrAddrImage.fill(0xffffff);
-    QPainter painter(&qrAddrImage);
-    painter.drawImage(0, 0, qrImage.scaled(QR_IMAGE_SIZE, QR_IMAGE_SIZE));
+    {
+        QPainter painter(&qrAddrImage);
+        painter.drawImage(QR_IMAGE_MARGIN, 0, qrImage.scaled(QR_IMAGE_SIZE, QR_IMAGE_SIZE));
 
-    if (!text.isEmpty()) {
-        QFont font = GUIUtil::fixedPitchFont();
-        font.setStyleStrategy(QFont::NoAntialias);
-        QRect paddedRect = qrAddrImage.rect();
+        if (!text.isEmpty()) {
+            QRect paddedRect = qrAddrImage.rect();
+            paddedRect.setHeight(QR_IMAGE_SIZE + QR_IMAGE_TEXT_MARGIN);
 
-        // calculate ideal font size
-        qreal font_size = GUIUtil::calculateIdealFontSize(paddedRect.width() - 20, text, font);
-        font.setPointSizeF(font_size);
+            QFont font = GUIUtil::fixedPitchFont();
+            font.setStretch(QFont::SemiCondensed);
+            font.setLetterSpacing(QFont::AbsoluteSpacing, 1);
+            const qreal font_size = GUIUtil::calculateIdealFontSize(paddedRect.width() - 2 * QR_IMAGE_TEXT_MARGIN, text, font);
+            font.setPointSizeF(font_size);
 
-        painter.setFont(font);
-        paddedRect.setHeight(QR_IMAGE_SIZE+12);
-        painter.drawText(paddedRect, Qt::AlignBottom|Qt::AlignCenter, text);
+            painter.setFont(font);
+            painter.drawText(paddedRect, Qt::AlignBottom | Qt::AlignCenter, text);
+        }
     }
 
-    painter.end();
     setPixmap(QPixmap::fromImage(qrAddrImage));
 
     return true;

src/qt/qrimagewidget.h

@@ -12,7 +12,9 @@
 static const int MAX_URI_LENGTH = 255;
 
 /* Size of exported QR Code image */
-static const int QR_IMAGE_SIZE = 300;
+static constexpr int QR_IMAGE_SIZE = 300;
+static constexpr int QR_IMAGE_TEXT_MARGIN = 10;
+static constexpr int QR_IMAGE_MARGIN = 2 * QR_IMAGE_TEXT_MARGIN;
 
 QT_BEGIN_NAMESPACE
 class QMenu;