Draw with alpha atop of QImage from blob

SeeLook

Halo everyone,

I need to draw a few things atop of QImage created from unsigned char* array.
But drawing doesn't respect alpha channel and anti-aliasing.

void processFrame(unsigned char* frameData, int w, int h, int lineSize) {
    QImage frameImage(frameData, w, h, lineSize, QImage::Format_ARGB32);
    QPainter p(&frameImage);
    p.setRenderHint(QPainter::Antialiasing);

    QColor c(255, 255, 255, 150); // white and translucent
    p.setPen(c);
    p.setBrush(c);

    p.drawRoundedRect(rectX, rectY, rectW, rectH, rectW / 35.0, rectH / 35.0);
    p.end();
}

I get only white, opaque rectangle.
And for the record - this function is called from C code through C++ wrapper - so there is no QGuitApplication - if it matters.
Did I miss something?

Pl45m4

@SeeLook said in Draw with alpha atop of QImage from blob:

Did I miss something?

Yes :)

QImage::Format_ARGB32

Have a look here....

QColor c(255, 255, 255, 150); // white and translucent

... and then look back here again :)

Are you sure the format is correct?

See here:

https://forum.qt.io/topic/94166/qimage-argb-and-rgba-alpha-channel-is-never-first-in-raw-bits

SeeLook

@Pl45m4 thanks for the answer.
I wish it would be so easy...
First of all, whatever blob data format is, QColor is QColor and its constructor takes arguments in that exact order (R, G, B, A).
And I can get blob data in that format (RGBA8888), I've tried already - the same result.

When I tried with color (127, 127, 127, 127) - the same, no transparency at all.

Also there is no anti-aliasing - drawing doesn't react with underlying pixels.
And I tested this QImage instance - pixel colors are as they should be - it looks like QPianter has problem with that painting device.

mchinand

Have you tried different Composition Modes?

JoeCFD

QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing);
painter.drawImage(rectX, rectY, rectW, rectH,frameImage );
your color setting is for background.

SeeLook

Have you tried different Composition Modes?

Yes.
And it works when it comes to cover source (rectangle) but not for blending with blob image pixels.

SeeLook

QPainter painter(this);

Maybe here is the thing... There is no any this.
this is used when QPainter is used in GUI app - it is QWidget then or QGraphicsSceneItem or so.

Here is only a library which uses bare QImage which is QPiantDevice for QPainter.

JoeCFD

@SeeLook Yes, "this" is the canvas for painting. I do not know where you paint it.

SeeLook

@JoeCFD - over pixels of QImage from blob/raw data.
And those raw pixels are changed - as example image shows but QPainter doesn't react with original ones - just overwrites them.

JoeCFD

@SeeLook
add this to see what happens?
painter.drawImage(rectX, rectY, rectW, rectH,frameImage );

It seems only background is painted.

SeeLook

@JoeCFD
In example image I attached before - background is pink and rectangle is as it is.
And I also tried to draw that entire image again into painter:

p.drawImage(0, 0, frameImage);

then draw that rectangle atop of all - still the same difference - no alpha.