About class CRect & Rect, Width = right - left

It is a problem about C++ and mfc. For example, left = 3, right = 8. Doesn't it mean there are 6 pixel from left to right? Why the width = right - left? If I know a rect which represents the image rect, when I allocate memory for the image data, which one should I use? Width = right-left, or Width = right-left+1? I am a beginner of image process. It really confuses me. Thank you for your help!

2 Answers

If we are talking about CRect and RECT the documentation is clear.

By convention, the right and bottom edges of the rectangle are normally considered exclusive. In other words, the pixel whose coordinates are ( right, bottom ) lies immediately outside of the rectangle. For example, when RECT is passed to the FillRect function, the rectangle is filled up to, but not including, the right column and bottom row of pixels. This structure is identical to the RECTL structure.

The principles of "inclusive lower bound, exclusive upper bound" is used here to. So the number of elements is always the difference between the boundaries.

Another way to think about this is that the width of the rectangle is a measure of DISTANCE from left to right. When left equals right (e.g.: left = 1 and right = 1), the distance between them is zero (note that the distance can be negative).

When using a RECT to represent pixel coordinates, we often want to know the count of pixels going from left to right. When left equals right (e.g.: left = 1 and right = 1), we know we have only one pixel in the left/right direction. There isn't a pre-made function to compute this count, so you need take the absolute value of the width and add 1.

In C/C++:

 int count = abs(myRect.right - myRect.left) + 1;