【opencv_python学习之二.五】如何查看opencv_python的函数说明

1、网上手册查询

A. 打开Welcome to opencv documentation

B. 在左侧输入要查询的函数, 如图1.1，假设我们要查询resize函数

图 1.1

C. 点击Search按钮后可以看到结果，如图1.2

图 1.2

D. 点开第一条函数说明，进去查找到resize函数，如图1.3

图 1.3

从图1.3中可以看到在python中的resize函数说明为

cv2.resize(src, dsize[, dst[, fx[, fy[, interpolation]]]]) → dst

• [, dst[, fx[, fy[, interpolation]]]])意味着这些参数是可选参数

• → dst意味着函数返回dst

• 参数说明Parameters可以看到参数的具体含义
  D.A: src – input image, src为输入图片
  D.B: dst, dst为输出图片
  D.C: dsize, dsize为输出图片大小，当它为0时，要计算dsize = Size(round(fx*src.cols), round(fy&src.rows))，由这句话可以得出当dsize=0时，要同时输入src、fx、fy
  D.D: fx, 水平缩放倍率，当它为0时则有fx = (double)dsize.width/src.clos
  D.E: fy, 垂直缩放倍率，当它为0时则有fy = (double)dsize.height/src.rows
  D.F: interpolation插值，主要有以下几个选项

INTER_NEAREST
INTER_LINEAR
INTER_AREA
INTER_CUBIC
INTER_LANCZOS4

• 根据D.C、D.D、D.E可知道定有dsize和fx、fy两种方式可以指定输出图片大小。

dsize方式

dsize=(180, 180)

fx、fy方式

fx=2, fy=2

如果dsize和fx、fy都存在，先计算dsize，如果dsize=None, 则输出图片大小为fx、fy，如果dsize！=None, 则为dsize。

• 阅读了上面的说明，我们知道resize可以像下面这样用

# 只指定src和dsize
res = cv2.resize(src=img, dsize=(2*width, 2*height))

# 只指定src、dsize和interpolation
res = cv2.resize(src=img, dsize=(2*width, 2*height), interpolation = cv2.INTER_LINEAR)

# 只指定src、dsize、fx、fy
res = cv2.resize(src=img, dsize=None, fx=0.5, fy = 0.5)

# 只指定src、dsize、fx、fy和interpolation
res = cv2.resize(src=img, dsize=None, fx=0.5, fy = 0.5, interpolation = cv2.INTER_LINEAR)

2、使用help命令查看

A. 在CMD中运行python进入python模式中

F:\QAQ\python\opencv>python
Python 3.5.2 (v3.5.2:4def2a2901a5, Jun 25 2016, 22:18:55) [MSC v.1900 64 bit (AM
D64)] on win32
Type "help", "copyright", "credits" or "license" for more information.

B. 导入cv2模块

>>> import cv2

C. 使用help查看cv2.resize函数

>>> help(cv2.resize)
Help on built-in function resize:

resize(...)
    resize(src, dsize[, dst[, fx[, fy[, interpolation]]]]) -> dst
    .   @brief Resizes an image.
    .
    .   The function resize resizes the image src down to or up to the specified
 size. Note that the
    .   initial dst type or size are not taken into account. Instead, the size a
nd type are derived from
    .   the `src`,`dsize`,`fx`, and `fy`. If you want to resize src so that it f
its the pre-created dst,
    .   you may call the function as follows:
    .   @code
    .   // explicitly specify dsize=dst.size(); fx and fy will be computed from
that.
    .   resize(src, dst, dst.size(), 0, 0, interpolation);
    .   @endcode
    .   If you want to decimate the image by factor of 2 in each direction, you
can call the function this
    .   way:
    .   @code
    .   // specify fx and fy and let the function compute the destination image
size.
    .   resize(src, dst, Size(), 0.5, 0.5, interpolation);
    .   @endcode
    .   To shrink an image, it will generally look best with cv::INTER_AREA inte
rpolation, whereas to
    .   enlarge an image, it will generally look best with cv::INTER_CUBIC (slow
) or cv::INTER_LINEAR
    .   (faster but still looks OK).
    .
    .   @param src input image.
    .   @param dst output image; it has the size dsize (when it is non-zero) or
the size computed from
    .   src.size(), fx, and fy; the type of dst is the same as of src.
    .   @param dsize output image size; if it equals zero, it is computed as:
    .   \f[\texttt{dsize = Size(round(fx*src.cols), round(fy*src.rows))}\f]
    .   Either dsize or both fx and fy must be non-zero.
    .   @param fx scale factor along the horizontal axis; when it equals 0, it i
s computed as
    .   \f[\texttt{(double)dsize.width/src.cols}\f]
    .   @param fy scale factor along the vertical axis; when it equals 0, it is
computed as
    .   \f[\texttt{(double)dsize.height/src.rows}\f]
    .   @param interpolation interpolation method, see cv::InterpolationFlags
    .
    .   @sa  warpAffine, warpPerspective, remap