如何根据OpenCV、Python中的图像大小调整cv2.putText的文本大小？

Question

fontScale = 1
fontThickness = 1

# make sure font thickness is an integer, if not, the OpenCV functions that use this may crash
fontThickness = int(fontThickness)

upperLeftTextOriginX = int(imageWidth * 0.05)
upperLeftTextOriginY = int(imageHeight * 0.05)

textSize, baseline = cv2.getTextSize(resultText, fontFace, fontScale, fontThickness)
textSizeWidth, textSizeHeight = textSize

# calculate the lower left origin of the text area based on the text area center, width, and height
lowerLeftTextOriginX = upperLeftTextOriginX
lowerLeftTextOriginY = upperLeftTextOriginY + textSizeHeight

# write the text on the image
cv2.putText(openCVImage, resultText, (lowerLeftTextOriginX, lowerLeftTextOriginY), fontFace, fontScale, Color,
            fontThickness)

It seems fontScale does not scale text according to the image width and height because the text is almost in the same size for different sized images. So how can I resize the text according to the image size so that all the text could fit in the image?

Answer 1

Here is the solution that will fit the text inside your rectangle. If your rectangles are of variable width, then you can get the font scale by looping through the potential scales and measuring how much width (in pixels) would your text take. Once you drop below your rectangle width you can retrieve the scale and use it to actually putText :

def get_optimal_font_scale(text, width):
    for scale in reversed(range(0, 60, 1)):
          textSize = cv.getTextSize(text, fontFace=cv.FONT_HERSHEY_DUPLEX, fontScale=scale/10, thickness=1)
          new_width = textSize[0][0]
          print(new_width)
    if (new_width <= width):
        return scale/10
    return 1

Answer 2

for this worked!

scale = 1 # this value can be from 0 to 1 (0,1] to change the size of the text relative to the image
fontScale = min(imageWidth,imageHeight)/(25/scale)

just keep in mind that the font type can affect the 25 constant

Answer 3

If you take fontScale = 1 for images with size approximately 1000 x 1000, then this code should scale your font correctly.

fontScale = (imageWidth * imageHeight) / (1000 * 1000) # Would work best for almost square images

If you are still having any problem, do comment.

Answer 4

I implemented a function to find best fitted centered location for text.

Take a look if these codes help you.

def findFontLocate(s_txt, font_face, font_thick, cv_bgd):
    best_scale = 1.0
    bgd_w = cv_bgd.shape[1]
    bgd_h = cv_bgd.shape[0]
    txt_rect_w = 0
    txt_rect_h = 0
    baseline = 0
    for scale in np.arange(1.0, 6.0, 0.2):
        (ret_w, ret_h), tmp_bsl = cv2.getTextSize(
            s_txt, font_face, scale, font_thick)
        tmp_w = ret_w + 2 * font_thick
        tmp_h = ret_h + 2 * font_thick + tmp_bsl
        if tmp_w >= bgd_w or tmp_h >= bgd_h:
            break
        else:
            baseline = tmp_bsl
            txt_rect_w = tmp_w
            txt_rect_h = tmp_h
            best_scale = scale
    lt_x, lt_y = round(bgd_w/2-txt_rect_w/2), round(bgd_h/2-txt_rect_h/2)
    rb_x, rb_y = round(bgd_w/2+txt_rect_w/2), round(bgd_h/2+txt_rect_h/2)-baseline
    return (lt_x, lt_y, rb_x, rb_y), best_scale, baseline

Note that, the function accept four arguments: s_txt (string to render), font_face , font_thick and cv_bgd (background image in ndarray format)

---

When you putText() , write codes as following:

cv2.putText(
    cv_bgd, s_txt, (lt_x, rb_y), font_face,
    best_scale, (0,0,0), font_thick, cv2.LINE_AA)

Answer 5

You can use get_optimal_font_scale function as bellow, to adjust font size according to the image size:

def get_optimal_font_scale(text, width):

    for scale in reversed(range(0, 60, 1)):
        textSize = cv2.getTextSize(text, fontFace=cv2.FONT_HERSHEY_DUPLEX, fontScale=scale/10, thickness=1)
        new_width = textSize[0][0]
        if (new_width <= width):
            return scale/10
    return 1

fontScale = 3*(img.shape[1]//6)
font_size = get_optimal_font_scale(text, fontScale)
cv2.putText(img, text, org, font, font_size, color, thickness, cv2.LINE_AA)

You can change fontScale for your image.

Answer 6

Approach

One way to approach this is to scale the font size proportionally to the size of the image. In my experience, more natural results are obtained when applying this not only to fontScale , but also to thickness . For example:

import math

import cv2

FONT_SCALE = 2e-3  # Adjust for larger font size in all images
THICKNESS_SCALE = 1e-3  # Adjust for larger thickness in all images

img = cv2.imread("...")
height, width, _ = img.shape

font_scale = min(width, height) * FONT_SCALE
thickness = math.ceil(min(width, height) * THICKNESS_SCALE)

Example

Let's take this free-to-use stock photo as an example. We create two versions of the base image by rescaling to a width of 2000px and 600px (keeping the aspect ratio constant). With the approach above, text looks appropriately sized to the image size in both cases (here shown in an illustrative use case where we label bounding boxes):

2000px

600px

Full code to reproduce ( but note: input images have to be preprocessed ):

import math

import cv2

FONT_SCALE = 2e-3  # Adjust for larger font size in all images
THICKNESS_SCALE = 1e-3  # Adjust for larger thickness in all images
TEXT_Y_OFFSET_SCALE = 1e-2  # Adjust for larger Y-offset of text and bounding box

img_width_to_bboxes = {
    2000: [
        {"xywh": [120, 400, 1200, 510], "label": "car"},
        {"xywh": [1080, 420, 790, 340], "label": "car"},
    ],
    600: [
        {"xywh": [35, 120, 360, 155], "label": "car"},
        {"xywh": [325, 130, 235, 95], "label": "car"},
    ],
}


def add_bbox_and_text() -> None:
    for img_width, bboxes in img_width_to_bboxes.items():
        # Base image from https://www.pexels.com/photo/black-suv-beside-grey-auv-crossing-the-pedestrian-line-during-daytime-125514/
        # Two rescaled versions of the base image created with width of 600px and 2000px
        img = cv2.imread(f"pexels-kaique-rocha-125514_{img_width}.jpg")
        height, width, _ = img.shape
        for bbox in bboxes:
            x, y, w, h = bbox["xywh"]
            cv2.rectangle(img, (x, y), (x + w, y + h), (0, 255, 0), 2)
            cv2.putText(
                img,
                bbox["label"],
                (x, y - int(height * TEXT_Y_OFFSET_SCALE)),
                fontFace=cv2.FONT_HERSHEY_TRIPLEX,
                fontScale=min(width, height) * FONT_SCALE,
                thickness=math.ceil(min(width, height) * THICKNESS_SCALE),
                color=(0, 255, 0),
            )
        cv2.imwrite(f"pexels-kaique-rocha-125514_{img_width}_with_text.jpg", img)


if __name__ == "__main__":
    add_bbox_and_text()

Answer 7

It`s work for me.

double calc_scale_rectbox(const char *txt, int box_width, int box_height, 
                          cv::Size &textSize, int &baseline)

{
       if (!txt) return 1.0;
       double scale = 2.0;
       double w_aprx = 0;
       double h_aprx = 0;
       do
       {
           textSize = cv::getTextSize(txt, FONT_HERSHEY_DUPLEX, scale, 2, 
                                      &baseline);
           w_aprx = textSize.width * 100 / box_width;
           h_aprx = textSize.height * 100 / box_height;
           scale -= 0.1;
        } while (w_aprx > 50 || h_aprx > 50);
        return scale;
 }

......

cv::Size textSize;

int baseline = 0;

double scale = calc_scale_rectbox(win_caption.c_str(), width, 
                                 height, textSize, baseline);

cv::putText(img, win_caption, Point(width / 2 - textSize.width / 2, 
           (height + textSize.height - baseline + 2) / 2), 
            FONT_HERSHEY_DUPLEX, scale, CV_RGB(255, 255, 255), 2);

Answer 8

A simple utility function:

def optimal_font_dims(img, font_scale = 2e-3, thickness_scale = 5e-3):
    h, w, _ = img.shape
    font_scale = min(w, h) * font_scale
    thickness = math.ceil(min(w, h) * thickness_scale)
    return font_scale, thickness

Usage:

font_scale, thickness = optimal_font_dims(image)
cv2.putText(image, "LABEL", (x, y), cv2.FONT_HERSHEY_SIMPLEX, font_scale, (255,0,0), thickness)