Method of Drawing with VC without blinking (dual buffering technology)

Drawing in VC is not flashing. You can use memory to draw a picture and copy the picture to the device. However, the result is that the image display speed is improved, but it is still very flashing. How to Avoid flashing the display image, how to Improve the display efficiency, and most people think that the MFC plot function is very inefficient, always

I want to seek other solutions. The drawing efficiency of MFC is indeed not high, but it is not bad, and its drawing function is very simple to use, as long as the use of appropriate methods, plus some skills, with MFC can get a very efficient drawing program.
1. Why does the displayed image flash?
Most of our drawing processes are stored in the ondraw or onpaint functions. ondraw calls onpaint for screen display. When the window needs to be re-painted for any reason, it always clears the display area with the background color before calling onpaint, while the background color

In this way, the background color and the display image appear alternately in a short period of time, making the display window appear flashing. If you set the background to null, the duplicate drawing will not flash.
Of course, this will make the display of the window messy, because during the re-painting, there is no background color to clear the original drawing, and a new image is superimposed.
Some people may say that the blinking is caused by the drawing speed being too slow or the display graphics being too complex. In fact, this is not true. The influence of the display speed of the drawing on the flashing is not fundamental. For example, in ondraw (CDC * PDC), write as follows:
PDC-> moveTo (0, 0 );
PDC-> lineto (100,100 );
This drawing process should be very simple and fast, but it will still flash when pulling the window changes. In fact, in principle, the drawing process is more complex, the slower the process, and the less the flickering process should be, because the time used for drawing is greater than the time used to clear the screen with the background.

People will feel less explicit about flashes. For example, if the screen time is 1 s, the drawing time is also 1 s, so that the continuous re-painting within 10 s will flash 5 times; if the screen time is 1 s, the drawing time is 9 s, so that the continuous re-painting within 10 s will only Flash once.

This can also be tested by writing in ondraw (CDC * PDC) as follows:
For (INT I = 0; I <100000; I ++)
{
PDC-> moveTo (0, I );
PDC-> lineto (1000, I );
}
The program is abnormal, but the problem can be explained. Some people may want to talk about it again. Why does a simple image look like a complex one? This is because the complex image occupies a large area and the contrast caused by the re-painting is relatively large, so it feels like a flash.

But the flicker frequency is low.
So why does the animation appear non-flashing when its re-painting frequency is high? Here, I want to emphasize again, what is flashing? Flashing means contrast. The larger the contrast, the more powerful the blinking. The animation does not seem to flash because the difference between two consecutive frames is very small. If you do not believe it, you can

It's strange to add a pure white frame in the middle of each frame of the animation.
2. How to Avoid blinking
After you know why the image flash, you can do the right thing. The first step is to remove the background rendering process provided by MFC. There are many implementation methods,
* When the window is formed, you can pay NULL for the registration background of the window.
* You can also modify the background after the formation.
Static cbrush brush (RGB (255, 0, 0 ));
Setclasslong (this-> m_hwnd, gcl_hbrbackground, (long) (hbrush) brush );
* To be simple, you can load onerasebkgnd (CDC * PDC) in the View class to directly return true.
The background is gone, and the result graphic display does not flash, but the display is also messy as mentioned above. What should I do? This requires the dual-Cache method. In addition to displaying images on the screen, dual buffering also displays images in the memory.

. We can plot the image to be displayed in the memory first, and then overwrite the image in the memory to the screen one by one point at a time (this process is very fast, because it is a very regular copy of memory ). In this way, you can use

The background color with a large contrast does not flash when it is cleared, because it cannot be seen. When it is attached to the screen, the final image in the memory is slightly different from the screen display image (if there is no motion, of course there is no difference), so that it will not flash.
3. How to implement dual Buffering
The implementation program is provided first, and then explained again, also in ondraw (CDC * PDC:
CDC memdc; // define a display device object
Cbitmap membitmap; // defines a bitmap object.
// Create a memory display device compatible with Screen Display
Memdc. createcompatibledc (null );
// No drawing yet, because there is no place to draw ^_^
// A bitmap compatible with screen display is created below. The size of the bitmap can be set to the size of the window.
Membitmap. createcompatiblebitmap (PDC, nwidth, nheight );
// Select the bitmap to the memory display device.
// Only the memory display device with the bitmap selected can draw a local image and draw it to the specified bitmap.
Cbitmap * poldbit = memdc. SelectObject (& membitmap );
// Use the background color to clear the bitmap. Here, I use white as the background color.
// You can also use your own color
Memdc. fillsolidrect (255,255,255, nwidth, nheight, RGB ));
// Drawing
Memdc. moveTo (......);
Memdc. lineto (......);
// Copy the image in the memory to the screen for display
PDC-> bitblt (0, 0, nwidth, nheight, & memdc, 0, srccopy );
// Cleanup after drawing is complete
Membitmap. deleteobject ();
Memdc. deletedc ();
The above comment should be very detailed, so I will not talk much about it.
4. How to Improve the drawing efficiency
I am mainly working on the CAD software for the Network Graphics of the power system. In a window, thousands of power components are usually displayed, and each component is composed of basic figures such as points, lines, and circles. If you really want to re-draw so many elements in a re-painting process, you can imagine this

The process is very long. If you have added the Image Browsing function, you need to re-paint a lot when you move the mouse to scroll the image. The speed will be too slow for users to endure. What should I do? We only need to study the Drawing Process of MFC.
In fact, not all charts drawn in ondraw (CDC * PDC) are displayed. For example, you have drawn two rectangles in ondraw, in one re-painting, although both the drawing functions of the two rectangles are executed, there may be only one display. This is because the MFC itself

The cropping area is set to improve the repainting efficiency. The purpose of the cropping area is to ensure that only the plotting process in this area is valid and that the plotting function is not displayed even if it is executed outside the area. In most cases, the re-painting of a window is large.

Most of the reason is that the part of the window is blocked or the window is rolling. The changed area is not the whole graph but only a small part. What needs to be changed is the cropping area in the PDC. Because the display (display to memory or video memory) is more time-consuming than the calculation of the drawing process.

More, with the cropping area, the display is only the part that should be displayed, greatly improving the display efficiency. However, this cropping area is set by MFC, which has improved the display efficiency. How can we further improve the efficiency when drawing complex images? You can only remove it.

The plotting process is now outside the cropping area. You can first use PDC-> getclipbox () to obtain the cropping area, and then determine whether your image is in this area during the drawing. If you are painting it, you will not draw it if you are not.
If your drawing process is not complex, this may not improve your drawing efficiency.
Moderator comments:
This article systematically explains how to prevent the image from flashing. In the final analysis, the final solution to prevent the flash is to re-paint the changed part. You must not erase all the elements with the background color before painting.