How to Read Values From a Ppm File C++
Solarian Programmer
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PPM image from scratch in Python 3
Posted on October 25, 2017 by Paul
Someone asked me if it is possible to write epitome files in pure Python, without any libraries. The answer is, of class, you can implement whatsoever epitome format in pure Python, every bit long every bit you have a articulate description of the paradigm format and you lot know how to write binary files.
In this article, I volition bear witness you lot how to write binary PPM images in pure Python. Linux and recent versions of macOS take native back up for PPM files, so you tin open a PPM prototype using the default OS image viewer. For Windows, you can employ a program like IrfanView. The PPM file format tin store uncompressed RGB images in binary or homo readable, ASCII format. As a side note, the PPM format is not an efficient format to store images, however information technology is relatively easy to implement and use, from a programming point of view.
A PPM image starts with a header that consists of:
- A magic number P3 or P6. P3 means that the file is stored in ASCII format, P6 means the file is stored in binary.
- Whitespace character.
- The width and meridian of the paradigm, formatted as 2 ASCII decimal characters, separated by a whitespace character.
- Whitespace character.
- The maximum color value, ASCII decimal characters, larger than zero and smaller than 65536. Typically, the maximum value is 255.
- Whitespace character. Usually the LF character.
Delight note, that in the above whitespace could be blank, TAB, CR, LF.
Side by side, we store the actual image data starting from the top left corner of the prototype, in row-major fashion, were each pixel consists of an (R, Yard, B) colour triplet. If the max color value is less than 256, each color value is stored as an unsigned integer of one byte length in binary. For max colour value larger than 256 we use ii bytes for the color data. You can read more about the PPM format at http://netpbm.sourceforge.net/physician/ppm.html.
In order to store the image information efficiently, in Python, nosotros volition use the array module. Arrays in Python are like to lists simply they can store only a single data blazon.
This is how y'all create an eight elements array of unsigned characters in Python:
1 import assortment ii three my_array = array . array ( 'B' , [ 0 ] * 8 ) 4 5 print ( "my_array has" , len ( my_array ), "elements" ) 6 impress ( "The size of ane element of my_array is" , my_array . itemsize , "bytes" ) 7 impress ( "my_array =" , my_array ) This is what I run into, on my automobile, if I run the to a higher place code:
1 ~ $ python examination.py two my_array has 8 elements iii The size of 1 chemical element of my_array is i bytes four my_array = array('B', [0, 0, 0, 0, 0, 0, 0, 0]) 5 ~ $ Delight annotation, that the type of my_array in the higher up example corresponds to uint8_t in recent versions of C and C++.
If you want to allocate space for an RGB image of (width x height) pixels, in Python, we can use:
ane import array ii 3 width = 256 four height = 128 5 half-dozen image = assortment . assortment ( 'B' , [ 0 , 0 , 0 ] * width * height ) We tin fill the header of a PPM image with:
1 width = 256 2 peak = 128 3 maxval = 255 4 5 # PPM header 6 ppm_header = f 'P6 {width} {meridian} {maxval} \n ' The final line from the higher up code snippet volition raise an error, if your version of Python is older than 3.half dozen. Here is an alternative for older Python versions:
1 # PPM header ii ppm_header = 'P6 ' + str ( width ) + ' ' + str ( height ) + ' ' + str ( maxval ) + ' \due north ' Once you accept the prototype data and the PPM header prepared, you lot can save them in a binary file. Here is a complete example that will create a PPM image of 256x128 pixels filled with the blueish color:
1 import array 2 3 # PPM header iv width = 256 five acme = 128 6 maxval = 255 7 ppm_header = f 'P6 {width} {height} {maxval} \n ' eight 9 # PPM image data (filled with blueish) 10 image = array . array ( 'B' , [ 0 , 0 , 255 ] * width * summit ) eleven 12 # Salvage the PPM image as a binary file 13 with open ( 'blue_example.ppm' , 'wb' ) every bit f : 14 f . write ( bytearray ( ppm_header , 'ascii' )) 15 epitome . tofile ( f ) 
If you want to alter a particular pixel value (x, y) in the in a higher place image, you tin can access it with something similar:
1 # Change the (x, y) pixel color to red 2 index = 3 * ( y * width + 10 ) 3 image [ index ] = 255 # blood-red channel 4 image [ index + 1 ] = 0 # light-green channel v image [ index + ii ] = 0 # blue aqueduct Please annotation, that x goes from left to right and y goes from top to bottom. To illustrate, here is how y'all can fill with red a rectangle with the origin at (10, 10), a width of 50 pixels and a height of eighty pixels:
1 import array 2 3 # PPM header 4 width = 256 5 superlative = 128 six maxval = 255 vii ppm_header = f 'P6 {width} {acme} {maxval} \n ' 8 nine # PPM image data (filled with blue) 10 image = array . assortment ( 'B' , [ 0 , 0 , 255 ] * width * height ) 11 12 # Fill with ruby the rectangle with origin at (10, 10) and width x top = fifty 10 fourscore pixels xiii for y in range ( 10 , 90 ): 14 for x in range ( 10 , lx ): 15 alphabetize = 3 * ( y * width + x ) 16 image [ index ] = 255 # red channel 17 image [ index + ane ] = 0 # green channel eighteen image [ alphabetize + 2 ] = 0 # blue channel xix 20 # Save the PPM paradigm as a binary file 21 with open ( 'blue_red_example.ppm' , 'wb' ) as f : 22 f . write ( bytearray ( ppm_header , 'ascii' )) 23 image . tofile ( f ) 
Encapsulating the in a higher place functionality in a PPM form is left as an practice for the reader.
If y'all want to learn more nigh Python, I recommend reading Python Crash Class past Eric Matthes:
Another proficient Python book, for more than advanced users, is Python Playground by Mahesh Venkitachalam:
Source: https://solarianprogrammer.com/2017/10/25/ppm-image-python-3/
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