Android Paint measureText vs getTextBounds

Question

I m measuring text using Paint getTextBounds    since I m interested in getting both the height and width of the text to be rendered  However  the actual text rendered is always a bit wider than the  width   of the Rect information filled by getTextBounds     To my surprise  I tested  measureText    and found that it returns a different  higher  value  I gave it a try  and found it correct   Why do they report different widths  How can I correctly obtain the height and width  I mean  I can use  measureText    but then I wouldn t know if I should trust the  height   returned by getTextBounds     As requested  here is minimal code to reproduce the problem   final String someText    Hello  I believe I m some text     Paint p   new Paint    Rect bounds   new Rect     for  float f   10  f  lt  40  f    1f        p setTextSize f        p getTextBounds someText  0  someText length    bounds        Log d  Test   String format           Size  f  measureText  f  getTextBounds  d           f          p measureText someText           bounds width               The output shows that the difference not only gets greater than 1  and is no last-minute rounding error   but also seems to increase with size  I was about to draw more conclusions  but it may be entirely font-dependent    D Test       607   Size 10 000000  measureText 135 000000  getTextBounds 134 D Test       607   Size 11 000000  measureText 149 000000  getTextBounds 148 D Test       607   Size 12 000000  measureText 156 000000  getTextBounds 155 D Test       607   Size 13 000000  measureText 171 000000  getTextBounds 169 D Test       607   Size 14 000000  measureText 195 000000  getTextBounds 193 D Test       607   Size 15 000000  measureText 201 000000  getTextBounds 199 D Test       607   Size 16 000000  measureText 211 000000  getTextBounds 210 D Test       607   Size 17 000000  measureText 225 000000  getTextBounds 223 D Test       607   Size 18 000000  measureText 245 000000  getTextBounds 243 D Test       607   Size 19 000000  measureText 251 000000  getTextBounds 249 D Test       607   Size 20 000000  measureText 269 000000  getTextBounds 267 D Test       607   Size 21 000000  measureText 275 000000  getTextBounds 272 D Test       607   Size 22 000000  measureText 297 000000  getTextBounds 294 D Test       607   Size 23 000000  measureText 305 000000  getTextBounds 302 D Test       607   Size 24 000000  measureText 319 000000  getTextBounds 316 D Test       607   Size 25 000000  measureText 330 000000  getTextBounds 326 D Test       607   Size 26 000000  measureText 349 000000  getTextBounds 346 D Test       607   Size 27 000000  measureText 357 000000  getTextBounds 354 D Test       607   Size 28 000000  measureText 369 000000  getTextBounds 365 D Test       607   Size 29 000000  measureText 396 000000  getTextBounds 392 D Test       607   Size 30 000000  measureText 401 000000  getTextBounds 397 D Test       607   Size 31 000000  measureText 418 000000  getTextBounds 414 D Test       607   Size 32 000000  measureText 423 000000  getTextBounds 418 D Test       607   Size 33 000000  measureText 446 000000  getTextBounds 441 D Test       607   Size 34 000000  measureText 455 000000  getTextBounds 450 D Test       607   Size 35 000000  measureText 468 000000  getTextBounds 463 D Test       607   Size 36 000000  measureText 474 000000  getTextBounds 469 D Test       607   Size 37 000000  measureText 500 000000  getTextBounds 495 D Test       607   Size 38 000000  measureText 506 000000  getTextBounds 501 D Test       607   Size 39 000000  measureText 521 000000  getTextBounds 515

User · Accepted Answer

You can do what I did to inspect such problem:

Study Android source code, Paint.java source, see both measureText and getTextBounds methods. You'd learn that measureText calls native_measureText, and getTextBounds calls nativeGetStringBounds, which are native methods implemented in C++.

So you'd continue to study Paint.cpp, which implements both.

native_measureText -> SkPaintGlue::measureText_CII

nativeGetStringBounds -> SkPaintGlue::getStringBounds

Now your study checks where these methods differ. After some param checks, both call function SkPaint::measureText in Skia Lib (part of Android), but they both call different overloaded form.

Digging further into Skia, I see that both calls result into same computation in same function, only return result differently.

To answer your question: Both your calls do same computation. Possible difference of result lies in fact that getTextBounds returns bounds as integer, while measureText returns float value.

So what you get is rounding error during conversion of float to int, and this happens in Paint.cpp in SkPaintGlue::doTextBounds in call to function SkRect::roundOut.

The difference between computed width of those two calls may be maximally 1.

EDIT 4 Oct 2011

What may be better than visualization. I took the effort, for own exploring, and for deserving bounty :)

enter image description here

This is font size 60, in red is bounds rectangle, in purple is result of measureText.

It's seen that bounds left part starts some pixels from left, and value of measureText is incremented by this value on both left and right. This is something called Glyph's AdvanceX value. (I've discovered this in Skia sources in SkPaint.cpp)

So the outcome of the test is that measureText adds some advance value to the text on both sides, while getTextBounds computes minimal bounds where given text will fit.

Hope this result is useful to you.

Testing code:

  protected void onDraw(Canvas canvas){
     final String s = "Hello. I'm some text!";

     Paint p = new Paint();
     Rect bounds = new Rect();
     p.setTextSize(60);

     p.getTextBounds(s, 0, s.length(), bounds);
     float mt = p.measureText(s);
     int bw = bounds.width();

     Log.i("LCG", String.format(
          "measureText %f, getTextBounds %d (%s)",
          mt,
          bw, bounds.toShortString())
      );
     bounds.offset(0, -bounds.top);
     p.setStyle(Style.STROKE);
     canvas.drawColor(0xff000080);
     p.setColor(0xffff0000);
     canvas.drawRect(bounds, p);
     p.setColor(0xff00ff00);
     canvas.drawText(s, 0, bounds.bottom, p);
  }

User · Answer

There is another way to measure the text bounds precisely  first you should get the path for the current Paint and text  In your case it should be like this   p getTextPath someText  0  someText length    0 0f  0 0f  mPath     After that you can call   mPath computeBounds mBoundsPath  true     In my code it always returns correct and expected values  But  not sure if it works faster than your approach

User · Answer

This is how I calculated the real dimensions for the first letter  you can change the method header to suit your needs  i e  instead of char   use String    private void calculateTextSize char   text  PointF outSize           use measureText to calculate width     float width   mPaint measureText text  0  1           use height from getTextBounds       Rect textBounds   new Rect        mPaint getTextBounds text  0  1  textBounds       float height   textBounds height        outSize x   width      outSize y   height      Note that I m using TextPaint instead of the original Paint class

User · Answer

DISCLAIMER  This solution is not 100  accurate in terms of determining the minimal width   I was also figuring out how to measure text on a canvas  After reading the great post from mice i had some problems on how to measure multiline text  There is no obvious way from these contributions but after some research i cam across the StaticLayout class  It allows you to measure multiline text  text with   n   and configure much more properties of your text via the associated Paint   Here is a snippet showing how to measure multiline text   private StaticLayout measure  TextPaint textPaint  String text  Integer wrapWidth         int boundedWidth   Integer MAX VALUE      if  wrapWidth    null  amp  amp  wrapWidth  gt  0            boundedWidth   wrapWidth            StaticLayout layout   new StaticLayout  text  textPaint  boundedWidth  Alignment ALIGN NORMAL  1 0f  0 0f  false        return layout      The wrapwitdh is able to determin if you want to limit your multiline text to a certain width   Since the StaticLayout getWidth   only returns this boundedWidth you have to take another step to get the maximum width required by your multiline text  You are able to determine each lines width and the max width is the highest line width of course   private float getMaxLineWidth  StaticLayout layout         float maxLine   0 0f      int lineCount   layout getLineCount        for  int i   0  i  lt  lineCount  i               if  layout getLineWidth  i    gt  maxLine                 maxLine   layout getLineWidth  i                        return maxLine

User · Answer

The mice answer is great    And here is the description of the real problem   The short simple answer is that Paint getTextBounds String text  int start  int end  Rect bounds  returns Rect which doesn t starts at  0 0   That is  to get actual width of text that will be set by calling Canvas drawText String text  float x  float y  Paint paint  with the same Paint object from getTextBounds   you should add the left position of Rect  Something like that   public int getTextWidth String text  Paint paint        Rect bounds   new Rect        paint getTextBounds text  0  end  bounds       int width   bounds left   bounds width        return width      Notice this bounds left - this the key of the problem   In this way you will receive the same width of text  that you would receive using Canvas drawText      And the same function should be for getting height of the text   public int getTextHeight String text  Paint paint        Rect bounds   new Rect        paint getTextBounds text  0  end  bounds       int height   bounds bottom   bounds height        return height      P s   I didn t test this exact code  but tested the conception     Much more detailed explanation is given in this answer

User · Answer

The different between getTextBounds and measureText is described with the image below   In short     getTextBounds is to get the RECT of the exact text  The measureText is the length of the text  including the extra gap on the left and right  If there are spaces between the text  it is measured in measureText but not including in the length of the TextBounds  although the coordinate get shifted  The text could be tilted  Skew  left  In this case  the bounding box left side would exceed outside the measurement of the measureText  and the overall length of the text bound would be bigger than measureText The text could be tilted  Skew  right  In this case  the bounding box right side would exceed outside the measurement of the measureText  and the overall length of the text bound would be bigger than measureText

User · Answer

My experience with this is that getTextBounds will return that absolute minimal bounding rect that encapsulates the text  not necessarily the measured width used when rendering  I also want to say that measureText assumes one line    In order to get accurate measuring results  you should use the StaticLayout to render the text and pull out the measurements   For example   String text    text   TextPaint textPaint   textView getPaint    int boundedWidth   1000   StaticLayout layout   new StaticLayout text  textPaint  boundedWidth   Alignment ALIGN NORMAL  1 0f  0 0f  false   int height   layout getHeight

User · Answer

Sorry for answering again on that question    I needed to embed the image   I think the results  mice found are missleading  The observations might be correct for the font size of 60 but they turn much more different when the text is smaller  Eg  10px  In that case the text is actually drawn BEYOND the bounds     Sourcecode of the screenshot      Override   protected void onDraw  Canvas canvas         for  int i   0  i  lt  20  i             int startSize   10        int curSize   i   startSize        paint setTextSize  curSize          String text   i   startSize     -     TEXT SNIPPET        Rect bounds   new Rect          paint getTextBounds  text  0  text length    bounds          float top   STEP DISTANCE   i   curSize        bounds top    top        bounds bottom    top        canvas drawRect  bounds  bgPaint          canvas drawText  text  0  STEP DISTANCE   i   curSize  paint

[android] Android Paint: .measureText() vs .getTextBounds()

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