Clearing a rectangle in a Graphics

[yfede]

I have a Graphics created on an Image, like this

Graphics g (image);

how do I clear a rectangle inside the Graphics, so that all the pixels in the area are replaced with Colours::transparentBlack, for example.

I have tried this:

g.setColour (Colours::transparentBlack);
g.fillRect (myArea);

but that does nothing (which has some kind of sense, as I am asking the Graphics to paint that rectangle with a dry brush, so to say, thus no pixels in that area change).

I have also tried this:

image.clear (myArea, Colours::transparentBlack);

which seemed more appropriate to actually replace the content of the area, but that’s not good either: turns out the implementation of that method does exactly the same steps above with fillRect.

I think that correcting the behaviour of Image::clear() now could break some existing code (despite it’s actually not-so-formally-correct), so what about adding a Graphics::clear() method that does what expected? Or maybe name it fillReplacing if you feel it’s more appropriate

[jules]

Graphics contexts can only paint over existing content, but Image::clear will certainly be able to clear a chunk of an image in the way you’re asking for.

[yfede]

I must be missing something then.

I expect the following (test) code to draw a yellow border around the the component, then

1. creates a 100x100 image
2. fills it with transparent red
3. clears only a portion of it, according to my calcs it should be the rectangle x=30, y=30, w=50, h=50
4. paints the image in (10, 10)

virtual void paint (Graphics& g) override
{
    g.setColour (Colours::yellow.withAlpha ((uint8)128));

    const int width = getWidth ();
    const int height = getHeight ();

    g.drawRect (0, 0, width, height);

    Image image (Image::ARGB, 100, 100, true);
    image.clear (image.getBounds (), Colours::red.withAlpha (uint8 (64)));
    image.clear ((image.getBounds () / 2).translated (30, 30));

    g.setOpacity (1.0f);
    g.drawImageAt (image, 10, 10);
}

This is what I get

As you can see, the red square has no “hole” inside it. What am I doing wrong?

[jules]

Hmm. Try “image.getBounds () / 2.0f” instead of 2

Looks like my new Rectangle operators don’t handle integer division very well - thanks, I’ll sort that out!

[yfede]

Oh… yes, I see it now, it’s that nasty “/ 2” that makes it 1 / 2 = 0.

Hmm one more thought about this, though: if integers are valid arguments for the * and / operators, what about changing their template typename to a generic Type rather than FloatType?

[jules]

It’s ok, I’ve sorted it out now so it should handle integers too.

[yfede]

Yeah, what I meant is: since the scale factor can also be an integer after these changes, what’s the point of calling its template type “FloatType”? It’s kinda misleading now since I now can divide a Rectangle by an integer constant and have its coordinates all divived as expected (according to integer division rules)

[jules]

@yfede

Yeah, what I meant is: since the scale factor can also be an integer after these changes, what’s the point of calling its template type “FloatType”? It’s kinda misleading now since I now can divide a Rectangle by an integer constant and have its coordinates all divived as expected (according to integer division rules)

True enough!