Qt/Images/SemiTransparentPixels

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main.cpp

#include <QApplication>
 
#include "form.h"
 
int main(int argc, char *argv[])
{
  QApplication app(argc, argv);
  Form form;
 
  form.show();
  return app.exec();
}

form.h

#ifndef FORM_H
#define FORM_H
 
#include "ui_form.h"
 
class Form : public QWidget, private Ui::Form
{
    Q_OBJECT
 
public:
    Form(QWidget *parent = 0);
 
};
 
#endif

form.cpp

#include <QtGui>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QPixmap>
 
#include "form.h"
 
Form::Form(QWidget *parent)
    : QWidget(parent)
{
  setupUi(this);
 
  QColor transparentRed(255,0,0, 0);
  QColor semiTransparentRed(255,0,0, 127);
  QColor opaqueRed(255,0,0, 255);
 
  QImage image(100,100,QImage::Format_ARGB32);
  image.fill(opaqueRed.rgba());
 
  // Set the top left corner to totally transparent
  for(unsigned int i = 0; i < 50; ++i)
    {
    for(unsigned int j = 0; j < 50; ++j)
      {
      image.setPixel(i,j,transparentRed.rgba());
      }
    }
 
  // Set the top right corner to semi-transparent red
  for(unsigned int i = 50; i < 100; ++i)
    {
    for(unsigned int j = 0; j < 50; ++j)
      {
      image.setPixel(i,j,semiTransparentRed.rgba());
      }
    }
 
  QPixmap pixmap = QPixmap::fromImage(image);
  QGraphicsScene* scene = new QGraphicsScene;
  scene->addPixmap(pixmap);
 
  this->graphicsView->setScene(scene);
 
}

form.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>Form</class>
 <widget class="QWidget" name="Form">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Form</string>
  </property>
  <widget class="QGraphicsView" name="graphicsView">
   <property name="geometry">
    <rect>
     <x>60</x>
     <y>50</y>
     <width>256</width>
     <height>192</height>
    </rect>
   </property>
  </widget>
 </widget>
 <resources/>
 <connections/>
</ui>

CMakeLists.txt

cmake_minimum_required(VERSION 2.6)
 
PROJECT(QPixmapOpacity)
 
FIND_PACKAGE(Qt4 REQUIRED)
INCLUDE(${QT_USE_FILE})
 
QT4_WRAP_UI(UISrcs form.ui)
QT4_WRAP_CPP(MOCSrcs form.h)
 
include_directories(${include_directories} ${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_CURRENT_BINARY_DIR})
 
ADD_EXECUTABLE(QPixmapOpacity main.cpp form.h form.cpp ${MOCSrcs} ${UISrcs})
TARGET_LINK_LIBRARIES(QPixmapOpacity ${QT_LIBRARIES})