On 03/05/2010 07:04 PM, Dr. David Kirkby wrote:
I just got a doc test failure on Solaris.
File
"/export/home/drkirkby/32/sage-4.3.4.alpha0/devel/sage/sage/plot/colors.py",
line 660:
sage: gold / pi + yellow * e
Expected:
RGB color (0.51829585732141792, 0.49333037605210095, 0.0)
Got:
RGB color (0.51829585732141814, 0.49333037605210117, 0.0)
Looking at the doc test I see this:
--------------------------------------
EXAMPLES::
sage: from __future__ import division
sage: from sage.plot.colors import yellow, gold
sage: yellow / 4
RGB color (0.25, 0.25, 0.0)
sage: yellow.__truediv__(4)
RGB color (0.25, 0.25, 0.0)
sage: gold / pi + yellow * e
RGB color (0.51829585732141792, 0.49333037605210095, 0.0)
----------------------------------------------
The is absolutely no justification given in the doc test for this
result, so how do we know it's right?
Printing the values of 'yellow' and 'gold' I get:
sage: print yellow
RGB color (1.0, 1.0, 0.0)
sage: print gold
RGB color (1.0, 0.84313725490196079, 0.0)
sage:
I personally don't understand how one can divide one colour by another,
but I'm not disputing that there can be some logic in this. I tried in
Mathematica to just divide the these as lists
In[44]:= yellow={1,1,0}
Out[44]= {1, 1, 0}
In[45]:= gold={1.0,0.84313725490196079, 0.0}
In[46]:= gold/Pi + yellow E
Out[46]= {3.03659, 2.98666, 0.}
With no idea what this division is supposed to do, I tried.
In[47]:= Normalize[%]
Out[47]= {0.712944, 0.701221, 0.}
but that gives totally different numbers.
So I'm none the wiser. Of course, I could create a ticket to check the
expected value to be
0.51829585732141..., 0.49333037605210...,0.0
but I'd have no justification for this.
Perhaps someone can enlighten me how one divides /multiples colours, and
can show me a high precision value for the result.
This is just one of several examples I've seen in Sage where the numeric
result from a doc test is not obvious. The "Expected" value is probably
what someone got on their computer. I "Got" a different value on my
computer. But who knows what the result should be? Without some
justification, I find it hard to believe this doc test achieves very
much. I've just waisted an hour trying to work out how I might reproduce
this, but can't
I assume we are talking about the patches up at #5601?
First, note that colors are multiplied by scalars, not by other colors.
The documentation for the multiplication and division methods talk
about what a multiplication or division means---scale the RGB values by
the scalar.
sage: colors.yellow
RGB color (1.0, 1.0, 0.0)
sage: colors.yellow/2
RGB color (0.5, 0.5, 0.0)
sage: colors.yellow*0.3
RGB color (0.29999999999999999, 0.29999999999999999, 0.0)
This makes some sort of sense, at least numerically. However, what does
not make sense is when a resulting component is more than one, the
fractional part is taken as the value. Therefore,
sage: colors.red*2
RGB color (0.0, 0.0, 0.0)
I think this is a problem (and missed this when I reviewed the colors
patch, so this getting in is partially my fault!) At least explains
what colors*scalar is. I think this ought to be changed to make more
sense when a resulting component is outside of [0,1] (maybe cap the
values, instead of just take the fractional part). The problem here is
in the rgbcolor() function when it creates a color from a tuple.
The addition of two colors is documented in the function for addition,
and says there that it uses the blend function. The documentation for
blend says
Return a color blended with the given ``color`` by a given
``fraction``. The algorithm interpolates linearly between the
colors' corresponding R, G, and B coordinates.
This is so that colors.red+colors.blue is approximately purple (though
not exactly colors.purple, since colors.purple is a specifically defined
value according to HTML standards). colors.yellow+colors.red is
approximately colors.orange, etc.
Thanks,
Jason
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