Proportionally filling a frame with an image in ConTeXt?

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

enter image description here


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.


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!

Tags:

Graphics

Scaling

Context

Floats

Recent Posts