Make a monocolor jpeg file encoded one bit per pixel? - imagemagick

Question

For example, I see the following output. I don't quite understand its complete meaning. But when I look at the figure, it is just black and white, without gray or other color.
$ file 0447.jpg
0447.jpg: JPEG image data, JFIF standard 1.01, aspect ratio, density 1x1, segment length 16, baseline, precision 8, 1725x2751, components 1
The file size is 652708. To encode, 1725x2751 bits, the file size should be no more than 1725 * 2751/8 = 593184.4. But the file size is bigger than this.
So it seems that a pixel is not encoded as one bit. Is there a way to shrink the size of the file (without loss of info) so that one pixel is one bit (maybe using other formats)?

Answer 1

Don't use IM identify to find the actual bits-per pixel. Use EXIFTOOL.
Input JPG:
Exiftool shows BitsPerSample of 8
exiftool -s -ee -g1 -u -n -D cow.jpg
---- ExifTool ----
- ExifToolVersion : 12.44
---- System ----
- FileName : cow.jpg
- Directory : .
- FileSize : 16214
- FileModifyDate : 2022:10:14 11:31:44-07:00
- FileAccessDate : 2022:10:14 11:31:44-07:00
- FileInodeChangeDate : 2022:10:14 11:31:44-07:00
- FilePermissions : 100644
---- File ----
- FileType : JPEG
- FileTypeExtension : JPG
- MIMEType : image/jpeg
- ImageWidth : 300
- ImageHeight : 300
- EncodingProcess : 0
- BitsPerSample : 8 <-------
- ColorComponents : 1
---- JFIF ----
0 JFIFVersion : 1 1
2 ResolutionUnit : 2
3 XResolution : 28
5 YResolution : 28
---- Composite ----
- ImageSize : 300 300
- Megapixels : 0.09
Threshold to GIF:
convert cow.jpg -threshold 50% cow_bw1.gif
Exiftool shows BitsPerPixel = 1
exiftool -s -ee -g1 -u -n -D cow_bw1.gif
---- ExifTool ----
- ExifToolVersion : 12.44
---- System ----
- FileName : cow_bw1.gif
- Directory : .
- FileSize : 2698
- FileModifyDate : 2022:10:14 11:44:17-07:00
- FileAccessDate : 2022:10:14 11:44:19-07:00
- FileInodeChangeDate : 2022:10:14 11:44:17-07:00
- FilePermissions : 100644
---- File ----
- FileType : GIF
- FileTypeExtension : GIF
- MIMEType : image/gif
---- GIF ----
- GIFVersion : 89a
0 ImageWidth : 300
2 ImageHeight : 300
4.1 HasColorMap : 1
4.2 ColorResolutionDepth : 8
4.3 BitsPerPixel : 1 <-------
5 BackgroundColor : 1
---- Composite ----
- ImageSize : 300 300
- Megapixels : 0.09
Threshold to PNG:
convert cow.jpg -threshold 50% cow_bw1.png
Exiftool shows BitDepth of 1
exiftool -s -ee -g1 -u -n -D cow_bw1.png
---- ExifTool ----
- ExifToolVersion : 12.44
- Warning : [minor] Text/EXIF chunk(s) found after PNG IDAT (may be ignored by some readers)
---- System ----
- FileName : cow_bw1.png
- Directory : .
- FileSize : 2251
- FileModifyDate : 2022:10:14 11:49:20-07:00
- FileAccessDate : 2022:10:14 11:49:21-07:00
- FileInodeChangeDate : 2022:10:14 11:49:20-07:00
- FilePermissions : 100644
---- File ----
- FileType : PNG
- FileTypeExtension : PNG
- MIMEType : image/png
---- PNG ----
0 ImageWidth : 300
4 ImageHeight : 300
8 BitDepth : 1 <-------
9 ColorType : 0
10 Compression : 0
11 Filter : 0
12 Interlace : 0
- Gamma : 2.2
- BackgroundColor : 1
- ModifyDate : 2022:10:14 18:49:20
- Datecreate : 2022-10-14T18:31:44+00:00
- Datemodify : 2022-10-14T18:31:44+00:00
- Datetimestamp : 2022-10-14T18:49:19+00:00
---- PNG-pHYs ----
0 PixelsPerUnitX : 2800
4 PixelsPerUnitY : 2800
8 PixelUnits : 1
---- Composite ----
- ImageSize : 300 300
- Megapixels : 0.09
ADDITION
Here is how to make a 2-bit TIFF file.
convert lena.jpg -colorspace gray -depth 2 -type palette lena_d2.tif
exiftool -s -ee -g1 -u -n -D lena_d2.tif
---- ExifTool ----
- ExifToolVersion : 12.44
---- System ----
- FileName : lena_d2.tif
- Directory : .
- FileSize : 16642
- FileModifyDate : 2022:10:14 16:42:42-07:00
- FileAccessDate : 2022:10:14 16:42:43-07:00
- FileInodeChangeDate : 2022:10:14 16:42:42-07:00
- FilePermissions : 100644
---- File ----
- FileType : TIFF
- FileTypeExtension : TIF
- MIMEType : image/tiff
- ExifByteOrder : II
---- IFD0 ----
256 ImageWidth : 256
257 ImageHeight : 256
258 BitsPerSample : 2 <------
259 Compression : 1
262 PhotometricInterpretation : 3
266 FillOrder : 1
273 StripOffsets : 8
274 Orientation : 1
277 SamplesPerPixel : 1
278 RowsPerStrip : 256
279 StripByteCounts : 16384
282 XResolution : 72
283 YResolution : 72
284 PlanarConfiguration : 1
296 ResolutionUnit : 2
297 PageNumber : 0 1
320 ColorMap : (Binary data 24 bytes, use -b option to extract)
---- Composite ----
- ImageSize : 256 256
- Megapixels : 0.065536