Beyond setDotsPerMeterY(): Alternative Approaches for Image Scaling and Information in Qt

Purpose

This information is crucial when using the image size for calculations within the user interface (UI).
QImage::setDotsPerMeterY() sets the number of pixels in a QImage object that correspond to one meter vertically.

Understanding Dots Per Meter (DPM)

Higher DPM values represent higher resolutions, meaning more pixels are packed into a smaller physical area.
DPM specifies the resolution of an image, indicating how many pixels represent a physical unit (in this case, meters) of length.

How setDotsPerMeterY() Works

You provide an integer value representing the desired number of pixels per meter vertically.
The QImage object stores this value internally.

Impact on UI Calculations

When working with the image size in UI layout calculations, you should use image.size() / image.devicePixelRatio() instead of directly using image.size(). This ensures that the calculations account for the device pixel ratio (DPR), which can vary on different displays.

Example Usage

#include <QImage>

int main() {
    QImage image(200, 100, QImage::Format_RGB32); // Create a 200x100 image

    // Set dots per meter vertically (e.g., assuming 300 pixels per meter)
    image.setDotsPerMeterY(300);

    // Calculate the actual size the image would occupy on a physical screen
    // (assuming a device pixel ratio of 2)
    qreal actualWidth = image.width() / image.devicePixelRatio();
    qreal actualHeight = image.height() / image.devicePixelRatio();

    // Use actualWidth and actualHeight in your UI layout calculations
    ...

    return 0;
}

Consider using it when the image represents a real-world entity with a specific physical size.
It works in conjunction with dotsPerMeterX() to define the image's intended scale and aspect ratio.
It doesn't directly affect the image data itself.
setDotsPerMeterY() is primarily used for informational purposes.

Example 1: Scaling an Image Based on Dots Per Meter

This example shows how to scale an image to a specific physical size on the screen using setDotsPerMeterY() and the device pixel ratio (DPR):

#include <QImage>
#include <QPainter>

int main() {
    QImage image(200, 100, QImage::Format_RGB32); // Create a 200x100 image

    // Set dots per meter vertically (e.g., assuming 300 pixels per meter)
    image.setDotsPerMeterY(300);

    // Desired physical width on the screen (e.g., 0.5 meters)
    qreal desiredWidth = 0.5;

    // Calculate the scaled image size based on DPM and DPR
    qreal scaleFactor = image.devicePixelRatio();
    qreal scaledWidth = desiredWidth * image.dotsPerMeterX() / scaleFactor;
    qreal scaledHeight = image.height() * scaledWidth / image.width();

    // Create a new scaled image
    QImage scaledImage(scaledWidth * scaleFactor, scaledHeight * scaleFactor, QImage::Format_RGB32);
    QPainter painter(&scaledImage);
    painter.drawImage(0, 0, image);
    painter.end();

    // Use scaledImage in your UI layout

    return 0;
}

Example 2: Displaying Image Information with DPM

This example shows how to display some information about an image, including its dots per meter:

#include <QImage>
#include <QDebug>

int main() {
    QImage image("path/to/your/image.png"); // Load an image

    // Check if image loading was successful
    if (image.isNull()) {
        qWarning() << "Failed to load image";
        return 1;
    }

    // Get and display image information
    int width = image.width();
    int height = image.height();
    int dpmX = image.dotsPerMeterX();
    int dpmY = image.dotsPerMeterY();

    qDebug() << "Image size:" << width << "x" << height;
    qDebug() << "Dots per meter (X):" << dpmX;
    qDebug() << "Dots per meter (Y):" << dpmY;

    return 0;
}

Modifying Image Size with QImage::scaled()

If you want to control the image's displayed size on the screen without changing the underlying data, you can use QImage::scaled(). This method creates a new image with the desired dimensions, effectively changing the apparent scale of the image based on the device pixel ratio (DPR).

Example

QImage scaledImage = image.scaled(desiredWidth, desiredHeight, Qt::KeepAspectRatio);
// Use scaledImage in your UI layout

Using QPainter for Transformations

The QPainter class allows you to perform various transformations on images within a widget's paint event. You can use scaling, rotation, and other transformations to control how the image appears without modifying the original data.

Example

void MyWidget::paintEvent(QPaintEvent *event) {
    QPainter painter(this);
    painter.drawImage(0, 0, image); // Draw the image (optional scaling and transformation)
}

Manipulating DPI Information (Limited Use)

While setDotsPerMeterY() doesn't directly affect the image data, some image formats (like TIFF) might store DPI information. If you're working with a format that supports embedded DPI settings, you might be able to modify them using format-specific libraries. However, this approach is not recommended for general use as it depends heavily on the image format and might not have consistent behavior across different formats.

For DPI Embedding (Limited Use)
Explore format-specific libraries if absolutely necessary, but proceed with caution due to potential compatibility issues.
For Transformations
Leverage QPainter's capabilities for various transformations like scaling, rotation, and flipping.
For UI Scaling
Use QImage::scaled() or a QPainter with scaling to control the image's displayed size without modifying the original data.