On page http://docs.opencv.org/doc/user_guide/ug_mat.html

<pre> ...
double minVal, maxVal;
minMaxLoc(sobelx, &minVal, &maxVal); //find minimum and maximum intensities
Mat draw;
sobelx.convertTo(draw, CV_8U, 255.0/(maxVal - minVal), -minVal);
</pre>
...

last line should be
<pre>

sobelx.convertTo(draw, CV_8U, 255.0/(maxVal - minVal), -minVal * 255.0/(maxVal - minVal));
</pre>
or
<pre>
const double alpha = 255.0/(maxVal - minVal);
sobelx.convertTo(draw, CV_8U, alpha, -minVal * alpha);
</pre>

Original line performs: @color_intensity(x,y) color_intensity(x,y) = color_intensity(x,y) * alpha - minVal;@ minVal; - which I believe isn't intended.