如何在 py-opencv 中保存 dpi 信息？

Question

import cv2

def clear(img):
    back = cv2.imread("back.png", cv2.IMREAD_GRAYSCALE)
    img = cv2.bitwise_xor(img, back)
    ret, img = cv2.threshold(img, 120, 255, cv2.THRESH_BINARY_INV)
    return img


def threshold(img):
    ret, img = cv2.threshold(img, 120, 255, cv2.THRESH_BINARY_INV)
    img = cv2.cvtColor(img, cv2.COLOR_RGB2GRAY)
    ret, img = cv2.threshold(img, 248, 255, cv2.THRESH_BINARY)
    return img


def fomatImage(img):
    img = threshold(img)
    img = clear(img)
    return img


img = fomatImage(cv2.imread("1566135246468.png",cv2.IMREAD_COLOR))
cv2.imwrite("aa.png",img)

这是我的代码。但是当我尝试用 tesseract-ocr 识别它时，我收到了警告。

Warning: Invalid resolution 0 dpi. Using 70 instead.

我应该如何设置 dpi？

Answer 1

AFAIK，OpenCV 不会设置它写入的 PNG 文件的 dpi，因此您正在寻找解决方法。这里有一些想法......

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方法 1 - 使用 PIL/Pillow 代替 OpenCV

PIL/Pillow 可以将dpi 信息写入PNG 文件。所以你会：

第 1 步 - 将您的 BGR OpenCV 图像转换为 RGB 以匹配 PIL 的通道顺序

from PIL import Image
RGBimage = cv2.cvtColor(BGRimage, cv2.COLOR_BGR2RGB)

第 2 步 - 将 OpenCV Numpy 数组转换为 PIL 图像

PILimage = Image.fromarray(RGBimage)

第 3 步 - 使用 PIL 编写

PILimage.save('result.png', dpi=(72,72))

正如 Fred 在 cmets 中提到的，您同样可以以几乎相同的方式使用 Python Wand。

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方法 2 - 使用 OpenCV 编写，然后使用一些工具进行修改

您可以使用 Python 的 subprocess 模块来封装，例如，ImageMagick 并像这样设置 dpi：

magick OpenCVImage.png -set units pixelspercentimeter -density 28.3 result.png

您需要知道的是，PNG 使用公制（每厘米点数）而不是英制（每英寸点数），每英寸有 2.54 厘米，因此 72 dpi 变为每厘米 28.3 点。

如果您的 ImageMagick 版本低于 v7，请将 magick 替换为 convert。

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方法 3 - 使用 OpenCV 编写并自己插入 dpi

您可以使用 OpenCV 的 imencode() 将文件写入内存。然后在文件中搜索IDAT（图像数据）块 - 这是包含图像像素的块，并在设置密度之前插入一个pHYs 块。然后写入磁盘。

实际上并不难 - 它只有 9 个字节，请参阅 here 并查看答案末尾的 pngcheck 输出。

此代码未经生产测试，但对我来说似乎工作得很好：

#!/usr/bin/env python3

import struct
import numpy as np
import cv2
import zlib

def writePNGwithdpi(im, filename, dpi=(72,72)):
   """Save the image as PNG with embedded dpi"""

   # Encode as PNG into memory
   retval, buffer = cv2.imencode(".png", im)
   s = buffer.tostring()

   # Find start of IDAT chunk
   IDAToffset = s.find(b'IDAT') - 4

   # Create our lovely new pHYs chunk - https://www.w3.org/TR/2003/REC-PNG-20031110/#11pHYs
   pHYs = b'pHYs' + struct.pack('!IIc',int(dpi[0]/0.0254),int(dpi[1]/0.0254),b"\x01" ) 
   pHYs = struct.pack('!I',9) + pHYs + struct.pack('!I',zlib.crc32(pHYs))

   # Open output filename and write...
   # ... stuff preceding IDAT as created by OpenCV
   # ... new pHYs as created by us above
   # ... IDAT onwards as created by OpenCV
   with open(filename, "wb") as out:
      out.write(buffer[0:IDAToffset])
      out.write(pHYs)
      out.write(buffer[IDAToffset:])

################################################################################
# main
################################################################################

# Load sample image
im = cv2.imread('lena.png')

# Save at specific dpi
writePNGwithdpi(im, "result.png", (32,300))

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无论你使用哪种方法，你都可以使用pngcheck --v image.png来检查你做了什么：

pngcheck -vv a.png

样本输出

File: a.png (306 bytes)
  chunk IHDR at offset 0x0000c, length 13
    100 x 100 image, 1-bit palette, non-interlaced
  chunk gAMA at offset 0x00025, length 4: 0.45455
  chunk cHRM at offset 0x00035, length 32
    White x = 0.3127 y = 0.329,  Red x = 0.64 y = 0.33
    Green x = 0.3 y = 0.6,  Blue x = 0.15 y = 0.06
  chunk PLTE at offset 0x00061, length 6: 2 palette entries
  chunk bKGD at offset 0x00073, length 1
    index = 1
  chunk pHYs at offset 0x00080, length 9: 255x255 pixels/unit (1:1). <-- THIS SETS THE DENSITY
  chunk tIME at offset 0x00095, length 7: 19 Aug 2019 10:15:00 UTC
  chunk IDAT at offset 0x000a8, length 20
    zlib: deflated, 2K window, maximum compression
    row filters (0 none, 1 sub, 2 up, 3 avg, 4 paeth):
      0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
      0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
      0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
      0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
      (100 out of 100)
  chunk tEXt at offset 0x000c8, length 37, keyword: date:create
  chunk tEXt at offset 0x000f9, length 37, keyword: date:modify
  chunk IEND at offset 0x0012a, length 0
No errors detected in a.png (11 chunks, 76.5% compression).

---

在编辑 PNG 块时，我还设法与作者一起设置了 tIME 块和 tEXt 块。他们是这样的：

# Create a new tIME chunk - https://www.w3.org/TR/2003/REC-PNG-20031110/#11tIME
year, month, day, hour, min, sec = 2020, 12, 25, 12, 0, 0    # Midday Christmas day 2020
tIME = b'tIME' + struct.pack('!HBBBBB',year,month,day,hour,min,sec)
tIME = struct.pack('!I',7) + tIME + struct.pack('!I',zlib.crc32(tIME))

# Create a new tEXt chunk - https://www.w3.org/TR/2003/REC-PNG-20031110/#11tEXt
Author = "Author\x00Sir Mark The Great"
tEXt = b'tEXt' + bytes(Author.encode('ascii'))
tEXt = struct.pack('!I',len(Author)) + tEXt + struct.pack('!I',zlib.crc32(tEXt))

# Open output filename and write...
# ... stuff preceding IDAT as created by OpenCV
# ... new pHYs as created by us above
# ... new tIME as created by us above
# ... new tEXt as created by us above 
# ... IDAT onwards as created by OpenCV
with open(filename, "wb") as out:
   out.write(buffer[0:IDAToffset])
   out.write(pHYs)
   out.write(tIME)
   out.write(tEXt)
   out.write(buffer[IDAToffset:])

关键字：OpenCV、PIL、Pillow、dpi、密度、imwrite、PNG、块、pHYs 块、Python、图像、图像处理、文本块、时间块、作者、评论