OpenCV-Python】滑動條Trackbar的創建和使用（createTrackbar())

一、前言

滑動條（Trackbar）It is a tool that can dynamically adjust parameters,它依附於窗口而存在.The interaction can be clearly felt,動態調參,針對尋找,rgb區間,hsv區間,etc. Algorithms where you can adjust parameters and see the effect,Scroll bars are a very useful tool

二、核心函數解析

There are two core functionscreateTrackbar(),getTrackbarPos()

1、createTrackbar() 參數解析

createTrackbar() This function is used to create a slider with adjustable values,並將滑動條附加到指定的窗口上.

該函數的參數有：
createTrackbar(const String& trackbarname, const String& winname, int *value, int count, TrackbarCallback onChange = 0, void *userdata = 0),具體含義為：

trackbarname：跟蹤欄名稱,The name of the created trackbar.
Winname：窗口的名字,表示這個軌跡條會依附到哪個窗口上,即對應namedWindow()The window name filled in when creating the window.
value：指向整數變量的可選指針,The value of this variable reflects the initial position of the slider.
count：表示滑塊可以達到的最大位置的值,The minimum position is always 0.
onChange：指向每次滑塊更改位置時要調用的函數的指針,有默認值0.此函數的原型應為void XXX (int, void *); ,The first parameter is the trackbar position,第二個參數是用戶數據（請參閱下一個參數）.如果回調是NULL指針,then no callback is called,and only update the value.
userdata：User data passed to the callback,有默認值0.It can be used to handle trackbar events,而無需使用全局變量.

注：如果使用的第三個參數valueArguments are global variables or constants,完全可以不去管這個userdata參數.
Only the first four are required

2、getTrackbarPos() 參數解析

對於 getTrackbarPos() 函數,It also takes multiple parameters：

第一個參數是Tarckbar 名稱;
第二個參數是它附加到的窗口名稱;
第三個參數是默認值;
第四個參數是最大值;
The fifth is a callback function that executes every time the track bar value changes.回調函數始終具有默認參數,即軌跡欄位置.

注：In general, it is only necessary to have the former2個即可

三、Simple practical code

在這裡,我們將創建一個簡單的應用程序,to display the assignment by sliding the track barR,G,G的顏色.默認情況下,初始顏色將設置為黑色,代碼如下：

import cv2 as cv
import numpy as np
cv.namedWindow('image',256)
#滾動條事件,默認為無,No need to move it to change anything
def task():
pass
def demo():
cv.createTrackbar('R','image',0,255,task)
cv.createTrackbar('G','image',0,255,task)
cv.createTrackbar('B','image',0,255,task)
img=np.zeros((640,480,3),np.uint8)
while True:
r=cv.getTrackbarPos('R','image')
g=cv.getTrackbarPos('G','image')
b=cv.getTrackbarPos('B','image')
img[:]=[r,g,b]
cv.imshow('image',img)
if cv.waitKey(1) &0xFF==27:
break
cv.destrayAllWindows()
if __name__=='__main__':
demo()