Understand corners_ Align, two perspectives for viewing pixels

Pixel aligning corners The concept is generally in the image Resize In the process of interpolation, we will encounter . In fact, this corresponds to two different perspectives on pixels , Affect the final coordinate mapping process .

1. How to treat pixels

Let's say I have a 5x5 Image

1. Centers-aligned： Regard pixels as a square with area , The center point of the square represents this pixel . In this way, the width of the image is equal to the number of pixels .

2. Corners-aligned： Think of the pixel as an ideal point , The position of this point represents this pixel . In this way, the width of the image is equal to the number of pixels minus one

Different perspectives on pixels

2. Resize

When doing Resize when , The scaling relationship between the two is different

1. Centers-aligned：

2. Corners-aligned：

​3X3 resize To 5X5

3. interpolation

1. Centers-aligned： Falling / During L sampling , The mapped coordinates will exceed the image boundary , The interpolation rules of these points will be different . As shown in the above figure, during the sampling process , The first pixel of the target image ( Green dot ) Exceed the first pixel of the original image ( Red dot ) Beyond the borders .

2. Corners-aligned： Because the pixel positions of the four corners are corresponding , There will be no situation beyond the boundary .

summary ：

	centers-gligned	corners-aligned
Pixels	Square with area	Ideal point
The width of the image	Equal number of pixels	Equal to the number of pixels minus one
Specify the scale	The number of pixels remains proportional	The number of pixels does not maintain proportion
Boundary interpolation problem	The boundary may be beyond the image range , Interpolation may have problems	The four points are perfectly aligned , There is no interpolation problem