[GUI development] browsing function implementation model of image processing software

Image processing software includes but is not limited to ：
Picture browser ,2D Map browser 、 Picture editor and other software .

To process large resolution images , It is generally used GDAL Load image ,GDAL Part of the image can be dynamically loaded , You can build image pyramids , Optimize loading speed .

General image processing software needs to enlarge the loaded image 、 narrow 、 Pan operation . First of all, clear ：

• Zoom in refers to an increase in image detail , A way of browsing with reduced visibility . English name ：Zoom In.
• Zoom out is the reduction in image detail , A browsing method that increases the visible range . English name ：Zoom Out.

Realization

Here's how to implement .
Before writing code , The mathematical model for image browsing needs to be established . As shown in the figure below ：

among , The display window refers to the window used to draw the current browsing content , The observation area refers to , The content in the current display window corresponds to the range of the content in the original drawing . Between the two must meet ： The aspect ratio of the display window is the same as that of the viewing area , In pixels .

Using the above model , You can write the corresponding code ：

• Image loading display ： Use GDAL In the original drawing, the rectangle of the observation area is taken as the range parameter , Read the corresponding pixel data in the original image , The output size is the size of the display window ;
• Zoom in ： The display window size does not change , Reduce the viewing area . The center point of the reduced observation area can be set according to the requirements ;
• Zoom out ： The display window size does not change , Zoom in on the viewing area . The enlarged center point of the observation area can be set as required ;
• Pan Browse ： The display window size does not change , The size of the observation area remains unchanged , However, the position of the observation area is modified according to the offset dragged by the mouse .

It is also convenient to calculate the so-called zoom ratio , namely

Zoom ratio = The width of the display window / The width of the observation area = The length of the display window / The length of the observation area
A unit of length ： Pixels

Zoom In、Zoom Out The explanation of

We know Z Axis Is perpendicular to the computer screen , Point to the person in front of the screen , The direction pointing to the inside of the screen is inside .
therefore ,Zoom In It means that people approach the screen to see the content on the screen , It is obviously an increase in detail 、 Zoom in operation with reduced range .
Zoom Out It means that people stay away from the screen to see the content on the screen , This is a reduction in detail . Zoom out of range increase .

Conclusion

In addition to the implementation scheme introduced in the article , There are other options . For example, the viewing area is always set to the same size as the image , Fixed , By changing the display window , To be correct, you should change the size of the drawing cache to zoom in and out . But it can be inferred that , For loading large drawings , If you zoom out in this case , Or occupy too much memory , Or the memory cannot be applied , The speed will also slow down .

Use the scheme introduced in the article , How to realize the requirement of image rotation ？ This is left to the readers to think for themselves .

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