imagemagick - 将图像转换为索引的 2 位灰度 BMP

Question

首先，我的问题与 How do I convert image to 2-bit per pixel? 不同。不幸的是，它的解决方案在我的情况下不起作用......

我需要将图像转换为每像素 2 位灰度 BMP 格式。示例图像具有以下属性:

Color Model: RGB
Depth: 4
Is Indexed: 1
Dimension: 800x600
Size: 240,070 bytes (4 bits per pixel but only last 2 bits are used to identify the gray scales as 0/1/2/3 in decimal or 0000/0001/0010/0011 in binary, plus 70 bytes BMP metadata or whatever)

示例 BMP 图像开头部分的 Hex 值:


3s 代表图像开头的白色像素。再往下有一些 0、1 和 2，分别代表黑色、深灰色和浅灰色:


使用下面的命令，

convert pic.png -colorspace gray +matte -depth 2 out.bmp

我可以获得视觉上正确的 4 级灰度图像，但每个像素的深度或大小错误:

Color Model: RGB
Depth: 8 (expect 4)
Dimension: 800x504
Size: 1,209,738 bytes (something like 3 bytes per pixel, plus metadata)
(no mention of indexed colour space)

请帮忙...

Answer 1

好的，我已经按照 Mark 的提示编写了一个 Python 脚本(请参阅原始问题下的评论)，以手动创建一个 4bpp 的 4 级灰度 BMP。这种特定的 BMP 格式结构适用于 WaveShare 制造的 4.3 英寸电子纸显示模块。可以在这里找到规范:http://www.waveshare.com/wiki/4.3inch_e-Paper

以下是如何将原始图像传输到我的代码并保存结果。

convert in.png -colorspace gray +matte -colors 4 -depth 2 -resize '800x600>' pgm:- | ./4_level_gray_4bpp_BMP_converter.py > out.bmp

4_level_gray_4bpp_BMP_converter.py的内容:

#!/usr/bin/env python

"""

### Sample BMP header structure, total = 70 bytes
### !!! little-endian !!!

Bitmap file header 14 bytes
42 4D          "BM"
C6 A9 03 00    FileSize = 240,070       <= dynamic value
00 00          Reserved
00 00          Reserved
46 00 00 00    Offset = 70 = 14+56

DIB header (bitmap information header)
BITMAPV3INFOHEADER 56 bytes
28 00 00 00    Size = 40
20 03 00 00    Width = 800              <= dynamic value
58 02 00 00    Height = 600             <= dynamic value
01 00          Planes = 1
04 00          BitCount = 4
00 00 00 00    compression
00 00 00 00    SizeImage
00 00 00 00    XPerlPerMeter
00 00 00 00    YPerlPerMeter
04 00 00 00    Colours used = 4
00 00 00 00    ColorImportant
00 00 00 00    Colour definition index 0
55 55 55 00    Colour definition index 1
AA AA AA 00    Colour definition index 2
FF FF FF 00    Colour definition index 3

"""

# to insert File Size, Width and Height with hex strings in order
BMP_HEADER = "42 4D %s 00 00 00 00 46 00 00 00 28 00 00 00 %s %s 01 00 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04 00 00 00 00 00 00 00 00 00 00 00 55 55 55 00 AA AA AA 00 FF FF FF 00"
BMP_HEADER_SIZE = 70
BPP = 4
BYTE = 8
ALIGNMENT = 4 # bytes per row

import sys
from re import findall

DIMENTIONS = 1
PIXELS = 3

BLACK     = "0"
DARK_GRAY = "1"
GRAY      = "2"
WHITE     = "3"

# sample data:
# ['P5\n', '610 590\n', '255\n', '<1 byte per pixel for 4 levels of gray>']
# where item 1 is always P5, item 2 is width heigh, item 3 is always 255, items 4 is pixels/colours
data = sys.stdin.readlines()

width = int(data[DIMENTIONS].strip().split(' ')[0])
height = int(data[DIMENTIONS].strip().split(' ')[1])


if not width*height == len(data[PIXELS]):
    print "Error: pixel data (%s bytes) and image size (%dx%d pixels) do not match" % (len(data[PIXELS]),width,height)
    sys.exit()

colours = [] # enumerate 4 gray levels
for p in data[PIXELS]:
    if not p in colours:
        colours.append(p)
        if len(colours) == 4:
            break

# it's possible for the converted pixels to have less than 4 gray levels

colours = sorted(colours) # sort from low to high

# map each colour to e-paper gray indexes
# creates hex string of pixels
# e.g. "0033322222110200....", which is 4 level gray with 4bpp

if len(colours) == 1: # unlikely, but let's have this case here
    pixels = data[PIXELS].replace(colours[0],BLACK)
elif len(colours) == 2: # black & white
    pixels = data[PIXELS].replace(colours[0],BLACK)\
                         .replace(colours[1],WHITE)
elif len(colours) == 3:
    pixels = data[PIXELS].replace(colours[0],DARK_GRAY)\
                         .replace(colours[1],GRAY)\
                         .replace(colours[2],WHITE)
else: # 4 grays as expected
    pixels = data[PIXELS].replace(colours[0],BLACK)\
                         .replace(colours[1],DARK_GRAY)\
                         .replace(colours[2],GRAY)\
                         .replace(colours[3],WHITE)

# BMP pixel array starts from last row to first row
# and must be aligned to 4 bytes or 8 pixels
padding = "F" * ((BYTE/BPP) * ALIGNMENT - width % ((BYTE/BPP) * ALIGNMENT))
aligned_pixels = ''.join([pixels[i:i+width]+padding for i in range(0, len(pixels), width)][::-1])

# convert hex string to represented byte values
def Hex2Bytes(hexStr):
    hexStr = ''.join(hexStr.split(" "))
    bytes = []
    for i in range(0, len(hexStr), 2):
        byte = int(hexStr[i:i+2],16)
        bytes.append(chr(byte))
    return ''.join(bytes)

# convert integer to 4-byte little endian hex string
# e.g. 800 => 0x320 => 00000320 (big-endian) =>20030000 (little-endian)
def i2LeHexStr(i):
    be_hex = ('0000000'+hex(i)[2:])[-8:]
    n = 2 # split every 2 letters
    return ''.join([be_hex[i:i+n] for i in range(0, len(be_hex), n)][::-1])

BMP_HEADER = BMP_HEADER % (i2LeHexStr(len(aligned_pixels)/(BYTE/BPP)+BMP_HEADER_SIZE),i2LeHexStr(width),i2LeHexStr(height))

sys.stdout.write(Hex2Bytes(BMP_HEADER+aligned_pixels))

编辑:有关此电子纸显示器的所有内容以及我在其上显示内容的代码都可以在这里找到: https://github.com/yy502/ePaperDisplay