Drawing rotated text on a HTML5 canvas

Question

Part of a web application I m developing requires me to create bar graphs to display various information  I figured  if the user s browser is capable  I would draw them using the HTML5 canvas element  I have no problem drawing lines and bars for my graphs  but when it comes to labeling the axes  the bars  or the lines I ran into a snag  How do I draw rotated text onto a canvas element so that it lines up with the item it is labeling  A couple examples include    Rotate text 90 degrees counter clockwise to label the y-axis Rotate text 90 degrees counter clockwise to label bars on a vertical bar graph Rotate text an arbitrary amount to label lines on a line graph   Any pointers would be appreciated

User · Answer

Posting this in an effort to help others with similar problems. I solved this issue with a five step approach -- save the context, translate the context, rotate the context, draw the text, then restore the context to its saved state.

I think of translations and transforms to the context as manipulating the coordinate grid overlaid on the canvas. By default the origin (0,0) starts in the upper left hand corner of the canvas. X increases from left to right, Y increases from top to bottom. If you make an "L" w/ your index finger and thumb on your left hand and hold it out in front of you with your thumb down, your thumb would point in the direction of increasing Y and your index finger would point in the direction of increasing X. I know it's elementary, but I find it helpful when thinking about translations and rotations. Here's why:

When you translate the context, you move the origin of the coordinate grid to a new location on the canvas. When you rotate the context, think of rotating the "L" you made with your left hand in a clockwise direction the amount indicated by the angle you specify in radians about the origin. When you strokeText or fillText, specify your coordinates in relation to the newly aligned axes. To orient your text so it's readable from bottom to top, you would translate to a position below where you want to start your labels, rotate by -90 degrees and fill or strokeText, offsetting each label along the rotated x axis. Something like this should work:

 context.save();
 context.translate(newx, newy);
 context.rotate(-Math.PI/2);
 context.textAlign = "center";
 context.fillText("Your Label Here", labelXposition, 0);
 context.restore();

.restore() resets the context back to the state it had when you called .save() -- handy for returning things back to "normal".

User · Answer

While this is sort of a follow up to the previous answer  it adds a little  hopefully    Mainly what I want to clarify is that usually we think of drawing things like draw a rectangle at 10  3   So if we think about that like this  move origin to 10  3  then draw rectangle at 0  0  Then all we have to do is add a rotate in between   Another big point is the alignment of the text   It s easiest to draw the text at 0  0  so using the correct alignment can allow us to do that without measuring the text width   We should still move the text by an amount to get it centered vertically  and unfortunately canvas does not have great line height support  so that s a guess and check thing   correct me if there is something better     I ve created 3 examples that provide a point and a text with 3 alignments  to show what the actual point on the screen is where the font will go     var font  lineHeight  x  y   x   100  y   100  font   20  lineHeight   15     this is guess and check as far as I know this context font   font    px Arial        Right Aligned this context save    this context translate x  y   this context rotate -Math PI   4    this context textAlign    right   this context fillText  right   0  lineHeight   2    this context restore     this context fillStyle    red   this context fillRect x  y  2  2        Center this context fillStyle    black   x   150  y   100   this context save    this context translate x  y   this context rotate -Math PI   4    this context textAlign    center   this context fillText  center   0  lineHeight   2    this context restore     this context fillStyle    red   this context fillRect x  y  2  2        Left this context fillStyle    black   x   200  y   100   this context save    this context translate x  y   this context rotate -Math PI   4    this context textAlign    left   this context fillText  left   0  lineHeight   2    this context restore     this context fillStyle    red   this context fillRect x  y  2  2     The line this context fillText  right   0  lineHeight   2   is basically 0  0  except we move slightly for the text to be centered near the point

User · Answer

Here s an HTML5 alternative to homebrew   http   www rgraph net   You might be able to reverse engineer their methods      You might also consider something like Flot  http   code google com p flot   or GCharts   http   www maxb net scripts jgcharts include demo  1   It s not quite as cool  but fully backwards compatible and scary easy to implement

User · Answer

Like others have mentioned  you probably want to look at reusing an existing graphing solution  but rotating text isn t too difficult   The somewhat confusing bit  to me  is that you rotate the whole context and then draw on it   ctx rotate Math PI 2  i 6      The angle is in radians   The code is taken from this example  which I believe was made for the transformations part of the MDC canvas tutorial   Please see the answer below for a more complete solution

User · Answer

Funkodebat posted a great solution which I have referenced many times  Still  I find myself writing my own working model each time I need this  So  here is my working model    with some added clarity   First of all  the height of the text is equal to the pixel font size  Now  this was something I read a while ago  and it has worked out in my calculations  I m not sure if this works with all fonts  but it seems to work with Arial  sans-serif   Also  to make sure that you fit all of the text in your canvas  and don t trim the tails off of your  p  s  you need to set context textBaseline    You will see in the code that we are rotating the text about its center  To do this  we need to set context textAlign    center  and the context textBaseline to bottom  otherwise  we trim off parts of our text   Why resize the canvas  I usually have a canvas that isn t appended to the page  I use it to draw all of my rotated text  then I draw it onto another canvas which I display  For example  you can use this canvas to draw all of the labels for a chart  one by one  and draw the hidden canvas onto the chart canvas where you need the label  context drawImage hiddenCanvas  0  0      IMPORTANT NOTE  Set your font before measuring your text  and re-apply all of your styling to the context after resizing your canvas  A canvas s context is completely reset when the canvas is resized   Hope this helps    x000D   x000D  var c   document getElementById  myCanvas    x000D  var ctx   c getContext  2d    x000D  var font  text  x  y  x000D   x000D  text    Mississippi   x000D   x000D    Set font size before measuring x000D  font   20  x000D  ctx font   font    px Arial  sans-serif   x000D    Get width of text x000D  var metrics   ctx measureText text   x000D    Set canvas dimensions x000D  c width   font   The height of the text  The text will be sideways  x000D  c height   metrics width   The measured width of the text x000D    After a canvas resize  the context is reset  Set the font size again x000D  ctx font   font    px Arial   x000D    Set the drawing coordinates x000D  x   font 2  x000D  y   metrics width 2  x000D    Style x000D  ctx fillStyle    black   x000D  ctx textAlign    center   x000D  ctx textBaseline    bottom   x000D    Rotate the context and draw the text x000D  ctx save    x000D  ctx translate x  y   x000D  ctx rotate -Math PI   2   x000D  ctx fillText text  0  font   2   x000D  ctx restore    x000D   lt canvas id  myCanvas  width  300  height  150  style  border 1px solid  d3d3d3   gt  x000D   x000D   x000D

[javascript] Drawing rotated text on a HTML5 canvas

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