Re: [Interest] Canvas keeping artifacts

[Jason H]

By the way, reset() is not a W3C function, and it's also not in the QML Canvas documentation.

 

I'm wondering how you found it?

 

 

 

Sent: Thursday, March 24, 2016 at 9:42 AM
From: "Jason H" <jh...@gmx.com>
To: "Jérôme Godbout" <jer...@bodycad.com>
Cc: "interest@qt-project.org Interest" <interest@qt-project.org>
Subject: Re: [Interest] Canvas keeping artifacts

ctx.reset(); fixes it.

 

Again, I think you have meaningless statements.

ctx.beginPath(); // It's not a path

ctx.fillStyle = 'rgba(0,0,0,0)'; // it's not using fillStyle

ctx.clearRect(0,0,width,height); // this works

ctx.fill(); // there is no closed path to fill

 

Sent: Wednesday, March 23, 2016 at 6:21 PM
From: "Jérôme Godbout" <jer...@bodycad.com>
To: "Jason H" <jh...@gmx.com>
Cc: "interest@qt-project.org Interest" <interest@qt-project.org>
Subject: Re: [Interest] Canvas keeping artifacts

Canvas

{

        id: component

        //...

        renderStrategy: Canvas.Immediate

        renderTarget: Canvas.FramebufferObject

 

        onWidthChanged: requestPaint()

        onHeightChanged: requestPaint()

        onPaint: paintHandler()

 

        function paintHandler()

{

var ctx = getContext('2d');

ctx.reset();

// clear with transparent

ctx.beginPath();

ctx.fillStyle = 'rgba(0,0,0,0)';

ctx.clearRect(0,0,width,height);

ctx.fill();

// Set general style

                ...

// Call draw functions here

...

}

}

 

At a few overhead near, this is what I do, the thing is clear, I guess my ctx.reset() is the missing key here, I had problems too with this clear thing with Qml Canvas where Html canvas where working perfectly fine with the same code.

 

My guess is that the Widgets try to redraw as little as possible somehow, not sure if a replacement to the requestPaint() or a function that will:

function myRequestPaint()

{

      markDirty(Qt.rect(0,0,width,height));

      requestPaint();

}

 

onWidthChanged: myRequestPaint()

onHeightChanged: myRequestPaint()

 

Hope this help you out, let me know if the makeDirty() change anything, I may change my code for it too.

 

Jerome

 

On Wed, Mar 23, 2016 at 5:26 PM, Jason H <jh...@gmx.com> wrote:

Thanks for the tip, but a few things:

1. I don't think clearRect uses fillStyle

2. I don't think clearRect needs a fill, as "clear" implies both color and fill op. http://www.w3schools.com/tags/tryit.asp?filename=tryhtml5_canvas_clearrect

 

 

I had played with that, including your code, no luck.

It looks like the Black Sabbath - Paraoid album cover. 

 

 

Sent: Wednesday, March 23, 2016 at 5:11 PM
From: "Jérôme Godbout" <jer...@bodycad.com>
To: "Jason H" <jh...@gmx.com>
Cc: "interest@qt-project.org Interest" <interest@qt-project.org>
Subject: Re: [Interest] Canvas keeping artifacts

You may want to clear fill with blank transparency:

 

ctx.fillStyle = 'rgba(0,0,0,0)';

ctx.clearRect(0,0,width,height);

ctx.fill();

 

for me it did the trick.

 

On Wed, Mar 23, 2016 at 4:54 PM, Jason H <jh...@gmx.com> wrote:

I have a very simple pause button:
        Canvas {
                id: pauseCanvas
                anchors.fill: parent
                onWidthChanged: requestPaint()
                onHeightChanged: requestPaint()
                onPaint: {
                        var ctx = getContext('2d');
                        ctx.save()
                        ctx.clearRect(0,0, width,height)
                        ctx.fillStyle = color;
                        ctx.rect(0.125*width, 0.05*height, 0.25*width, 0.90*height);
                        ctx.fill();
                        ctx.rect(0.625*width, 0.05*height, 0.25*width, 0.90*height);
                        ctx.fill();
                        ctx.restore();
                }
        }

The problem is when I resize the window (which in turn resizes the canvas), I get artifacts of previous draws. If the window is made larger (corner drag or maximize) I can see the smaller original rectangles. And vice-versa.

How can I get Canvas to play nice?


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