Pikchr

Tutorial Analysis Of A Pikchr Diagram
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Let's look closely at an example Pikchr diagram to better understand how they work. For this analysis, we will use a diagram that depicts a rebase operation in a version control system. The original diagram is found in the Rebase Considered Harmful document of the Fossil documentation. The version shown here is modified slightly from the original, for example by adding line number comments. Click on the diagram to see the Pikchr code.

C0 C1 C2 C4 C6 C3 C5 C3' C5' trunk feature branch
/* 01 */ scale = 0.8
/* 02 */ fill = white
/* 03 */ linewid *= 0.5
/* 04 */ circle "C0" fit
/* 05 */ circlerad = previous.radius
/* 06 */ arrow
/* 07 */ circle "C1"
/* 08 */ arrow
/* 09 */ circle "C2"
/* 10 */ arrow
/* 11 */ circle "C4"
/* 12 */ arrow
/* 13 */ circle "C6"
/* 14 */ circle "C3" at dist(C2,C4) heading 30 from C2
/* 15 */ arrow
/* 16 */ circle "C5"
/* 17 */ arrow from C2 to C3 chop
/* 18 */ C3P: circle "C3'" at dist(C4,C6) heading 30 from C6
/* 19 */ arrow right from C3P.e
/* 20 */ C5P: circle "C5'"
/* 21 */ arrow from C6 to C3P chop
/* 22 */ box height C3.y-C2.y \
/* 23 */     width (C5P.e.x-C0.w.x)+linewid \
/* 24 */     with .w at 0.5*linewid west of C0.w \
/* 25 */     behind C0 \
/* 26 */     fill 0xc6e2ff thin color gray
/* 27 */ box same width previous.e.x - C2.w.x \
/* 28 */     with .se at previous.ne \
/* 29 */     fill 0x9accfc
/* 30 */ "trunk" below at 2nd last box.s
/* 31 */ "feature branch" above at last box.n

Hint: Copy the Pikchr source text and paste it into the /pikchrshow page in a separate browser window or tab so that you can make minor changes and see the effect of those changes as we work through the text.

Lines 01 through 03 - modifying object size defaults

The script begins by setting some global property variables. The "scale = 0.8" line simply makes the whole diagram a little smaller so that it fits better within its host document. Try commenting out that line (by adding a "#" or "//") and seeing the difference.

The "fill = white" line causes all objects on the graph to have a default background fill color of white. Without this line, the objects are not filled at all, and so the background colors (to be inserted on lines 22 through 29) show through. The result is still legible, but less pleasing. Try commenting out line 02 to see what happens. We could have added a "fill white" attribute on every circle in the diagram instead, but it seems easier just to set the default fill color once.

The "linewid *= 0.5" on line 03 shortens the default length of lines and arrows by 50%. Try commenting out that line. You will see that the arrows become twice as long, which makes the graph more spread out and harder to read. Shortening the arrows is an aesthetic improvement.

Even though they appear first in the script, lines like these are typically added at the end of writing, in order to clean up a diagram after the basic structure is established. Do not feel like you need to start out by setting a bunch of variables. Write the object definitions first, and then perhaps go back and tweak the appearance by adjusting some variable settings.

Lines 04 and 05 - establishing the prototype node circle

Line 04 creates a circle sized to fit its label "C0". We want all the circles in this diagram to be the same size, so after sizing the first one to fit the text, line 05 sets the new default circle radius for all subsequent circles to be same as the first circle. This saves us from having to add a "fit" on every line. And it means that all of the circles will be of a uniform size, even if they contain varying amounts of text.

Lines 06 through 13 - the bottom row of nodes

After the initial node has been established, lines 06 through 13 create a sequence of nodes, C1, C2, C4, and C6, connected by arrows and moving to the right. The default layout direction for the graph is "right" so everything is placed automatically.

Line 14 - drawing the first node of the first branch

We want the C3-C5 branch to be above and slightly to the right of the C2 node. For a pleasing appearance, it seems best to make the distance from C2 to C3 be the same as the distance from C2 to C4. This is accomplished by setting the location of C3 using a clause of the form:

The basis is C2. The distance is the same as the distance from C2 to C4, and so we use the expression "dist(C2,C4)". Notice here that we are able to refer to the nodes using their text annotations because the text annotations have the form of a valid object label - they begin with a capital letter and consists of alphanumerics and underscores. The angle is a compass heading - 0 to 360 degrees clockwise from from north. A heading of 30 degrees means that there is a 60-degree angle between C2-C4 and C2-C3, thus establishing C2, C3, and C4 as the vertexes of an equilateral triangle.

C0 C1 C2 C4 C6 C3 30° distance C2 to C4
linewid *= 0.5
circle "C0" fit
circlerad = previous.radius
arrow
circle "C1"
arrow
circle "C2"
arrow
circle "C4"
arrow
circle "C6"
circle "C3" at dist(C2,C4) heading 30 from C2

d1 = dist(C2,C3.ne)+2mm
line thin color gray from d1 heading 30 from C2 \
   to d1+1cm heading 30 from C2
line thin color gray from d1 heading 0 from C2 \
   to d1+1cm heading 0 from C2
spline thin color gray <-> \
   from d1+8mm heading 0 from C2 \
   to d1+8mm heading 10 from C2 \
   to d1+8mm heading 20 from C2 \
   to d1+8mm heading 30 from C2 \
   "30&deg;" aligned above small

X1: line thin color gray from circlerad+1mm heading 300 from C3 \
        to circlerad+6mm heading 300 from C3
X2: line thin color gray from circlerad+1mm heading 300 from C2 \
        to circlerad+6mm heading 300 from C2
line thin color gray <-> from X2 to X1 "distance" aligned above small \
    "C2 to C4" aligned below small

Lines 15 through 17 - completing the first branch

Lines 15 and 16 add the arrow and C5 node.

The arrow from C2 to C3 is drawn by line 17. The "chop" attribute causes the arrow to begin and end on node boundaries. If you remove the "chop" (try it!) the arrow will go between the centers of the two nodes.

Lines 18 through 21 - nodes of the second branch

Lines 18 through 21 are mostly a repeat of lines 14 through 17. The differences are (1) the branch is connected to C6 instead of C2 and (2) the nodes of the branch have text attributes "C3'" and "C5'" Because the addition of the "'" characters, text attributes are no longer valid object label names and we cannot use them to refer to objects any longer. Therefore, the nodes of this second branch are given explicit labels "C3P" and "C5P". Do not be bashful about adding labels to objects. The use of labels often makes the script much easier to read and maintain.

Lines 22 through 26 - background color for trunk

Lines 22 through 26 implement a single box object that provides background color for the trunk. Note the use of backslash ("\") to continue the definition of this object across multiple lines. It is not required to break up the definition of the box across multiple lines. Splitting the object definition to multiple lines merely is merely an aid to human understanding. Pikchr does not care.

Some tricky calculations are involved here. We need to figure out an appropriate width and height for the box so that it encloses the sequence of circles and arrows that represent the trunk, with a comfortable margin, and we have to position the box so that the circles and arrows are approximately centered. The height is "C3.y-C2.y". That is the equivalent of the distance between the bottom and second row. In this way, the division between the bottom and top row can occur right in the middle of the two, and the margins above and below the bottom row will be the same. The width is sufficient to span the entire row, plus one extra "linewid" for margin, to be evenly divided between both ends.

Line 24 positions the background color box. That line says that the extreme western end of the background color box should be half a linewid to the west of the extreme western end of the first node of the graph. Recall that we allowed for one linewid of margin to be split between both ends, so the western side is half that margin to the left of the leftmost end of the graph.

Normally, Pikchr draws elements in the order that they appear in the graph. So, normally, this new background color box would paint on top of the objects that come before. That would obscure the graph nodes. To prevent this, the "behind C0" on line 25 tells Pikchr to paint this box before it paints the C0 circle, so that the background color box occurs in the background rather than on top of the graph. Try commenting out the "behind C0" and see what happens!

Finally, line 26 changes the fill color for the box to a light shade of blue, and makes the border line thin with color gray.

Lines 27 through 29 - background color for the branches

Lines 27 through 29 create a second box to provide background color to the upper branches. The second box definition begins with the keyword "same". The "same" means that all of the settings to the new box are initialized to values from the previous box. That means we don't have to set the height, or set "behind C0" or "thin" or "color gray". All those attributes are inherited. The second box only has to change the width (because it is shorter) and adjust the background color, and set the position.

We want the right edge of both background boxes to align. And we want the branch background to begin at a little to the left of C3. The left edge of C2 seems like a reasonable starting point, so we set the width to "previous.e.x - C2.w.x" on line 27. The "previous" refers to the previous background color box, of course. Be careful that you do not insert any new objects in between the two boxes, and thus mess up the "previous". Perhaps it would be better to label the prior color box and refer to it by name, like this:

         ...
/* 22 */ BG1: box height C3.y-C2.y \
/* 23 */     width (C5P.e.x-C0.w.x)+linewid \
/* 24 */     with .w at 0.5*linewid west of C0.w \
/* 25 */     behind C0 \
/* 26 */     fill 0xc6e2ff thin color gray
/* 27 */ box same as BG1 width BG1.e.x - C2.w.x \
/* 28 */     with .se at BG1.ne \
/* 29 */     fill 0x9accfc
         ...

Line 28 positions the second box. The southeast (.se) corner of the second box is set to align with the northeast (.ne) corner of the previous box. This causes the two boxes to be flush right and stacked directly on top of each other.

Line 29 adjusts the background color to a darker shade of blue.

Lines 30 and 31 - labeling the branches

Lines 30 and 31 create a pair of text objects to identify the two branches depicted in the diagram.

Overview

A 31-line Pikchr script might look intimidating at first glance. But as we see here, it is really quite simple. No coordinates are involved, nor any hard-coded distances. Everything is laid out and sized relative to other elements and to the system defaults. This makes the diagram portable and adjustments easy.

Exercises

Practice your Pikchr-script writing skills by modifying the example script as follows:

  1. Add a new "C7" node to the right of "C6"

  2. Put the feature branch below the trunk rather than above it.

  3. Add "C8" to the right of "C7". This one is harder because it will involve expanding the background color boxes.

  4. Move the "feature branch" and "trunk" labels to the left ends of their respective boxes, rather than centering them.

  5. Add another branch above the "feature branch" that adds nodes "C9", "C10", and "C11" that fork off from "C5'". You will probably need to find a new place to put the "feature branch" label in order to get it out of the way.

  6. Add a new node and dashed line from "C5'" that illustrates "C5'" being merged back into trunk.

  7. Rotate the graph so that it goes bottom-up rather than left-to-right.