将QImage转换为灰度 [英] Convert a QImage to grayscale

问题描述

我有一个QImage，我需要将其转换为灰度，然后再涂上颜色。我发现一个 allGray（）和 isGrayScale（）函数检查图像是否已经是灰度， c $ c> toGrayScale（）或类似命名的函数。

I have a QImage and I need to convert it to grayscale, then later paint over that with colors. I found an allGray() and isGrayScale() function to check if an image is already grayscale, but no toGrayScale() or similarly-named function.

现在我使用这个代码，但它没有非常好的效果：

Right now I'm using this code, but it's does not have a very good performance:

for (int ii = 0; ii < image.width(); ii++) {
    for (int jj = 0; jj < image.height(); jj++) {
        int gray = qGray(image.pixel(ii, jj));
        image.setPixel(ii, jj, QColor(gray, gray, gray).rgb());
    }
}

什么是最好的方式，将QImage转换为灰度？

What would be the best way, performance-wise, to convert a QImage to grayscale?

推荐答案

而不是使用 slow functions QImage :: pixel 和 QImage :: setPixel ，使用
QImage :: scanline 来访问数据。扫描（水平线）上的像素是连续的。假设你有一个32 bpp的图像，你可以使用QRgb迭代扫描。最后总是把x坐标放在内循环中。其中：

Rather than using the slow functions QImage::pixel and QImage::setPixel, use QImage::scanline to access the data. Pixels on a scan (horizontal line ) are consecutive. Assuming you have a 32 bpp image, you can use QRgb to iterate over the scan. Finally always put the x coordinate in the inner loop. Which gives :

for (int ii = 0; ii < image.height(); ii++) {
    uchar* scan = image.scanLine(ii);
    int depth =4;
    for (int jj = 0; jj < image.width(); jj++) {

        QRgb* rgbpixel = reinterpret_cast<QRgb*>(scan + jj*depth);
        int gray = qGray(*rgbpixel);
        *rgbpixel = QColor(gray, gray, gray).rgba();
    }
}

3585 x 2386图片

A quick test with an 3585 x 2386 image gave

********* Start testing of TestImage *********
Config: Using QTest library 4.7.4, Qt 4.7.4
PASS   : TestImage::initTestCase()

RESULT : TestImage::grayscaleOp():
     390 msecs per iteration (total: 390, iterations: 1)
PASS   : TestImage::grayscaleOp()

RESULT : TestImage::grayscaleFast():
     125 msecs per iteration (total: 125, iterations: 1)
PASS   : TestImage::grayscaleFast()

PASS   : TestImage::cleanupTestCase()
Totals: 4 passed, 0 failed, 0 skipped
********* Finished testing of TestImage *********

源代码：
testimage.h文件：

Source code: testimage.h file:

#ifndef TESTIMAGE_H
#define TESTIMAGE_H

#include <QtTest/QtTest>

#include <QObject>
#include <QImage>

class TestImage : public QObject
{
    Q_OBJECT
public:
    explicit TestImage(QObject *parent = 0);

signals:

private slots:
    void grayscaleOp();

    void grayscaleFast();

private:
    QImage imgop;
    QImage imgfast;
};

#endif // TESTIMAGE_H

testimage.cpp档案：

testimage.cpp file:

#include "testimage.h"

TestImage::TestImage(QObject *parent)
    : QObject(parent)
    , imgop("path_to_test_image.png")
    , imgfast("path_to_test_image.png")
{
}


void TestImage::grayscaleOp()
{
    QBENCHMARK
    {
        QImage& image = imgop;

        for (int ii = 0; ii < image.width(); ii++) {
            for (int jj = 0; jj < image.height(); jj++) {
                int gray = qGray(image.pixel(ii, jj));
                image.setPixel(ii, jj, QColor(gray, gray, gray).rgb());
            }
        }
    }
}

void TestImage::grayscaleFast()
{

    QBENCHMARK {

    QImage& image = imgfast;


    for (int ii = 0; ii < image.height(); ii++) {
        uchar* scan = image.scanLine(ii);
        int depth =4;
        for (int jj = 0; jj < image.width(); jj++) {

            QRgb* rgbpixel = reinterpret_cast<QRgb*>(scan + jj*depth);
            int gray = qGray(*rgbpixel);
            *rgbpixel = QColor(gray, gray, gray).rgba();
        }
    }

    }
}

QTEST_MAIN(TestImage)

专业文件：

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = QImageTest
TEMPLATE = app

CONFIG  += qtestlib

SOURCES += testimage.cpp

HEADERS += testimage.h

重要注意事项：

Important note:

• 通过反转环路。在这个测试案例中，它是〜90ms 。


• 您可以使用其他库如opencv进行灰度转换，从opencv缓冲区。我期望更好的性能改进。

这篇关于将QImage转换为灰度的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！