bmp file properties
The size of the bmp file is 1921078 bytes, but the red box on the picture shows 00 00 00 00 ?
bmp file format
I searched for answers to similar questions, but couldn't find an answer
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The image properties for this image say that the width and height are respectively 340 pixels and 471 pixels. The bit depth is 24 bits. My understanding was that this means that the value associated with each pixel is encoded using 24 bits. So I expected the file size to be around 471 * 340 * 24 = 3843360 bits = 480420 bytes =480 KB. But then one of the image properties says the size of the file is 9.98 KB. Why the big difference?
I am reading the images into a convolutional neural network where I need to supply the input shape.
Below is a screenshot of image properties
Below is a screenshot of actual image:
The PNG image format is designed to support lossless data compression. The file size you calculated is completely uncompressed, just raw image data. To save disk space, the file is compressed, and given your example of the image, this can be done very effectively because most of the image is exactly the same color.
I have an image stored as BytesIO of Pillow and I need to save it to a file with some header information (containing textual attributes) that I need to add specific to my problem. I need the bytes to be represented according to some image compression format. Would that be possible? If yes, how it can be done?
I also need to store more than one image in the file.
Storing more than one image in a file is problematic for PNG, JPEG and the most of the common formats. One option for that is TIFF - not sure if that works for you?
Here's how you can store some additional text in a PNG at least:
#!/usr/bin/env python3
from PIL.PngImagePlugin import Image, PngInfo
# Create empty metadata and add a couple of text strings
metadata = PngInfo()
metadata.add_text("Key1:","Value1")
metadata.add_text("Key2:","Value2")
# Create red image and save with metadata embedded
im = Image.new('RGB',(64,64),'red')
im.save("result.png", pnginfo=metadata)
If you check that with pngcheck you will see:
pngcheck -7v result.png
Sample Output
File: result.png (200 bytes)
chunk IHDR at offset 0x0000c, length 13
64 x 64 image, 24-bit RGB, non-interlaced
chunk tEXt at offset 0x00025, length 12, keyword: Key1:
Value1
chunk tEXt at offset 0x0003d, length 12, keyword: Key2:
Value2
chunk IDAT at offset 0x00055, length 95
zlib: deflated, 32K window, default compression
chunk IEND at offset 0x000c0, length 0
No errors detected in result.png (5 chunks, 98.4% compression).
Here's how to save 3 images and a comment in a single TIFF file:
from PIL import Image
from PIL.TiffImagePlugin import ImageFileDirectory_v2, TiffTags
# Create a structure to hold meta-data
ifd = ImageFileDirectory_v2()
ifd[270] = 'Some Funky Comment'
ifd.tagtype[270] = TiffTags.ASCII
# Create red image and save with metadata embedded
im1 = Image.new('RGB',(50,50),'red')
im2 = Image.new('RGB',(64,64),'green')
im3 = Image.new('RGB',(80,80),'blue')
im1.save("result.tif", append_images[im2,im3], save_all=True, tiffinfo=ifd)
And check that with:
tiffinfo -v result.tif
Sample Output
TIFF Directory at offset 0x8 (8)
Image Width: 50 Image Length: 50
Bits/Sample: 8
Compression Scheme: None
Photometric Interpretation: RGB color
Samples/Pixel: 3
Rows/Strip: 50
Planar Configuration: single image plane
ImageDescription: Some Funky Comment
TIFF Directory at offset 0x1e08 (7688)
Image Width: 64 Image Length: 64
Bits/Sample: 8
Compression Scheme: None
Photometric Interpretation: RGB color
Samples/Pixel: 3
Rows/Strip: 64
Planar Configuration: single image plane
ImageDescription: Some Funky Comment
TIFF Directory at offset 0x4eb8 (20152)
Image Width: 80 Image Length: 80
Bits/Sample: 8
Compression Scheme: None
Photometric Interpretation: RGB color
Samples/Pixel: 3
Rows/Strip: 80
Planar Configuration: single image plane
ImageDescription: Some Funky Comment
You can then extract the images on the command-line with ImageMagick like this.
To extract first image:
magick result.tif[0] first.png
To extract last image:
magick result.tif[-1] last.png
To extract all three images:
magick result.tif image-%d.png
Result
-rw-r--r-- 1 mark staff 457 21 Jan 08:11 image-0.png
-rw-r--r-- 1 mark staff 458 21 Jan 08:11 image-1.png
-rw-r--r-- 1 mark staff 460 21 Jan 08:11 image-2.png
Note: Use convert in place of magick above if you are running v6 ImageMagick.
Keywords: Python, PIL, image processing, multiple images, TIF, comment, tiffinfo, IFD, PNG tEXt.
I would like to confirm my interpretation of the PNG file format description is correct with color type 0x02 and with tRNS chunk present. It says on PNG website that:
For colour types 0 or 2, two bytes per sample are used regardless of the image bit depth (see 7.1: Integers and byte order). Pixels of the specified grey sample value or RGB sample values are treated as transparent (equivalent to alpha value 0); all other pixels are to be treated as fully opaque (alpha value 2bitdepth-1). If the image bit depth is less than 16, the least significant bits are used and the others are 0.
Give this same tRNS Chunk:
Length: 6
Type: tRNS
Data: 00 ff 00 ff 00 ff
CRC: 37581b7d
Does this mean for RRGGGBBB: 0xFFFFFF or the white color is treated as transparent and all other colors are as is? If so, for the general case, I need to construct an array of such color (say array of int) and during the decoding of the IDAT chunk, I need to look up if the color is one of the transparent color, if so, it is treated as (RRGGBBAA) 0xRRGGBB00, or fully transparent? What would be a good LUT for such case?
Thanks!
Your interpretation is correct.
I need to look up if the color is one of the transparent color,
Actually not "one of" but the transparent colour: in the truecolor+TRNS scenario all colors are opaque except one.
What would be a good LUT for such case?
I don't think that scenario has to do with LUTs. Think of this format as an efficient way to store a true color image with "hard" (yes/no) transparency. The decoder will typically either decode the image adding an alpha channel (and then the colour would be factually ignored), (for example you can see lineToARGB32 method here) or either by ignoring the TRNS.
Notice that this truecolour+TRNS case is not very usual, there are not many PNG images with that format, and many readers just ignore the TRNS in that case.
I have to convert a .pdf file containing scanned images into .txt files. The tesseract ocr converts only images to .txt, but I need to first extract the .tif images and then convert it. Can anyone help me with this?
Use Imagemagick:
convert -density 600 input.pdf output.tif
Density is in DPI, from my experience 600 DPI works the best.
A 24 bit .png file with transparency, as those that can be generated with Photoshop, has really 24 bits distributed across each color plus the alpha ? or the 24 bit refer only to the colors and ignores the alpha (RGBA 8888).
Is there any tool to examine a PNG file and verify this kind of information? Does Photoshop have any options to verify or configure this?
24 bit + alpha is actually 32 bits per pixel. Meaning you have the Red, Green, Blue and Alpha channels, each being 8 bit, allowing for 256 shades per channel translating to 256 x 256 x 256 x 256 possible colour combinations. That's what the "millions of colours" and "billions of colours" mean in certain graphics and video software.
As I understand it, there are three kinds of "24 bit" pngs:
24 bits with no transparency. No alpha information, truly 24 bits per pixel.
24 bits per pixel with alpha transparency. This would be 24 bits of color information with 8 bits of alpha (allows for various levels of transparency) - 32 bits per pixel total.
24 bits per pixel with binary transparency. This would be 24 bits of color information with 1 bit of alpha (transparent or not transparent) - 25 bits per pixel total.
24 bit PNG doesn't say much. An image has a pixel format. The pixel format describes the Colorspace used (such as CMYK, RGB) and bits per channel information (i.e. how many bits are allocated to represent each channel of the colorspace in use).
Go to File > File Info > Advanced. That should tell you what you are looking for.
After dissecting the exported file myself (from Photoshop CS6), I found that the "24 bit" file generated by Photoshop is actually still 8 bit. The RGBA is still one byte per channel. The IHDR PNG chunk still says that it's 8 bits per channel.
It's an 8 bit PNG.
The exported PNG also contains about 825 bytes of useless marketing text data (per PNG image).
See the image (with the byte for "bits per channel" selected):
See the specification for more details:
http://www.libpng.org/pub/png/spec/1.2/png-1.2.pdf