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Clean up updated docos (Issue #78)

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Michael R Sweet
2024-10-25 17:32:38 -04:00
parent 21b8e3b06f
commit 21ac2b52d1
1 changed files with 79 additions and 56 deletions
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135
doc/pdfio.md
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@@ -118,17 +118,20 @@ that are defined in a separate header file:
```c
#include <pdfio-content.h>
```
Understanding PDF Files
-----------------------
A PDF file provides data and commands for displaying pages of graphics and text,
and is structured in a way that allows it to be displayed in the same way across
multiple devices and platforms.
The following is a PDF which shows "Hello, World!" on one page:
A PDF file provides data and commands for displaying pages of graphics and text,
and is structured in a way that allows it to be displayed in the same way across
multiple devices and platforms. The following is a PDF which shows "Hello,
World!" on one page:
```
%PDF-1.0 %Header starts here
%PDF-1.0 % Header starts here
%âãÏÓ
1 0 obj %Body starts here
1 0 obj % Body starts here
<<
/Kids [2 0 R]
/Count 1
@@ -175,7 +178,7 @@ endobj
/Type /Catalog
>>
endobj
xref %Cross-reference table starts here
xref % Cross-reference table starts here
0 6
0000000000 65535 f
0000000015 00000 n
@@ -183,7 +186,7 @@ xref %Cross-reference table starts here
0000000192 00000 n
0000000291 00000 n
0000000409 00000 n
trailer %Trailer starts here
trailer % Trailer starts here
<<
/Root 5 0 R
/Size 6
@@ -193,29 +196,40 @@ startxref
%%EOF
```
### Header
This is the first line of a PDF File. This specifies the version of PDF Format used.
For Example: '%PDF-1.0'
Since PDF files almost always contain binary data, they can become corrupted if line
endings are changed (for example, if the file is transferred over FTP in text mode).
To allow legacy file transfer programs to determine that the file is binary, it is
usual to include some bytes withcharacter codes higher than 127 in the header.
- For example: %âãÏÓ
- The percent sign indicates another header line, the other few bytes are arbitrary
character codes in excess of 127. So, the whole header in our example is:
### Header
The header is the first line of a PDF file that specifies the version of the PDF
format that has been used, for example `%PDF-1.0`.
Since PDF files almost always contain binary data, they can become corrupted if
line endings are changed. For example, if the file is transferred using FTP in
text mode or is edited in Notepad on Windows. To allow legacy file transfer
programs to determine that the file is binary, the PDF standard recommends
including some bytes with character codes higher than 127 in the header, for
example:
```
%PDF-1.0
%âãÏÓ
```
### Body
The file body consists of a sequence of objects, each preceded by an object number,
generation number, and the obj keyword on one line, and followed by the endobj keyword
on another. For Example:
The percent sign indicates a comment line while the other few bytes are
arbitrary character codes in excess of 127. So, the whole header in our example
is:
```
```
%PDF-1.0
%âãÏÓ
```
### Body
The file body consists of a sequence of objects, each preceded by an object
number, generation number, and the obj keyword on one line, and followed by the
endobj keyword on another. For example:
```
1 0 obj
<<
/Kids [2 0 R]
@@ -225,51 +239,60 @@ on another. For Example:
endobj
```
Here, the object number is 1, and the generation number is 0 (it almost always is).
The content for object 1 is in between the two lines 1 0 obj and endobj.
In this case, it’s the dictionary <</Kids [2 0 R] /Count 1 /Type /Pages>>
In this example, the object number is 1 and the generation number is 0, meaning
it is the first version of the object. The content for object 1 is between the
initial `1 0 obj` and trailing `endobj` lines. In this case, the content is the
dictionary `<</Kids [2 0 R] /Count 1 /Type /Pages>>`.
### Cross-Reference Table
The cross-reference table lists the byte offset of each object in the file body.
This allows random access to objects, meaning they don't have to be read in order.
Objects that are not used are never read, making the process efficient.
Operations like counting the number of pages in a PDF document are fast, even in large files.
Each object has an object number and a generation number.
- Generation numbers are used when a cross-reference table entry is reused.
- For simplicity, we will assume generation numbers to be always zero and ignore them.
The cross-reference table consists of:
- Header line that indicates the number of entries.
- Special entry (the first entry).
- One line for each of the object in the file body.
This allows random access to objects, meaning they don't have to be read in
order. Objects that are not used are never read, making the process efficient.
Operations like counting the number of pages in a PDF document are fast, even in
large files.
Each object has an object number and a generation number. Generation numbers
are used when a cross-reference table entry is reused. For simplicity, we will
assume generation numbers to be always zero and ignore them. The
cross-reference table consists of a header line that indicates the number of
entries, a free entry line for object 0, and a line for each of the objects in
the file body. For example:
```
0 6 %Six entries in table, starting at 0
0000000000 65535 f %Special entry
0000000015 00000 n %Object 1 is at byte offset 15
0000000074 00000 n %Object 2 is at byte offset 74
0000000192 00000 n %etc...
0000000291 00000 n
0000000409 00000 n %Object 5 is at byte offset 409
0 6 % Six entries in table, starting at 0
0000000000 65535 f % Free entry for object 0
0000000015 00000 n % Object 1 is at byte offset 15
0000000074 00000 n % Object 2 is at byte offset 74
0000000192 00000 n % etc...
0000000291 00000 n
0000000409 00000 n % Object 5 is at byte offset 409
```
### Trailer
The first line of the trailer is just the trailer keyword. This is followed by the trailer dictionary,
which contains at least the /Size entry (Number of entries in the cross-reference table)
and the /Root entry (Object number of the document catalog, which is the root element
of the graph of objects in the body).
There follows a line with just the startxref keyword, a line with a single number (the byte offset of
the start of the cross-reference table within the file), and then the line %%EOF, which signals the
end of the PDF file.
The first line of the trailer is just the `trailer` keyword. This is followed
by the trailer dictionary which contains at least the `/Size` entry specifying
the number of entries in the cross-reference table and the `/Root` entry which
references the object for the document catalog which is the root element of the
graph of objects in the body.
There follows a line with just the `startxref` keyword, a line with a single
number specifying the byte offset of the start of the cross-reference table
within the file, and then the line `%%EOF` which signals the end of the PDF
file.
```
trailer %Trailer keyword
<< %The trailer dictinonary
trailer % Trailer keyword
<< % The trailer dictinonary
/Root 5 0 R
/Size 6
>>
startxref %startxref keyword
459 %Byte offset of cross-reference table
%%EOF %End-of-file marker
startxref % startxref keyword
459 % Byte offset of cross-reference table
%%EOF % End-of-file marker
```