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Writing programs which create image files

Writing programs which create image files. Writing programs which create image files. We have seen that an image file just contains a sequence of bytes We know, in detail, the rules that apply to the sequence of bytes in a BMP file

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Writing programs which create image files

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  1. Writing programs which create image files

  2. Writing programs which create image files • We have seen that an image file just contains a sequence of bytes • We know, in detail, the rules that apply to the sequence of bytes in a BMP file • Most programming languages allow us to create new files and to write data into such files • PHP is one such programming language • We will use it to create some image files

  3. Creating image files with PHP • An example PHP program which writes two bytes of data, 42hex and 4Dhex, into a file calledfile1 <?php $fp=fopen(“file1”,”w”); $byte=chr(0x42); fwrite($fp,$byte); $byte=chr(0x4D); fwrite($fp,$byte); fclose($fp); ?>

  4. Folder before this program is executed

  5. Browser window when this program is executed

  6. Folder after this program is executed • The file called file1 has not been created • Why not? • Because the web server (which executes the PHP program) does not have permission to write into the folder /j.bowen/cs1107/exercises/tests/demo

  7. Unix/Linux Permissions • The computer on which the web server runs uses Unix • Log in and view the folder • Using the command ls –l we can see detailed information about the program demo.php • The permissions associated with demo.php are -rw-r--r--

  8. Unix/Linux Permissions (contd.) • The set of possible permissions that a file or directory (folder) can have is drwxrwxrwx • The first position contains d if and only if the item is a directory (folder) so the hyphen in the first position of the demo.php permission confirms that demo.php is a file, not a folder • The other positions come in groups of three -rwxrwxrwx

  9. Unix/Linux Permissions (contd.) • The permissions come in sets of three -rwxrwxrwx • The first set specifies whether the user who owns the item can read, write or execute the item • The second set specifies whether the group to which the owning user belongs can read, write or execute the item • The third set specifies whether other users can read, write or execute the item • The permissions associated with demo.php are -rw-r--r-- • So the user who owns demo.php can read and write the file; but the group and others can only read it.

  10. Permissions for /j.bowen/cs1107/exercises/tests/demo • Use the cd .. command to go up to the parent directory • Using the command ls –l we can see detailed information about the contents of the parent directory • The permissions associated with our target folder are drwxr-xr-x • So only the user can write into the target folder

  11. Permissions for /j.bowen/cs1107/exercises/tests/demo • Use the chmod g+w command to let the group write into the folder chmod g+w demo • Using the command ls –l we can see that the permissions associated with our target folder are now drwxrwxr-x • So the user and his group can write into the target folder

  12. Permissions for /j.bowen/cs1107/exercises/tests/demo • The permissions associated with our target folder are now drwxrwxr-x • The web server (which runs PHP programs) is part of each user’s group • so a program controlled by the web server can now write into the target folder • So, let’s execute our program again and see what happens

  13. Folder before this program is executed

  14. Browser window when this program is executed

  15. Folder after this program is executed • The file called file1 has been created

  16. Let’s inspect the file with XVI32 • Right-click on the file and download it • Then, …

  17. Let’s inspect the file with XVI32 • … then open the file with XVI32 • We see that the contents are as expected <?php $fp=fopen(“file1”,”w”); $byte=chr(0x42); fwrite($fp,$byte); $byte=chr(0x4D); fwrite($fp,$byte); fclose($fp); ?>

  18. We can use PHP to create BMP files • We have just seen that we can use PHP to write arbitrary bytes into a file • Therefore, we can use PHP to create BMP files • For example, we can easily create a BMP file by following the rules we saw earlier for the absolutely basic 24-bit depth BMP format

  19. Absolutely basic 24-bit depth BMP format • The minimum we must do when making a 24-bit depth BMP file is • assign the fixed values shown below to the bytes marked green • put the image width and depth in bytes 12-15hex and 16-19hex, respectively • specify the pixels from byte 36hex onwards, 3 bytes per pixel, padding the lines if necessary to make each line a multiple of four bytes • put the overall file size in bytes 02-05hex

  20. PHP program to create a 24-bit depth BMP file (part 1) <?php $fp=fopen("file2.bmp","w"); $byte=chr(0x42); fwrite($fp,$byte); $byte=chr(0x4D); fwrite($fp,$byte); // The next four bytes give the file size $byte=chr(0x66); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); $byte=chr(0x36); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); $byte=chr(0x28); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte);

  21. Creating a 24-bit depth BMP file (part 1, abbreviated) <?php $fp=fopen("file2.bmp","w"); $byte=chr(0x42); fwrite($fp,$byte); $byte=chr(0x4D); fwrite($fp,$byte); // The next four bytes give the file size $byte=chr(0x66); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0x36); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0x28); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte);

  22. PHP program to create a 24-bit depth BMP file (part 2) fwrite($fp,$byte); fwrite($fp,$byte); // The next four bytes give the width $byte=chr(0x04); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); // The next four bytes give the height $byte=chr(0x04); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0x01); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); $byte=chr(0x18); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte);

  23. PHP program to create a 24-bit depth BMP file (part 3) fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte);

  24. PHP program to create a 24-bit depth BMP file (part 4) fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); //The pixel data start here $byte=chr(0xFF); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte);

  25. PHP program to create a 24-bit depth BMP file (part 5) $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte);

  26. PHP program to create a 24-bit depth BMP file (part 6) fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte); $byte=chr(0x00); fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF); fwrite($fp,$byte);

  27. PHP program to create a 24-bit depth BMP file (part 7) $byte=chr(0x00);fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF);fwrite($fp,$byte); $byte=chr(0x00);fwrite($fp,$byte); fwrite($fp,$byte); $byte=chr(0xFF);fwrite($fp,$byte); fclose($fp); ?>

  28. Folder before this program is executed

  29. Browser window when this program is executed

  30. Folder after this program is executed • The file called file2.bmp has been created • Let’s open it in the browser …

  31. Browser window when file2.bmp is opened • Let’s zoom in …

  32. Browser window when zoom level is 400%

  33. Higher-level PHP constructs for manipulating image files • We know how to create image files by specifying individual bytes • But this is very tedious • PHP provides higher-level constructs for creating and/or editing image files • We will look at these in the next set of slides

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