OpenCV putText() new line character

Question

I am using cv2.putText() to draw a text string on an image.

When I write:

cv2.putText(img, "This is n some text", (50,50), cv2.FONT_HERSHEY_SIMPLEX, 1, 2)

The text drawn on the image is:

This is ? some text

I was expecting the text to be printed in the new line as n is an escape character for newline but it draws ? on the image instead.

Why this is happening ? am I doing something wrong ?

Asked By: Vipul

||

Source

Answer 1

Unfortunately putText doesn’t correctly handle n symbols. See the relevant rejected pull request. You need to split your text yourself and make several putText calls, something like:

text = "This is n some text"
y0, dy = 50, 4
for i, line in enumerate(text.split('n')):
    y = y0 + i*dy
    cv2.putText(img, line, (50, y ), cv2.FONT_HERSHEY_SIMPLEX, 1, 2)

Answered By: elyase

Answer 2

Here’s another example:

https://gist.github.com/EricCousineau-TRI/596f04c83da9b82d0389d3ea1d782592

Basically, same as elyase’s answer, but using getTextSize() as well (though I had to increase line size a bit larger than expected).

Answered By: Eric Cousineau

Answer 3

The approach posed by elyase worked well for me in OpenCV 4.5.1.38. I did verify that the text strings can still be joined with "+" and numeric values can be inserted using "str(value)". The start of the newline is just prepended with the "n" and everything behaved as expected.

y0, dy, text = 185,50, "FPS: "+str(framerate)+"nMIN: "+str(frMIN)+"nMAX: "+str(frMAX)+"nAVG: "+str(frAVG)
for i, line in enumerate(text.split('n')):
    y = y0 + i*dy
    cv2.putText(currStack, line, (50, y ), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 0, 255), 2, cv2.LINE_AA, False)

Answered By: drmikeburns

Answer 4

I’ve wrote a utility function for this porpuse:

from typing import Optional, Tuple

import cv2
import numpy as np


def add_text_to_image(
    image_rgb: np.ndarray,
    label: str,
    top_left_xy: Tuple = (0, 0),
    font_scale: float = 1,
    font_thickness: float = 1,
    font_face=cv2.FONT_HERSHEY_SIMPLEX,
    font_color_rgb: Tuple = (0, 0, 255),
    bg_color_rgb: Optional[Tuple] = None,
    outline_color_rgb: Optional[Tuple] = None,
    line_spacing: float = 1,
):
    """
    Adds text (including multi line text) to images.
    You can also control background color, outline color, and line spacing.

    outline color and line spacing adopted from: https://gist.github.com/EricCousineau-TRI/596f04c83da9b82d0389d3ea1d782592
    """
    OUTLINE_FONT_THICKNESS = 3 * font_thickness

    im_h, im_w = image_rgb.shape[:2]

    for line in label.splitlines():
        x, y = top_left_xy

        # ====== get text size
        if outline_color_rgb is None:
            get_text_size_font_thickness = font_thickness
        else:
            get_text_size_font_thickness = OUTLINE_FONT_THICKNESS

        (line_width, line_height_no_baseline), baseline = cv2.getTextSize(
            line,
            font_face,
            font_scale,
            get_text_size_font_thickness,
        )
        line_height = line_height_no_baseline + baseline

        if bg_color_rgb is not None and line:
            # === get actual mask sizes with regard to image crop
            if im_h - (y + line_height) <= 0:
                sz_h = max(im_h - y, 0)
            else:
                sz_h = line_height

            if im_w - (x + line_width) <= 0:
                sz_w = max(im_w - x, 0)
            else:
                sz_w = line_width

            # ==== add mask to image
            if sz_h > 0 and sz_w > 0:
                bg_mask = np.zeros((sz_h, sz_w, 3), np.uint8)
                bg_mask[:, :] = np.array(bg_color_rgb)
                image_rgb[
                    y : y + sz_h,
                    x : x + sz_w,
                ] = bg_mask

        # === add outline text to image
        if outline_color_rgb is not None:
            image_rgb = cv2.putText(
                image_rgb,
                line,
                (x, y + line_height_no_baseline),  # putText start bottom-left
                font_face,
                font_scale,
                outline_color_rgb,
                OUTLINE_FONT_THICKNESS,
                cv2.LINE_AA,
            )
        # === add text to image
        image_rgb = cv2.putText(
            image_rgb,
            line,
            (x, y + line_height_no_baseline),  # putText start bottom-left
            font_face,
            font_scale,
            font_color_rgb,
            font_thickness,
            cv2.LINE_AA,
        )
        top_left_xy = (x, y + int(line_height * line_spacing))

    return image_rgb

Here are example results:

image = 200 * np.ones((550, 410, 3), dtype=np.uint8)

image = add_text_to_image(
    image,
    "New linenDouble new linennLine too longggggggggggggggggggggg",
    top_left_xy=(0, 10),
)
image = add_text_to_image(
    image,
    "Different font scale",
    font_scale=0.5,
    font_color_rgb=(0, 255, 0),
    top_left_xy=(0, 150),
)
image = add_text_to_image(
    image,
    "New line with bgnDouble new linennLine too longggggggggggggggggggggg",
    bg_color_rgb=(255, 255, 255),
    font_color_rgb=(255, 0, 0),
    top_left_xy=(0, 200),
)
image = add_text_to_image(
    image,
    "Different line specing,noutline and font face",
    font_color_rgb=(0, 255, 255),
    outline_color_rgb=(0, 0, 0),
    top_left_xy=(0, 350),
    line_spacing=1.5,
    font_face=cv2.FONT_HERSHEY_SCRIPT_SIMPLEX,
)
import matplotlib.pyplot as plt

plt.imshow(image)
plt.show()

Answered By: YoniChechik

OpenCV putText() new line character

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