QImage Class

QImage provides several ways of loading an image file: The file can be loaded when constructing the QImage object, or by using the load() or loadFromData() functions later on. QImage also provides the static fromData() function, constructing a QImage from the given data. When loading an image, the file name can either refer to an actual file on disk or to one of the application's embedded resources. See The Qt Resource System overview for details on how to embed images and other resource files in the application's executable.

Simply call the save() function to save a QImage object.

The complete list of supported file formats are available through the QImageReader::supportedImageFormats() and QImageWriter::supportedImageFormats() functions. New file formats can be added as plugins. By default, Qt supports the following formats:

QImage provides a collection of functions that can be used to obtain a variety of information about the image:

The functions used to manipulate an image's pixels depend on the image format. The reason is that monochrome and 8-bit images are index-based and use a color lookup table, while 32-bit images store ARGB values directly. For more information on image formats, see the Image Formats section.

In case of a 32-bit image, the setPixel() function can be used to alter the color of the pixel at the given coordinates to any other color specified as an ARGB quadruplet. To make a suitable QRgb value, use the qRgb() (adding a default alpha component to the given RGB values, i.e. creating an opaque color) or qRgba() function. For example:

QImage image(3, 3, QImage::Format_RGB32);
QRgb value;

value = qRgb(189, 149, 39); // 0xffbd9527
image.setPixel(1, 1, value);

value = qRgb(122, 163, 39); // 0xff7aa327
image.setPixel(0, 1, value);
image.setPixel(1, 0, value);

value = qRgb(237, 187, 51); // 0xffedba31
image.setPixel(2, 1, value);

In case of a 8-bit and monchrome images, the pixel value is only an index from the image's color table. So the setPixel() function can only be used to alter the color of the pixel at the given coordinates to a predefined color from the image's color table, i.e. it can only change the pixel's index value. To alter or add a color to an image's color table, use the setColor() function.

An entry in the color table is an ARGB quadruplet encoded as an QRgb value. Use the qRgb() and qRgba() functions to make a suitable QRgb value for use with the setColor() function. For example:

QImage image(3, 3, QImage::Format_Indexed8);
QRgb value;

value = qRgb(122, 163, 39); // 0xff7aa327
image.setColor(0, value);

value = qRgb(237, 187, 51); // 0xffedba31
image.setColor(1, value);

value = qRgb(189, 149, 39); // 0xffbd9527
image.setColor(2, value);

image.setPixel(0, 1, 0);
image.setPixel(1, 0, 0);
image.setPixel(1, 1, 2);
image.setPixel(2, 1, 1);

For images with more than 8-bit per color-channel. The methods setPixelColor() and pixelColor() can be used to set and get with QColor values.

QImage also provide the scanLine() function which returns a pointer to the pixel data at the scanline with the given index, and the bits() function which returns a pointer to the first pixel data (this is equivalent to scanLine(0)).

Each pixel stored in a QImage is represented by an integer. The size of the integer varies depending on the format. QImage supports several image formats described by the Format enum.

Monochrome images are stored using 1-bit indexes into a color table with at most two colors. There are two different types of monochrome images: big endian (MSB first) or little endian (LSB first) bit order.

8-bit images are stored using 8-bit indexes into a color table, i.e. they have a single byte per pixel. The color table is a QList<QRgb>, and the QRgb typedef is equivalent to an unsigned int containing an ARGB quadruplet on the format 0xAARRGGBB.

32-bit images have no color table; instead, each pixel contains an QRgb value. There are three different types of 32-bit images storing RGB (i.e. 0xffRRGGBB), ARGB and premultiplied ARGB values respectively. In the premultiplied format the red, green, and blue channels are multiplied by the alpha component divided by 255.

An image's format can be retrieved using the format() function. Use the convertToFormat() functions to convert an image into another format. The allGray() and isGrayscale() functions tell whether a color image can safely be converted to a grayscale image.

QImage supports a number of functions for creating a new image that is a transformed version of the original: The createAlphaMask() function builds and returns a 1-bpp mask from the alpha buffer in this image, and the createHeuristicMask() function creates and returns a 1-bpp heuristic mask for this image. The latter function works by selecting a color from one of the corners, then chipping away pixels of that color starting at all the edges.

The mirrored() function returns a mirror of the image in the desired direction, the scaled() returns a copy of the image scaled to a rectangle of the desired measures, and the rgbSwapped() function constructs a BGR image from a RGB image.

The scaledToWidth() and scaledToHeight() functions return scaled copies of the image.

The transformed() function returns a copy of the image that is transformed with the given transformation matrix and transformation mode: Internally, the transformation matrix is adjusted to compensate for unwanted translation, i.e. transformed() returns the smallest image containing all transformed points of the original image. The static trueMatrix() function returns the actual matrix used for transforming the image.

There are also functions for changing attributes of an image in-place:

See also QImageReader, QImageWriter, QPixmap, QSvgRenderer, Image Composition Example, Scribble Example

The following image formats are available in Qt. See the notes after the table.

See Also▲

This enum type is used to describe how pixel values should be inverted in the invertPixels() function.

See Also▲

QImage QImage::convertToFormat(QImage::Format format, Qt::ImageConversionFlags flags = Qt::AutoColor) const &

Returns a copy of the image in the given format.

The specified image conversion flags control how the image data is handled during the conversion process.

See Also▲

[since 6.0] QImage QImage::convertedTo(QImage::Format format, Qt::ImageConversionFlags flags = Qt::AutoColor) const &

Returns a copy of the image in the given format.

The specified image conversion flags control how the image data is handled during the conversion process.

This function was introduced in Qt 6.0.

See Also▲

QImage QImage::mirrored(bool horizontal = false, bool vertical = true) const &

Returns a mirror of the image, mirrored in the horizontal and/or the vertical direction depending on whether horizontal and vertical are set to true or false.

Note that the original image is not changed.

See Also▲

QImage QImage::rgbSwapped() const &

Returns a QImage in which the values of the red and blue components of all pixels have been swapped, effectively converting an RGB image to an BGR image.

The original QImage is not changed.

See Also▲

Constructs an image from the given xpm image.

Make sure that the image is a valid XPM image. Errors are silently ignored.

Note that it's possible to squeeze the XPM variable a little bit by using an unusual declaration:

static const char * const start_xpm[] = {
    "16 15 8 1",
    "a c #cec6bd",
    // etc.
};

The extra const makes the entire definition read-only, which is slightly more efficient (e.g., when the code is in a shared library) and able to be stored in ROM with the application.

Constructs a shallow copy of the given image.

For more information about shallow copies, see the Implicit Data Sharing documentation.

See Also▲

Move-constructs a QImage instance, making it point at the same object that other was pointing to.

Destroys the image and cleans up.

Returns the size of the color table for the image.

Notice that colorCount() returns 0 for 32-bpp images because these images do not use color tables, but instead encode pixel values as ARGB quadruplets.

See Also▲

Returns a sub-area of the image as a new image.

The returned image is copied from the position (rectangle.x(), rectangle.y()) in this image, and will always have the size of the given rectangle.

In areas beyond this image, pixels are set to 0. For 32-bit RGB images, this means black; for 32-bit ARGB images, this means transparent black; for 8-bit images, this means the color with index 0 in the color table which can be anything; for 1-bit images, this means Qt::color0.

If the given rectangle is a null rectangle the entire image is copied.

See Also▲

This is an overloaded function.

The returned image is copied from the position (x, y) in this image, and will always have the given width and height. In areas beyond this image, pixels are set to 0.

Fills the entire image with the given pixelValue.

If the depth of this image is 1, only the lowest bit is used. If you say fill(0), fill(2), etc., the image is filled with 0s. If you say fill(1), fill(3), etc., the image is filled with 1s. If the depth is 8, the lowest 8 bits are used and if the depth is 16 the lowest 16 bits are used.

If the image depth is higher than 32bit the result is undefined.

See Also▲

This is an overloaded function.

Fills the entire image with the given color.

If the depth of the image is 1, the image will be filled with 1 if color equals Qt::color1; it will otherwise be filled with 0.

If the depth of the image is 8, the image will be filled with the index corresponding the color in the color table if present; it will otherwise be filled with 0.

This is an overloaded function.

Fills the image with the given color, described as a standard global color.

This is an overloaded function.

Constructs a QImage from the first size bytes of the given binary data.

This is an overloaded function.

Constructs a QImage from the given QByteArray data.

Returns a QImage that is equivalent to the given hbitmap.

HBITMAP does not store information about the alpha channel.

In the standard case, the alpha channel is ignored and a fully opaque image is created (typically of format QImage::Format_RGB32).

There are cases where the alpha channel is used, though, for example for application icon or systray icons. In that case, reinterpretAsFormat(QImage::Format_ARGB32) should be called on the returned image to ensure the format is correct.

This function was introduced in Qt 6.0.

See Also▲

Returns a QImage that is equivalent to the given icon.

This function was introduced in Qt 6.0.

See Also▲

Returns the height of the image.

See Also▲

Returns true if it is a null image, otherwise returns false.

A null image has all parameters set to zero and no allocated data.

This is an overloaded function.

Loads an image from the first len bytes of the given binary data.

This is an overloaded function.

Loads an image from the given QByteArray data.

Mirrors of the image in the horizontal and/or the vertical direction depending on whether horizontal and vertical are set to true or false.

This function was introduced in Qt 6.0.

See Also▲

Returns the number of pixels by which the image is intended to be offset by when positioning relative to other images.

See Also▲

Returns the color of the pixel at the given position.

If the position is not valid, the results are undefined.

See Also▲

Returns the color of the pixel at the given position as a QColor.

If the position is not valid, an invalid QColor is returned.

See Also▲

Returns the QImage::Format as a QPixelFormat

Returns the pixel index at the given position.

If position is not valid, or if the image is not a paletted image (depth() > 8), the results are undefined.

See Also▲

Returns the enclosing rectangle (0, 0, width(), height()) of the image.

See Also▲

Swaps the values of the red and blue components of all pixels, effectively converting an RGB image to an BGR image.

This function was introduced in Qt 6.0.

See Also▲

Returns a copy of the image scaled to a rectangle defined by the given size according to the given aspectRatioMode and transformMode.

• If aspectRatioMode is Qt::IgnoreAspectRatio, the image is scaled to size.

• If aspectRatioMode is Qt::KeepAspectRatio, the image is scaled to a rectangle as large as possible inside size, preserving the aspect ratio.

• If aspectRatioMode is Qt::KeepAspectRatioByExpanding, the image is scaled to a rectangle as small as possible outside size, preserving the aspect ratio.

If the given size is empty, this function returns a null image.

See Also▲

This is an overloaded function.

Returns a copy of the image scaled to a rectangle with the given width and height according to the given aspectRatioMode and transformMode.

If either the width or the height is zero or negative, this function returns a null image.

Returns a scaled copy of the image. The returned image is scaled to the given height using the specified transformation mode.

This function automatically calculates the width of the image so that the ratio of the image is preserved.

If the given height is 0 or negative, a null image is returned.

See Also▲

Returns a scaled copy of the image. The returned image is scaled to the given width using the specified transformation mode.

This function automatically calculates the height of the image so that its aspect ratio is preserved.

If the given width is 0 or negative, a null image is returned.

See Also▲

Sets the color at the given index in the color table, to the given to colorValue. The color value is an ARGB quadruplet.

If index is outside the current size of the color table, it is expanded with setColorCount().

See Also▲

Sets the number of pixels by which the image is intended to be offset by when positioning relative to other images, to offset.

See Also▲

Sets the pixel index or color at the given position to index_or_rgb.

If the image's format is either monochrome or paletted, the given index_or_rgb value must be an index in the image's color table, otherwise the parameter must be a QRgb value.

If position is not a valid coordinate pair in the image, or if index_or_rgb >= colorCount() in the case of monochrome and paletted images, the result is undefined.

See Also▲

Sets the color at the given position to color.

If position is not a valid coordinate pair in the image, or the image's format is either monochrome or paletted, the result is undefined.

See Also▲

Sets the image text to the given text and associate it with the given key.

If you just want to store a single text block (i.e., a "comment" or just a description), you can either pass an empty key, or use a generic key like "Description".

The image text is embedded into the image data when you call save() or QImageWriter::write().

Not all image formats support embedded text. You can find out if a specific image or format supports embedding text by using QImageWriter::supportsOption(). We give an example:

    QImageWriter writer;
    writer.setFormat("png");
    if (writer.supportsOption(QImageIOHandler::Description))
        qDebug() << "Png supports embedded text";

You can use QImageWriter::supportedImageFormats() to find out which image formats are available to you.

See Also▲

Returns the size of the image, i.e. its width() and height().

See Also▲

Swaps image other with this image. This operation is very fast and never fails.

Creates a CGImage equivalent to this QImage.

Returns a CGImageRef handle.

The returned CGImageRef partakes in the QImage implicit sharing, and holds a reference to the QImage data. CGImage is immutable and will never detach the QImage. Writing to the QImage will detach as usual.

This function is fast, and does not copy or convert image data.

The following image formats are supported, and will be mapped to a corresponding native image type:

Other formats are not supported; this function returns a null CGImageRef for those cases. Users of this function may then convert the QImage to a supported format first, for example Format_ARGB32_Premultiplied.

The CGImageRef color space is set to the sRGB color space.

Creates a HBITMAP equivalent of the QImage.

Returns the HBITMAP handle.

It is the caller's responsibility to free the HBITMAP data after use.

For usage with with standard GDI calls, such as BitBlt(), the image should have the format QImage::Format_RGB32.

When using the resulting HBITMAP for the AlphaBlend() GDI function, the image should have the format QImage::Format_ARGB32_Premultiplied (use convertToFormat()).

When using the resulting HBITMAP as application icon or a systray icon, the image should have the format QImage::Format_ARGB32.

This function was introduced in Qt 6.0.

See Also▲

Creates a HICON equivalent of the QPixmap, applying the mask mask.

If mask is not null, it needs to be of format QImage::Format_Mono. Returns the HICON handle.

It is the caller's responsibility to free the HICON data after use.

This function was introduced in Qt 6.0.

See Also▲

Converts format into a QImage::Format

Converts format into a QPixelFormat

Returns the actual matrix used for transforming an image with the given width, height and matrix.

When transforming an image using the transformed() function, the transformation matrix is internally adjusted to compensate for unwanted translation, i.e. transformed() returns the smallest image containing all transformed points of the original image. This function returns the modified matrix, which maps points correctly from the original image into the new image.

Unlike the other overload, this function creates transformation matrices that can be used to perform perspective transformations on images.

See Also▲

Returns true if pos is a valid coordinate pair within the image; otherwise returns false.

See Also▲

Returns the width of the image.

See Also▲

Returns the image as a QVariant.

Returns true if this image and the given image have different contents; otherwise returns false.

The comparison can be slow, unless there is some obvious differ