Proportionally filling a frame with an image in ConTeXt?

Question

I have been trying to a achieve in ConTeXt what's called "Auto-Fit" in Adobe InDesign, specifically the "Fill Frame Proportionally" kind of Auto-Fit. And from what I gather from the description in ConTeXt Garden this should be possible. Unfortunately, however, I haven't been able to make it work, which is why I would like to ask for help.

Here's a MWE (btw, this resembles an older question discussed here):

\setuppapersize[A4][A4]
\setuplayout
[
  backspace=11.67mm,        width=131.25mm,
  topspace=21.21mm,         height=254.57mm,
  headerdistance=13pt,      header=13pt,
  footerdistance=13pt,      footer=13pt,
  rightmargindistance=13pt, rightmargin=39.5mm,
]

\setuppagenumbering[alternative=doublesided, location=]

\definehead[PageSection][section]
\setuphead[PageSection][
  page=right,
  continue=no,
  style=\ssb\bf,
]

\starttext

\chapter{Testing Proportional Fitting}

\dorecurse{3}{\input{knuth}}

\placefigure[leftpage,none]{}
  {\externalfigure[http://www.hardwickagriculture.org/blog/wp-content/uploads/placeholder.jpg] [
      width=\textwidth,
      height=\textheight,
  ]}


\PageSection{Here's a Section on a New Page}

\dorecurse{3}{\input{knuth}}

\stoptext

And this is what it looks like:

As you can see, the image is scaled to the size of the type area, which is almost what I want. I would like for the image to scale proportionally, though (and be cropped as necessary). So, I did some research and found the parameter factor: max here, which seems to be perfectly suited ("aspect ratio is kept, the image is scaled to the bigger (oversized) variant"). However, the result of adding the parameter...

\placefigure[leftpage,none]{}
  {\externalfigure[http://www.hardwickagriculture.org/blog/wp-content/uploads/placeholder.jpg] [
      width=\textwidth,
      height=\textheight,
      factor=max,
  ]}

is this:

Indeed, the image is scaling proportionally now, but it's not filling the frame anymore. So, I thought, I must have misunderstood the meaning of the factor and also tried factor: fit. Surprisingly, the result is exactly the same (i.e. no difference).

Can anybody offer any ideas as to how to achieve this? Am I on the right track trying the factor parameter? Is this a bug? Or are there other solutions?

Thanks in advance for any help.

Answer 1

This is not an answer but a comment. The options to \externalfigure never clip the image, so you need to use \clip as in Henri Menke's answer.

I have always struggled to understand how the keywords given to factor work. Based on the code (grph-trf.mkiv), this is a high level explanation of what is going on (in pseudocode).

If factor is set to max, fit, broad or auto then the following calculations are done (I am ignoring broad because it is somewhat complicated).

1. First, x_size and y_size are calculated. That depends on the scale parameter, but we can approximate it as being equal to the natural width and natural height of the box. In case of images, if neither height or width are specified, x_size and y_size equal to the natural size of the image; if one is specified, say width, then x_size equals width and y_size is scaled proportionally; if both are specified, then x_size equals width and y_size equals height.

2. Then, outer_v_size, h_size and v_size are calculated:

   if maxheight is not set
      outer_v_size = textheight
      if inner or insidefloat or inpagebody
         outer_v_size = \vsize
         scrachdimen = \vsize
      else
        if \pagegoal < \maxdimen
           if \pagetotal < \pagegoal
              scratchdimen = \pagegoal - \pagetotal
           else
              scratchdimen = outer_v_size % \textheight
           end
        else
           scratchdimen = outer_v_size % \textheight
        end
     else % maxheight is set
        scratchdimen = maxheight
        outer_v_size = scratchdimen
     end
   end  
   
   if height is empty
     v_size = scratchdimen
   else
     v_size = height
   end
   
   if width is empty 
      h_size = \hsize
   else
      h_size = width 
   end
   

3. Then used_x_size and used_y_size are calculated.

   function calculate_norm(used, factor, maxdim, size, _size)
     switch(factor)
       case max: used = size
       case fit: used = _size
       case auto: used = maxdim
     end
   end
   
   if x_size > y_size
      caclulate_norm(used_x_size, factor, maxwidth, hsize, h_size)
      scale = used_x_size / x_size
      used_y_size = scale * y_size
   else 
      calculate_norm(used_y_size, factor, maxheight, outer_v_size, v_size)
      scale = used_y_size / y_size
      used_x_size = scale * x_size
   end
   

4. Finally, the box or image is scaled to used_x_size and used_y_size.

Now, the key point to note is that the scaling depends on whether x_size > y_size. So, you can get the desired behaviour by setting the height and width of the image.

Here is an example to play with:

   \setupexternalfigures[location={local,default}]
\useexternalfigure
  [placeholder]
  [http://www.hardwickagriculture.org/blog/wp-content/uploads/placeholder.jpg]

\defineexternalfigure[tall][width=2\textheight, height=\textheight]
\defineexternalfigure[wide][width=\textwidth,  height=2\textwidth]

\setuppapersize[S4][S4]

\showframe
\startbuffer
  \title{None - max}
  \page
  \externalfigure[placeholder][factor=max]
  \page
  \externalfigure[mill][factor=max]

  \title{Tall - max}
  \page
  \externalfigure[placeholder][tall][factor=max]
  \page
  \externalfigure[mill][tall][factor=max]

  \title{Wide - max}
  \page
  \externalfigure[placeholder][wide][factor=max]
  \page
  \externalfigure[mill][wide][factor=max]

  \title{None - fit}
  \page
  \externalfigure[placeholder][factor=fit]
  \page
  \externalfigure[mill][factor=fit]


  \title{Tall - fit}
  \page
  \externalfigure[placeholder][tall][factor=fit]
  \page
  \externalfigure[mill][tall][factor=fit]

  \title{Wide - fit}
  \page
  \externalfigure[placeholder][wide][factor=fit]
  \page
  \externalfigure[mill][wide][factor=fit]
  \page
\stopbuffer
\starttext

\getbuffer

\setuppapersize[A4][A4]
\getbuffer
\stoptext

Note that the image is never clipped. So, if you want to clip an image, then you will need to use \clip as in Henri Menke's answer.

Answer 2

Use \clip. You might want to play with the hoffset parameter to get the right section of the image.

I also noticed that there is a spurious vertical space of .5\baselineskip above the picture. I couldn't find out where it comes from but I asked on the mailing list. EDIT: Herbert got the answer, add high to the location.

\starttext

\input knuth


\startplacefigure
  [location={leftpage,high,none}]

  \useexternalfigure
    [placeholder]
    [http://www.hardwickagriculture.org/blog/wp-content/uploads/placeholder.jpg]

  \clip
    [width=\textwidth,height=\textheight]
    {%
      \externalfigure
        [placeholder]
        [height=\textheight]
    }

\stopplacefigure

\stoptext

Answer 3

Following mickep's suggestion (thank you very much!) I am going to post an answer to this question myself - just in case, anybody else is having the same problem.

So, originally, I thought that some setting for the factor parameter should achieve in ConTeXt what's called "Fill Frame Proportionally" in Adobe InDesign (i.e. scale the image proportionally until it completely fills the intended space, then clip it), but that doesn't seem to be the case. Indeed, there's even a suggestion for Hans to "add this functionality to the scale macro with factor=clip" here.

Anyway, to solve my problem at hand, I copied the code suggested on this page and added it as a module (file t-scaleandclip.tex) in the right directory:

\unprotect

\newdimen\d_scaleandclip_actual_wd
\newdimen\d_scaleandclip_actual_ht

\newdimen\d_scaleandclip_requested_wd
\newdimen\d_scaleandclip_requested_ht

\newbox\scaleandclip_box

\installnamespace{scaleandclip}
\installcommandhandler \????scaleandclip {scaleandclip} \????scaleandclip

\setupscaleandclip
   [width=\textwidth,
    height=\textheight]

\unexpanded\def\scaleandclip{\dodoubleempty\doscaleandclip}

\def\doscaleandclip[#1][#2]%
   {\bgroup
    \ifsecondargument
       \edef\currentscaleandclip{#1}%
       \setupcurrentscaleandclip[#2]%
    \else\iffirstargument
       \doifassignmentelse{#1}
           {\let\currentscaleandclip\empty
            \setupcurrentscaleandclip[#1]}
           {\edef\currentscaleandclip{#1}}
     \else
       \let\currentscaleandclip\empty
     \fi\fi
     \dowithnextboxcs\scaleandclip_finish\hbox}

\def\scaleandclip_finish
     {%
       \d_scaleandclip_requested_wd \dimexpr\scaleandclipparameter\c!width\relax
       \d_scaleandclip_requested_ht \dimexpr\scaleandclipparameter\c!height\relax
       %
       \d_scaleandclip_actual_wd\wd\nextbox
       \d_scaleandclip_actual_ht\dimexpr\ht\nextbox + \dp\nextbox\relax
       %
       \ifdim\dimexpr\d_scaleandclip_actual_wd*100/\d_scaleandclip_requested_wd <
             \dimexpr\d_scaleandclip_actual_ht*100/\d_scaleandclip_requested_ht \relax
             \setbox\scaleandclip_box\hbox
                 {\scale[\c!width=\d_scaleandclip_requested_wd]{\box\nextbox}}%
             \scratchdimen=\the\dimexpr(\ht\scaleandclip_box - \d_scaleandclip_requested_ht)/2\relax
             \clip
               [
                 \c!voffset=\scratchdimen,
                 \c!height=\d_scaleandclip_requested_ht,
               ]{\box\scaleandclip_box}%
       \else
             \setbox\scaleandclip_box\hbox
                 {\scale[\c!height=\d_scaleandclip_requested_ht]{\box\nextbox}}%
             \scratchdimen=\the\dimexpr(\wd\scaleandclip_box - \d_scaleandclip_requested_wd)/2\relax
             \clip
               [
                 \c!hoffset=\scratchdimen,
                 \c!width=\d_scaleandclip_requested_wd,
               ]{\box\scaleandclip_box}%
       \fi
       \egroup}

\protect

The usage is:

\scaleandclip[width=..., height=...]{ ... any box ...}

simimlar to the usage of \scale macro.


\starttext

\dontleavehmode
\scaleandclip[width=3cm, height=3cm]{\externalfigure[cow.pdf][width=3cm,frame=on]}
\scale[maxwidth=3cm, maxheight=3cm]{\externalfigure[cow.pdf][width=3cm,frame=on]}

\dontleavehmode
\scaleandclip[width=3cm, height=3cm]{\externalfigure[cow.pdf][width=3cm,height=10cm,frame=on]}
\scale[maxwidth=3cm, maxheight=3cm]{\externalfigure[cow.pdf][width=3cm,height=10cm,frame=on]}

\dontleavehmode
\scaleandclip[width=3cm, height=3cm]{\externalfigure[cow.pdf][width=10cm,height=3cm,frame=on]}
\scale[maxwidth=3cm, maxheight=3cm]{\externalfigure[cow.pdf][width=10cm,height=3cm,frame=on]}

\stoptext

Then, after loading the module with \usemodule[scaleandclip] I can use it like this:

\placefigure[leftpage,high,none]{}{
  \scaleandclip[width=\textwidth, height=\textheight]
    {\externalfigure[http://www.hardwickagriculture.org/blog/wp-content/uploads/placeholder.jpg] [
       width=\textwidth,
       height=\textheight,
     ]}
  }

Indeed, I find that when combined with the \offset macro this even gives me an easier solution than the \bleed macro to achieve full page (bleeding) figures as discussed here.

Hope that helps. Thanks to anybody offering solutions!