[200 opencv routines] 210 Are there so many holes in drawing a straight line?

OpenCV routine 200 piece General catalogue

【youcans Of OpenCV routine 200 piece 】210. There are so many holes in drawing a straight line ？

I'm not a headline party .

Drawing straight lines is of course the simplest drawing , What kind of pit can there be ？ Students who don't believe in it can directly jump to the precautions . Even if you are OpenCV A great god , If this 10 You can master all the questions , Then I really took it .

7.1 Basic parameters of drawing function

OpenCV It provides drawing function , You can draw a line on the image 、 rectangular 、 round 、 Ellipse and other geometric figures .

function cv.line()、cv.rectangle()、cv.circle()、cv.polylines() And so on are used to draw straight lines in the image 、 rectangular 、 circular 、 Polygon and other geometric shapes , There are some setting parameters in these drawing functions , Introduce the following ：

• img： Input / output image , Format is unlimited
• color： The color of the drawing line ,(b,g,r) Tuple of format , Or a scalar representing the gray value
• • thickness： Draw the thickness of the line , The default value is 1px,-1 Indicates internal padding
• lineType： Draw the linear of the line segment , The default is LINE_8
  • cv.FILLED： Inside filling （ Solid graphics ）
  • cv.LINE_4：4 Adjacency Linetype
  • cv.LINE_8：8 Adjacency Linetype
  • cv.LINE_AA： Anti aliasing , Smoother image
• shift： The number of decimal places of point coordinates , The default is 0

7.2 Draw a straight line

The function prototype

function cv.line() Used to draw lines in an image , function cv.arrowedLine() Used to draw straight lines with arrows in the image .

cv.line(img, pt1, pt2, color[, thickness=1, lineType=LINE_8, shift=0]) → img
cv.arrowedLine(img, pt1, pt2, color[, thickness=1, line_type=8, shift=0, tipLength=0.1]) → img

function cv.line() Draw the midpoint of the image pt1 With the point pt2 The line between , function cv.arrowedLine() Draw the midpoint of the image pt1 With the point pt2 Line segment with arrow between .

Parameter description ：

• img： Input / output image , Allows single channel grayscale images or multi-channel color images
• pt1： Coordinates of the first point of the line segment ,(x1, y1)
• pt2： Coordinates of the second point of the line segment ,(x2, y2)
• tipLength： The ratio of the length of the arrow part to the length of the line segment , The default is 0.1

matters needing attention ：

1. The drawing operation will be performed directly on the incoming image img Make changes , Whether to accept the return value of the function or not . If you want to keep the input image unchanged, use img.copy() replicate .
2. Use the starting point of the line segment pt1、 End pt2 Draw a straight line , Note that the format of the start and end coordinates is (x,y) instead of (y,x).
3. The arrow starts from the starting point pt1 Point to the end pt2, By swapping the starting point pt1 And the end pt2 Redraw , Can generate the line segment of the double arrow .
4. If the end coordinates exceed the image boundary , The drawn line segment is clipped by the image boundary . That is, the shape of the straight line is determined by pt1、pt2 decision , But the end of the line segment is a straight line pt1-pt2 Intersection with image boundary . If the end coordinates exceed the image boundary , The arrow may not be fully displayed or displayed .
5. Draw on a color image , line color color You can tuple (b,g,r) Express , Such as (0,0,255) It means red ; It can also be scalar b, But it doesn't mean grayscale lines , It means color (b,0,0).
6. Only gray lines can be drawn on a single channel gray image , Cannot draw colored lines . however , line color color It can be scalar b, It can also be tuples (b,g,r), Will be interpreted as grayscale values b. The parameters of the last two channels in the tuple are invalid .
7. If set thickness, key word “lineType” It can be omitted ; If not set thickness, Then the key words “lineType” Don't omit , Otherwise, the input linetype parameters will be incorrectly interpreted as lineweight .
8. You cannot select a fill linetype when drawing a line “cv.FILLED”, Otherwise, an error will be reported .
9. When the image size is small ,LINE_4 The linetype has obvious sawtooth ,LINE_AA The linetype is smoother . But if the image size is large , Then the influence of linetype is not great , Default values are recommended LINE_8.
10. The angle between the arrow and the line is $\pm 45^o$,tipLength Represents the ratio of the length of the arrow part to the length of the line segment .

These precautions , Please review it again against the routine and the result diagram .

routine A4.1： Draw a line on the image

    # A4.1  Draw a line on the image 
    height, width, channels = 200, 120, 3
    img = np.ones((height, width, channels), np.uint8)*160  #  Create a black image  RGB=0

    #  Be careful  pt1, pt2  The format of coordinates is  (x,y)  instead of  (y,x)
    img1 = img.copy()
    cv.line(img1, (0,0), (200,150), (0,0,255), 1)  #  Red  R=255
    cv.line(img1, (0,0), (150,200), (0,255,0), 1)  #  green  G=255
    cv.line(img1, (0,50), (200,50), (128,0,0), 2)  #  Navy Blue  B = 128
    cv.line(img1, (0,100), (200,100), 128, 2)  # color=128  Equivalent to  (128,0,0)
    cv.line(img1, (0,150), (200,150), 255, 2)  # color=255  Equivalent to  (255,0,0)

    img2 = img.copy()
    # img2 = cv.line(img2, (0,120), (100,180), (0,0,255), lineType=cv.FILLED) # cv.FILLED  illegal 
    img2 = cv.line(img2, (0,80), (150,160), (0,0,255), lineType=cv.LINE_4)  #  Red ,cv.LINE_4
    img2 = cv.line(img2, (0,40), (150,120), (0,0,255), lineType=cv.LINE_8)  #  Red ,cv.LINE_8
    img2 = cv.line(img2, (0,0), (150,80), (0,0,255), lineType=cv.LINE_AA)  #  Red ,cv.LINE_AA
    #  If set  thickness, key word  "lineType"  It can be omitted 
    img2 = cv.line(img2, (150,10), (20,50), (0,255,0), 1, cv.LINE_8)  #  green 
    img2 = cv.line(img2, (150,60), (20,100), (0,255,0), 1, cv.LINE_AA)  #  green 
    #  If not set  thickness, Then the key words  "lineType"  Don't omit 
    img2 = cv.line(img2, (150,110), (30,150), (255,0,0), cv.LINE_8)  #  Blue , cv.LINE  Recognized as line width 
    img2 = cv.line(img2, (150,160), (30,200), (255,0,0), cv.LINE_AA)  #  Blue , cv.LINE  Recognized as line width 

    # img3 = img.copy()
    # tipLength  It refers to the ratio of the length of the arrow part to the length of the entire line segment 
    img3 = cv.arrowedLine(img.copy(), (10,0), (100,30), (0,0,255), tipLength=0.05)  #  from  pt1  Point to  pt2
    img3 = cv.arrowedLine(img3, (10,50), (100,80), (0,0,255), tipLength=0.1)
    img3 = cv.arrowedLine(img3, (10,100), (100,130), (0,0,255), tipLength=0.2)  #  Double arrow 
    img3 = cv.arrowedLine(img3, (100,130), (10,100), (0,0,255), tipLength=0.2)  #  Double arrow 
    img3 = cv.arrowedLine(img3, (10,150), (200,200), (0,0,255), tipLength=0.1)  #  End point out of bounds , The arrow does not show 

    #  Not used  img.copy(),  Will directly change  img, And interact with each other 
    img4 = cv.line(img, (0,100), (150,100), (0,255,0), 1)  #  Level , y=100
    img5 = cv.line(img, (75,0), (75,200), (0,0,255), 1)  #  Vertical line , x= 60

    #  Drawing lines can be used for grayscale images , Parameters  color  Only the first channel value is valid , And is set to the gray value 
    gray = np.zeros((height, width), np.uint8)  #  Create a grayscale image 
    img6 = cv.line(gray, (0,10), (200,10), (0,255,255), 2)
    img6 = cv.line(gray, (0,30), (200,30), (64,128,255), 2)
    img6 = cv.line(gray, (0,60), (200,60), (128,64,255), 2)
    img6 = cv.line(gray, (0,100), (200,100), (255,0,255), 2)
    img6 = cv.line(gray, (20,0), (20,200), 128, 2)
    img6 = cv.line(gray, (60,0), (60,200), (255,0,0), 2)
    img6 = cv.line(gray, (100,0), (100,200), (255,255,255), 2)
    print(gray.shape, img6.shape)

    plt.figure(figsize=(9, 6))
    plt.subplot(231), plt.title("img1"), plt.axis('off')
    plt.imshow(cv.cvtColor(img1, cv.COLOR_BGR2RGB))
    plt.subplot(232), plt.title("img2"), plt.axis('off')
    plt.imshow(cv.cvtColor(img2, cv.COLOR_BGR2RGB))
    plt.subplot(233),plt.title("img3"), plt.axis('off')
    plt.imshow(cv.cvtColor(img3, cv.COLOR_BGR2RGB))
    plt.subplot(234), plt.title("img4"), plt.axis('off')
    plt.imshow(cv.cvtColor(img4, cv.COLOR_BGR2RGB))
    plt.subplot(235), plt.title("img5"), plt.axis('off')
    plt.imshow(cv.cvtColor(img5, cv.COLOR_BGR2RGB))
    plt.subplot(236),plt.title("img6"), plt.axis('off')
    plt.imshow(img6, cmap="gray")
    plt.tight_layout()
    plt.show()

Routine description ：

The running result of this routine is shown in the figure below . The precautions in the previous article , You can read it by referring to the comments in the routine and the run result graph .

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【 At the end of this section 】

Copyright notice ：
reference ： Use the Photoshop Levels adjustment (adobe.com)
[email protected] Original works , Reprint must be marked with the original link ：(https://blog.csdn.net/youcans/article/details/125432101)
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Crated：2022-6-20
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210. There are so many holes in drawing a straight line ？