[reportlab-users] Basic usage question

[Tim Roberts]

On Sat, 3 Sep 2005 06:25:20 -0500, Kent Tenney <ktenney at gmail.com> wrote:

>I want to lay out business cards on letter size paper.
>If the cards are vertical, 12 can fit on a page;
>3 rows of 3.5" tall, 4 columns of 2" wide: 1/8" border.
>  
>

Yes.  This is how my print shop prefers to receive orders for business 
cards.  I found Visio a bit easier than ReportLab for this task, which 
is somewhat unusual for me...

>This has led me to questions about how the coordinate system 
>works, the relationships between canvas, drawing, widget, group
>and what can be placed on what.
>  
>

I don't think of PostScript as having "widgets".  PostScript is just 
instructions that apply ink to paper.  You can pile things on top of 
each other to your heart's content, just as you can overprint documents 
and drawings on top of one another, and the effect is the same.  More ink.

At any point in its list of instructions, PostScript has a "current 
transform", which is a 2x3 matrix that converts whatever coordinates you 
provide into absolute points on the piece of paper.  The default 
transform has (0,0) at the lower left, and coordinates are in units of 
points.  There are several canvas methods that alter the transform 
(rotate, scale, translate), or you can set the 6-element matrix 
directly.  If you're doing to alter the transform, you often want to 
wrap those alterations with saveState and restoreState, so that you 
don't provide nasty surprises to the next code.

>I am sorry if there is documentation which covers these topics,
>I haven't been able to find it.
>
>Using the RLBusinessCard demo as a starting point
>IE: a card as a horizontal widget.
>
>Questions;
>
>- it looks like I need to make 12 copies of the card widget
>  and place them, I can't place the same widget in 12 places ... correct?
>  
>

I don't see the difference.  A PostScript widget isn't an object that 
exists with a life of its own.  It's just a set of instructions that 
apply ink to paper.  Somehow, you are going to have to tell PostScript 
to apply that ink 12 times.  In Python, your business card is either a 
Python function or a Python object that knows how to render itself in 
PostScript.  It's up to you to decide whether you create 12 such objects 
and have them each render onto the canvas, or create one object that can 
render itself and then be moved to a new location.  The latter is 
probably less resource-intensive, but at this level, who cares.  If the 
"12 separate object" model makes more sense to you, go for it.

>- how would I rotate the card 90 degrees and do the step and repeat?
>  
>

You can do this by changing the transform.  You can use the 
canvas.rotate method to rotate by 90 degrees, and then use the 
canvas.translate method to move the current origin by 2" or 3.5".  That 
way, your object just draws starting at (0,0), and the transform ensures 
that it draws in 12 different locations.

Alternatively, you can leave the PostScript transform with the rotation 
but no translation, and pass an offset into your business card routine, 
which it can add to all of its coordinates.  Both schemes have their 
benefits.

    canvas.rotate( 90 )
    for x in range(3):
        for y in range(4):
            canvas.saveState()
            canvas.translate( x * 3.5 * INCH, y * 2 * INCH )
            DrawBusinessCard( canvas )
            canvas.restoreState()

vs.

    canvas.rotate( 90 )
    for x in range(3):
        for y in range(4):
            DrawBusinessCard( canvas, x * 3.5 * INCH, y * 2 * INCH )

-- 
Tim Roberts, timr at probo.com
Providenza & Boekelheide, Inc.