Raw data to QImage

Question

I'm new to graphics programming (pixels, images, etc..) I'm trying to convert Raw data to QImage and display it on QLabel. The problem is that, the raw data can be any data (it's not actually image raw data, it's binary file.) The reason if this is that, to understand deeply how pixels and things like that work, I know I'll get random image with weird results, but it will work. I'm doing something like this, but I think I'm doing it wrong!

QImage *img = new QImage(640, 480, QImage::Format_RGB16); //640,480 size picture.
//here I'm trying to fill newly created QImage with random pixels and display it.
for(int i = 0; i < 640; i++)
{
    for(int u = 0; u < 480; u++)
    {
        img->setPixel(i, u, rawData[i]);
    }
}
ui->label->setPixmap(QPixmap::fromImage(*img));

am I doing it correctly? By the way, can you point me where should I learn these things? Thank you!

Kealey

no need to create img dynamically if you are going to use QPixmap::fromImage as it allocate heap memory for the pixmap. And QImage use copy on write. – Farl

Answer 1

Overall it's correct. QImage is a class that allows to manipulate its own data directly, but you should use correct pixel format.

A bit more efficient example:

QImage* img = new QImage(640, 480, QImage::Format_RGB16);
for (int y = 0; y < img->height(); y++)
{
    memcpy(img->scanLine(y), rawData[y], img->bytesPerLine());
}

Where rawData is a two-dimension array.

Arboreous

Hi, when I tried your example, all the raw data shows same result. rawData is QByteArray and I'm using it like &rawData[y], I'm wrong right? – Kealey

No, &rawData[y] isn't correct, because you will copy the same pixels many times. You should use: rawData.constData() + y * img->bytesPerLine(). In this case on the each iteration new peace of data will be copied. – Arboreous

I faced another problem here. I don't know how long picture should be, I mean width and height and sometimes it crashes when I pass less than 2kb data.. What is the best way to calculate it? – Kealey

How about creating new question? Where from are you reading data? Why don't you know the size of picture? Could you describe in detail what you are doing? – Arboreous

Okay, will write new question :) – Kealey

Answer 2

This is how I saved a raw BGRA frame to the disk:

QImage image((const unsigned char*)pixels, width, height, QImage::Format_RGB32);
image.save("out.jpg");

Gyrostatics

Should work for RGB32 data, but one have to keep in mind that for smaller pixel types (for example Format_Grayscale8), lines have to be 32-bytes aligned. – Hershel

Answer 3

Syntactically, your code appears to be correct.

Reading the class signature, you may want to call setPixel in the following manner:

img->setPixel(i, u, QRbg(##FFRRGGBB));

Where ##FFRRGGBB is a color quadruplet, unless, of course, you want monochrome 8 bit support.

Additionally, declaring a naked pointer is dangerous. The following code is equivalent:

QImage image(640, 480, QImage::Format_something);
QPixmap::fromImage(image);

And will deallocate appropriately upon function completion.

Qt Examples directory is a great place to search for functionality. Also, peruse the class documentation because they're littered with examples.

Goodish