How to use QImage repeatedly?

Question

I have a horizontal sprite of images. This one for example:

All the images have the same width and height. I have stored it in my resources file, and declared it in my application by a define :

#define SPRITE QImage(":/sprite.png")

So there are 4 images that I will need several times in my application. In order to do this, I implemented this function which retrieves the image at the position n in the sprite.

QImage getNthImageFromSprite(int n, QImage sprite)
{
    int size = sprite.height();
    return sprite.copy((n - 1) * size, 0, size, size);
}

Then, I declared a general enum to put names on the position:

enum eImage
{
    eImage_black = 1,
    eImage_red,
    eImage_orange,
    eImage_green
}

And finally, the function that I'm using anywhere in my application:

QImage getSpriteImage(eImage img)
{
    return getNthImageFromSprite(img, SPRITE);
}

This works well .

But I have the impression that this is not very good, since I call the constructor of QImage each time I want to get a specific image. Knowing that a sprite can contain +40 images and that I will need these images several times, should I cache an image the first time it get called, or the way I'm doing it is good?

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Note: I need QImage for various reasons, but a comparison with QPixmap would be appreciated.

Answer 1

Replace the define with an object:

static QImage SPRITE(":/sprite.png");

It will be initialized only once on startup.

You can even put it in the method:

QImage getSpriteImage(eImage img)
{
    static QImage SPRITE(":/sprite.png");
    return getNthImageFromSprite(img, SPRITE);
}

In this case, it is initialized on first usage.

Howerver, you sill create new objects for every call to getNthSpriteImage . You could use a local static cache to re-use already returned objects:

QImage getNthSpriteImage(int n, QImage img) {
    static QMap<int, QImage> cache;
    if (!cache.contains(n)) {
        int size = sprite.height();
        cache[n] = sprite.copy((n - 1) * size, 0, size, size);
    }
    return cache[n];
}

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As for the difference for QImage/QPixmap, this is the main difference:

QImage is designed and optimized for I/O, and for direct pixel access and manipulation, while QPixmap is designed and optimized for showing images on screen.

So I would recommend to use QPixmap if you don't want special image formats (RGBA, premultiplied RGBA, indexed, ...) or use direct pixel manipulation.