QT QImage, how to extract RGB?

Question

I want to extract RGB from each pixel in QImage. Ideally, I want to use the img.bits() function.

QImage img;

if( img.load("Red.jpg") )
{
    uchar *bits = img.bits();

    for (int i = 0; i < 12; i++)
    {
        std::cout << (int) bits[i] << std::endl;
    }
}

How to manipulate the returned bits? I expected all red because the picture is a pure red image created in Paint. However, I get 36, 27, 237, 255, 36 etc...

Reading with `bits()` depends heavily on the image format. Try to convert the format to something more suitable for your scenario, e.g. `img = img.convert(QImage::Format_ARGB32)`. Then the bytes should come out as `0, 255, 0, 0, [loop]` if I'm not mistaken. — Stefan Majewsky, Sep 12 '12 at 09:13

Pie_Jesu · Accepted Answer · 2012-09-12T07:36:15.693

QImage img( "Red.jpg" );

if ( false == img.isNull() )
{
    QVector<QRgb> v = img.colorTable(); // returns a list of colors contained in the image's color table.

    for ( QVector<QRgb>::const_iterator it = v.begin(), itE = v.end(); it != itE; ++it )
    {
        QColor clrCurrent( *it );
        std::cout << "Red: " << clrCurrent.red()
                  << " Green: " << clrCurrent.green()
                  << " Blue: " << clrCurrent.blue()
                  << " Alpha: " << clrCurrent.alpha()
                  << std::endl;
    }
}

However this example above does returns the color table. Color table does not includes same colors twice. They will be added once in order of appearance.
If you want to get each pixels color, you can use next lines:

for ( int row = 1; row < img.height() + 1; ++row )
    for ( int col = 1; col < img.width() + 1; ++col )
    {
        QColor clrCurrent( img.pixel( row, col ) );

        std::cout << "Pixel at [" << row << "," << col << "] contains color ("
                  << clrCurrent.red() << ", "
                  << clrCurrent.green() << ", "
                  << clrCurrent.blue() << ", "
                  << clrCurrent.alpha() << ")."
                  << std::endl;
    }

Cause I was remembering that image or pixmap starts indexing from 1, not 0. Probably mistaking. Did you tried it? Starts from 0 or what? — Pie_Jesu, Sep 13 '12 at 05:36
Tried it on QT5. `row` and `col` should be in range from 0 to width-1/height-1. And also you mixed up `width()` and `height()` in for loops. — Astronavigator, Aug 26 '16 at 00:33
Qt documentation says that `QImage::pixel()` / `setPixel()` are slow. It recommends you to use `bits()` or `scanLine()` (and their respective const versions) if you plan to process a great deal of pixels. — Paulo Carvalho, May 21 '20 at 11:59

score 6 · Answer 2 · edited Sep 12 '12 at 09:07

Reference for bits() says:

Returns a pointer to the first pixel data. This is equivalent to scanLine(0).

So if you check reference for scanLine()

If you are accessing 32-bpp image data, cast the returned pointer to QRgb* (QRgb has a 32-bit size) and use it to read/write the pixel value. You cannot use the uchar* pointer directly, because the pixel format depends on the byte order on the underlying platform. Use qRed(), qGreen(), qBlue(), and qAlpha() to access the pixels.

One other option would probably be pixel() member function.

Hope that helps.

score 4 · Answer 3 · answered Sep 12 '12 at 20:23

4

One of the problems with using the bits() function is that you need to know the format of the original image. You should convert it to RGB by using convertToFormat.

img = img.convertToFormat(QImage::Format_RGB888);

Now, when you call bits(), the data will be in the RGB format with the proper data alignment.

uchar *bits = img.bits();

for (int i = 0; i < (img.width() * img.height() * 3); i++)
{
    std::cout << (int) bits[i] << std::endl;
}

answered Sep 12 '12 at 20:23

Keith

225
1
5

1

fyi: you can cast img.bits() to a pointer of type `QRgb`. So you doesn't need to know the type of the image. Just iterate over each pixel and use qRed(), qGreen(), qBlue...and so on – 501 - not implemented Jun 01 '15 at 17:55
@501-notimplemented how do you cast it? I'm trrying `const QRgb *rawPixelData = static_cast(pixelSource.constBits());` but apparently it's an invalid static cast... – Troyseph May 26 '16 at 12:16
hi @Troyseph, you can use `QRgb *rawPixelData= (QRgb*)(pixelSource.bits())` without static_cast. – 501 - not implemented May 27 '16 at 08:12
@501-notimplemented C style cast... gross =P doesn't that end up being a reinterpret_cast? – Troyseph May 27 '16 at 10:34
it must be reinterpret_cast. So use C-style cast! – Петър Петров Mar 21 '19 at 11:42

score 0 · Answer 4 · answered Jun 20 '20 at 08:07

0

In Qt 5.6 was introduced QColor QImage::pixelColor(int x, int y) method, so you can directly get color information from image pixel.

answered Jun 20 '20 at 08:07

Andrey Azazello Yaromenok

640
5
6

QT QImage, how to extract RGB?

4 Answers4