Six Degrees of Document Format

[ipdf/sam.git] / chapters / Background.tex

diff --git a/chapters/Background.tex b/chapters/Background.tex
index 8a85ce0..87d18ce 100644 (file)
--- a/chapters/Background.tex
+++ b/chapters/Background.tex
@@ -34,19 +34,34 @@ There are many different ways to define a spline. One approach is to specify ``k
 \subsubsection{Bezier Curves}
 \input{chapters/Background_Bezier}
 
 \subsubsection{Bezier Curves}
 \input{chapters/Background_Bezier}
 

+\subsection{Font Rendering}
+
+Donald Knuth's 1986 textbook ``Metafont'' blargh
+
+
+

 \subsection{Shading}
 
 Algorithms for shading on vector displays involved drawing equally spaced lines in the region with endpoints defined by the boundaries of the region\cite{brassel1979analgorithm}. Apart from being unrealistic, these techniques required a computationally expensive sorting of vertices\cite{lane1983analgorithm}.
 
 On raster displays, shading is typically based upon Lane's algorithm of 1983\cite{lane1983analgorithm}. Lane's algorithm relies on the ability to ``subtract'' fill from a region. This algorithm is now implemented in the GPU \rephrase{stencil buffer-y and... stuff} \cite{kilgard2012gpu}
 
 \subsection{Shading}
 
 Algorithms for shading on vector displays involved drawing equally spaced lines in the region with endpoints defined by the boundaries of the region\cite{brassel1979analgorithm}. Apart from being unrealistic, these techniques required a computationally expensive sorting of vertices\cite{lane1983analgorithm}.
 
 On raster displays, shading is typically based upon Lane's algorithm of 1983\cite{lane1983analgorithm}. Lane's algorithm relies on the ability to ``subtract'' fill from a region. This algorithm is now implemented in the GPU \rephrase{stencil buffer-y and... stuff} \cite{kilgard2012gpu}
 

-\subsection{Compositing}
+\subsection{Compositing and the Painter's Model}\label{Compositing and the Painter's Model}

 
 So far we have discussed techniques for rendering vector graphics primitives in isolation, with no regard to the overall structure of a document which may contain many thousands of primitives. A straight forward approach would be to render all elements sequentially to the display, with the most recently drawn pixels overwriting lower elements. Such an approach is particularly inconvenient for anti-aliased images where colours must appear to smoothly blur between the edge of a primitive and any drawn underneath it.
 
 
 So far we have discussed techniques for rendering vector graphics primitives in isolation, with no regard to the overall structure of a document which may contain many thousands of primitives. A straight forward approach would be to render all elements sequentially to the display, with the most recently drawn pixels overwriting lower elements. Such an approach is particularly inconvenient for anti-aliased images where colours must appear to smoothly blur between the edge of a primitive and any drawn underneath it.
 

-Most raster displays are based on an additive red-green-blue colour representation which matches the human eye's response to light\cite{citationneeded}. In 1984, Porter and Duff introduced a fourth colour channel to be used when combining rasterised images called the ``alpha'' channel, analogous to the transparency of a pixel\cite{porter1984compositing}. Elements can be rendered seperately, with the four colour channels of successively drawn elements being combined according to one of several possible operations described by Porter and Duff. 
+Colour raster displays are based on an additive red-green-blue $(r,g,b)$ colour representation which matches the human eye's response to light\cite{computergraphics2}. In 1984, Porter and Duff introduced a fourth colour channel for rasterised images called the ``alpha'' channel, analogous to the transparency of a pixel\cite{porter1984compositing}. In compositing models, elements can be rendered seperately, with the four colour channels of successively drawn elements being combined according to one of several possible operations.
+
+In the ``painter's model'' as described by the SVG standard, Porter and Duff's ``over'' operation is used when rendering one primitive over another\cite{svg2011-1.1}.
+Given an existing pixel $P_1$ with colour values $(r_1, g_1, b_1, a_1)$ and a pixel $P_2$ with colours $(r_2, g_2, b_2, a_2)$ to be painted over $P_1$, the resultant pixel $P_T$ has colours given by:
+\begin{align}
+       a_T &= 1 - (1-a_1)(1-a_2) \\
+       r_T &= (1 - a_2)r_1 + r_2 \quad \text{(similar for $g_T$ and $b_T$)}
+\end{align}
+It should be apparent that alpha values of $1$ correspond to an opaque pixel; that is, when $a_2 = 1$ the resultant pixel $P_T$ is the same as $P_2$.
+When the final pixel is actually drawn on an rgb display, the $(r, g, b)$ components are $(r_T/a_T, g_T/a_T, b_T/a_T)$.

 
 

-In the ``painter's model'' described by the SVG standard, the ``over'' operation is used when rendering one primitive over another; the red-green-blue components of overlapping pixels are added but the alpha component is set to that of the uppermost pixel\cite{svg2011-1.1}. The PostScript and PDF standards also use the ``painter's model''. The painter's model is demonstrated in Figure \ref{SVG} --- originally an SVG image but converted to a PDF for inclusion in this report\footnote{PDF and SVG formats may be converted but neither standard allows for importing the other directly}.
+The PostScript and PDF standards, as well as the OpenGL API also use a painter's model for compositing. However, PostScript does not include an alpha channel, so $P_T = P_2$ always\cite{plrm}. Figure \ref{SVG} illustrates the painter's model for partially transparent shapes as they would appear in both the SVG and PDF models.

 
 \subsection{Rasterisation on the CPU and GPU}
 
 
 \subsection{Rasterisation on the CPU and GPU}
 


@@ -58,11 +73,11 @@ Traditionally, vector graphics have been rasterized by the CPU before being sent
 
 The representation of information, particularly for scientific purposes, has changed dramatically over the last few decades. For example, Brassel's 1979 paper referenced earlier has been produced on a mechanical type writer. Although the paper discusses an algorithm for shading on computer displays, the figures illustrating this algorithm have not been generated by a computer, but drawn by Brassel's assistant\cite{brassel1979analgorithm}. In contrast, modern papers such as Barnes et. al's recent paper on embedding 3d images in PDF documents\cite{barnes2013embeddding} can themselves be an interactive proof of concept.
 
 
 The representation of information, particularly for scientific purposes, has changed dramatically over the last few decades. For example, Brassel's 1979 paper referenced earlier has been produced on a mechanical type writer. Although the paper discusses an algorithm for shading on computer displays, the figures illustrating this algorithm have not been generated by a computer, but drawn by Brassel's assistant\cite{brassel1979analgorithm}. In contrast, modern papers such as Barnes et. al's recent paper on embedding 3d images in PDF documents\cite{barnes2013embeddding} can themselves be an interactive proof of concept.
 

-\rephrase{Say some stuff about Knuth's Metafont and \TeX here}
+In this section we will consider various approaches and motivations to specifying the structure and appearance of a document, including: early interpreted formats (PostScript, \TeX, DVI), the Document Object Model popular in standards for web based documents (HTML, SVG), and Adobe's ubiquitous Portable Document Format (PDF). Some of these formats were discussed in a recent paper ``Pixels Or Perish'' by Hayes\cite{hayes2012pixelsor} who argues for greater interactivity in the PDF standard.

 
 

-Hayes' 2012 article ``Pixels or Perish'' discusses the recent history and current state of the art in documents for scientific publications\cite{hayes2012pixels}.
+\subsection{Interpreted Document Formats}
+\input{chapters/Background_Interpreted}

 
 

-\subsection{Interpreted Model}

 
 \begin{itemize}
        \item This model treats a document as the source code program which produces graphics
 
 \begin{itemize}
        \item This model treats a document as the source code program which produces graphics


@@ -78,19 +93,13 @@ Hayes' 2012 article ``Pixels or Perish'' discusses the recent history and curren
        \item Problems with security --- Turing complete, can be exploited easily
 \end{itemize}
 
        \item Problems with security --- Turing complete, can be exploited easily
 \end{itemize}
 

-\subsection{Crippled Interpreted Model}
-
-\rephrase{I'm pretty sure I made that one up}
-
-\begin{itemize}
-       \item PDF is PostScript but without the Turing Completeness
-       \item Solves security issues, more efficient
-\end{itemize}
-

 \pagebreak
 \subsection{Document Object Model}\label{Document Object Model}
 \input{chapters/Background_DOM}
 
 \pagebreak
 \subsection{Document Object Model}\label{Document Object Model}
 \input{chapters/Background_DOM}
 

+\subsection{The Portable Document Format}
+
+

 \subsection{Scientific Computation Packages}
 
 The document and the code that produces it are one and the same.
 \subsection{Scientific Computation Packages}
 
 The document and the code that produces it are one and the same.