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Q&A Iterating over pixels in QImage (Qt): which method adapts better for any image size?

I asked this in SE years ago. I'm aware of two methods in order to access all the pixels in a QImage called img. Method 1 for (int y = 0; y < img.height(); y++) { QRgb *line = (QRgb *)...

1 answer · posted 4d ago by aura-lsprog-86‭ · last activity 5h ago by InfiniteDissent‭

Question c++ qt qimage pixel-manipulation

#2: Post edited by

aura-lsprog-86‭ · 2025-02-17T07:55:04Z (4 days ago)
Minor grammar correction.

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_I asked this in [SE](https://stackoverflow.com/a/38340293/5397930) years ago._
<hr />
~~I'm aware of two methods in order to access to all the pixels in a `QImage` called `img`.~~
## Method 1
for (int y = 0; y < img.height(); y++) {
QRgb *line = (QRgb *) img.scanline(y);
for (int x = 0; x < img.width(); x++) {
// line[x] has an individual pixel
line[x] = QColor(255, 128, 0).rgb();
}
}
## Method 2
QRgb *st = (QRgb *) img.bits();
quint64 pixelCount = img.width() * img.height();
for (quint64 p = 0; p < pixelCount; p++) {
// st[p] has an individual pixel
st[p] = QColor(255, 128, 0).rgb();
}
I'm keen on using the second method as it only involves one loop, but am also concerned about any possible overflows on `pixelCount` if processing a "big enough" image.
Given this situation, I would like to ask:
_What is the most "scalable" way to iterate over all pixels stored in a QImage?_ By _scalable_ I mean that it will still work no matter what the image dimensions (width and height) are.
**NOTE:** I'm already aware there are "physical" limits in terms of memory usage. I just want to know whether both methods are capable of reaching such a limit.

_I asked this in [SE](https://stackoverflow.com/a/38340293/5397930) years ago._
<hr />
I'm aware of two methods in order to access all the pixels in a `QImage` called `img`.
## Method 1
for (int y = 0; y < img.height(); y++) {
QRgb *line = (QRgb *) img.scanline(y);
for (int x = 0; x < img.width(); x++) {
// line[x] has an individual pixel
line[x] = QColor(255, 128, 0).rgb();
}
}
## Method 2
QRgb *st = (QRgb *) img.bits();
quint64 pixelCount = img.width() * img.height();
for (quint64 p = 0; p < pixelCount; p++) {
// st[p] has an individual pixel
st[p] = QColor(255, 128, 0).rgb();
}
I'm keen on using the second method as it only involves one loop, but am also concerned about any possible overflows on `pixelCount` if processing a "big enough" image.
Given this situation, I would like to ask:
_What is the most "scalable" way to iterate over all pixels stored in a QImage?_ By _scalable_ I mean that it will still work no matter what the image dimensions (width and height) are.
**NOTE:** I'm already aware there are "physical" limits in terms of memory usage. I just want to know whether both methods are capable of reaching such a limit.

#1: Initial revision by

aura-lsprog-86‭ · 2025-02-17T07:54:28Z (4 days ago)

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Iterating over pixels in QImage (Qt): which method adapts better for any image size?

_I asked this in [SE](https://stackoverflow.com/a/38340293/5397930) years ago._

<hr />

I'm aware of two methods in order to access to all the pixels in a `QImage` called `img`.

## Method 1

    for (int y = 0; y < img.height(); y++) {
        QRgb *line = (QRgb *) img.scanline(y);
        for (int x = 0; x < img.width(); x++) {
            // line[x] has an individual pixel
            line[x] = QColor(255, 128, 0).rgb();
        }
    }

## Method 2

    QRgb *st = (QRgb *) img.bits();
    quint64 pixelCount = img.width() * img.height();

    for (quint64 p = 0; p < pixelCount; p++) {
        // st[p] has an individual pixel
        st[p] = QColor(255, 128, 0).rgb();
    }

I'm keen on using the second method as it only involves one loop, but am also concerned about any possible overflows on `pixelCount` if processing a "big enough" image.

Given this situation, I would like to ask:

_What is the most "scalable" way to iterate over all pixels stored in a QImage?_ By _scalable_ I mean that it will still work no matter what the image dimensions (width and height) are.

**NOTE:** I'm already aware there are "physical" limits in terms of memory usage. I just want to know whether both methods are capable of reaching such a limit.

c++ qt qimage pixel-manipulation

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