我有一个QImage,我需要将其转换为灰度,然后再用颜色绘制它。我找到了allGray()
和isGrayScale()
函数来检查图像是否已经是灰度,但没有toGrayScale()
或类似命名的函数。
现在我正在使用此代码,但它没有很好的性能:
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());
}
}
将QImage转换为灰度的最佳方法是什么,性能方面呢?
答案 0 :(得分:9)
使用,而不是使用slow functions QImage::pixel
和QImage::setPixel
QImage::scanline
访问数据。扫描(水平线)上的像素是连续的。假设您有32 bpp图像,则可以使用QRgb迭代扫描。最后总是将x坐标放在内部循环中。这给出了:
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();
}
}
使用3585 x 2386图片进行快速测试
********* 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:
#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文件:
#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)
专业档案:
QT += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = QImageTest
TEMPLATE = app
CONFIG += qtestlib
SOURCES += testimage.cpp
HEADERS += testimage.h
重要提示:
~90ms
。答案 1 :(得分:7)
从Qt 5.5起,您可以调用QImage::convertToFormat()将QImage转换为灰度,如下所示:
QImage image = ...;
image.convertToFormat(QImage::Format_Grayscale8);
答案 2 :(得分:4)
我将发布@ UmNyobe代码的略微修改版本。我只是为扫描线增加一个指针,而不是通过索引计算每个像素。
// We assume the format to be RGB32!!!
Q_ASSERT(image.format() == QImage::Format_RGB32);
for (int ii = 0; ii < image.height(); ii++) {
QRgb *pixel = reinterpret_cast<QRgb*>(image.scanLine(ii));
QRgb *end = pixel + image.width();
for (; pixel != end; pixel++) {
int gray = qGray(*pixel);
*pixel = QColor(gray, gray, gray).rgb();
}
}
答案 3 :(得分:0)
内部qt类QPixmapColorizeFilter
使用解决类似主题的函数grayscale
。
我从中派生了以下函数,这应该可以解决问题。
重要的一点是将图像转换为32位格式,因此您可以将每个像素视为32位值,而无需考虑位对齐。
您也可以直接使用bits
函数并迭代所有像素,而不是遍历行和列。通过此技巧,您可以避免在scanLine
函数中执行乘法。
QImage convertToGrayScale(const QImage &srcImage) {
// Convert to 32bit pixel format
QImage dstImage = srcImage.convertToFormat(srcImage.hasAlphaChannel() ?
QImage::Format_ARGB32 : QImage::Format_RGB32);
unsigned int *data = (unsigned int*)dstImage.bits();
int pixelCount = dstImage.width() * dstImage.height();
// Convert each pixel to grayscale
for(int i = 0; i < pixelCount; ++i) {
int val = qGray(*data);
*data = qRgba(val, val, val, qAlpha(*data));
++data;
}
return dstImage;
}