QT Learning Pathway (29): Drawing equipment

A drawing device is a subclass of an inherited Qpainterdevice. QT provides a total of four such classes, namely Qpixmap, Qbitmap, Qimage and Qpicture. Qpixmap specifically optimizes the display of the image on the screen, and Qbitmap is a subclass of Qpixmap, whose color depth is limited to 1, you can use the Qpixmap isqbitmap () function to determine if this qpixmap is a qbitmap. Qimage is specifically optimized for pixel-level access to images. Qpicture can record and reproduce the Qpainter commands. Below we will introduce these four kinds of drawing equipment in two parts.

Qpixmap inherits Qpaintdevice, so you can use Qpainter to draw graphs directly above. Qpixmap can also accept a string as a path to a file to display the file, such as you want to open in the program PNG, JPEG and other files, you can use the Qpixmap. Use the Qpainter Drawpixmap () function to draw this file to a Qlabel, Qpushbutton, or other device. Qpixmap is specially optimized for the screen, so it is closely related to the actual underlying display device. Notice that the display device here is not hardware, but the native drawing engine provided by the operating system. So, under the different operating system platform, the Qpixmap display may have the difference.

Qpixmap provides a static Grabwidget () and Grabwindow () function that draws its own image to the target. At the same time, when using Qpixmap, you can use the values directly and do not need to pass the pointer, because QPIXMAP provides "implicit data sharing." In this regard, we will describe in detail in a later chapter, as long as you know that passing qpixmap do not have to use pointers.

Qbitmap inherits from Qpixmap, so it has all the attributes of Qpixmap. The Qbitmap color depth is always 1. The concept of color depth is derived from computer graphics, which refers to the number of digits used to represent the color of the binary. We know that the data inside the computer is represented by the binary system. In order to represent a color, we also use binary. For example, we want to say 8 colors, need to use 3 bits, then we say color depth is 3. Therefore, the so-called color deep 1, that is, the use of 1 bits to represent color. 1 bits have only two states: 0 and 1, so there are two kinds of colors, black and white. So, Qbitmap is actually only black-and-white image data.

Because the Qbitmap color is deep and small, it takes up very little storage space, so it is suitable for cursor files and brushes.

Here we look at the different manifestations of the same image file under Qpixmap and Qbitmap:

void　PaintedWidget::paintEvent(QPaintEvent　*event)
{
QPainter　painter(this);
QPixmap　pixmap("Cat.png");
QBitmap　bitmap("Cat.png");
painter.drawPixmap(10,　10,　128,　128,　pixmap);
painter.drawPixmap(140,　10,　128,　128,　bitmap);
QPixmap　pixmap2("Cat2.png");
QBitmap　bitmap2("Cat2.png");
painter.drawPixmap(10,　140,　128,　128,　pixmap2);
painter.drawPixmap(140,　140,　128,　128,　bitmap2);
}

Let's take a look at the results of the operation: