POV-Ray : Newsgroups : povray.general : Trouble with +ER1 with Pov-Ray 3.5a : Trouble with +ER1 with Pov-Ray 3.5a Server Time: 25 Sep 2017 15:08:06 GMT
  Trouble with +ER1 with Pov-Ray 3.5a  
From: William F  Pokorny
Date: 3 Sep 2002 20:02:21
The following 3.5 code in povmsrec.cpp does not allow the command line
option of (+ER) to be 1, but rather forces the value, indirectly, to be
the last row in the image.  Basically it seems to be impossible to
render the top row of an image by itself.  Could someone please verify
that it happens for you too?  Thanks.

Here is the code which I think is the culprit and my suggested change:

   if(POVMSUtil_GetFloat(msg, kPOVAttrib_EndRow, &f) == 0)
      if(f > 0.0 && f <= 1.0)  /* Should be < 1.0 */
         opts.Last_Line = -1;
         opts.Last_Line_Percent = f;
         opts.Last_Line = (int)f;

I'd bet the bug crept in to allow for a 100% ending percentage, but just
not specifiying the ending percentage defaults to 100%.  Here is the
related documentation which is 'mostly clear' about how it should work.

                  Set last row to n pixels
                  Set last row to n percent of height
  +ERn or +En
                  Same as End_Row=n
  +ER0.n or +E0.n
                  Same as End_Row=0.n

When doing test rendering it is often convenient to define a small,
rectangular sub-section of the whole screen so you can quickly check out
one area of the image.  The Start_Row, End_Row, Start_Column and
End_Column options allow you to define the subset area to be rendered.
The default values are the full size of the image from (1,1) which is
the upper left to (w,h) on the lower right where w and h are the Width=n
and Height=n values you have set.

Note:  if the number specified is greater than 1 then it is interpreted
as an absolute row or column number in pixels.  If it is a decimal value
between 0.0 and 1.0 then it is interpreted as a percent of the total
width or height of the image.

For example:  Start_Row=0.75 and Start_Column=0.75 starts on a row 75%
down from the top at a column 75% from the left.  Thus it renders only
the lower-right 25% of the image regardless of the specified width and

I believe the change brings the code in line with the documented
behavior though I am also going to suggest the "Note" in the
documentation be changed to read "Note:  if the number specified is
greater than or equal to 1 then....."  It is currently self

The change also makes the +SR/+ER flags consistent with the behavior of the
+SC/+EC flags.
Bill P.

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