I have recently upgraded to QGIS 2.12 which has resulted in a problem with one of my projects, whereby the data dependent override of the line width and colour of a line pattern fill and SVG marker line pattern has ceased functioning.
I have looked in the QGIS 2.12 changelog and didn't find anything that should have affected this.
What should happen is that the colour and linewidth of the pattern fill should vary according to the topographic curvature and the average slope within a vector segment.
I use the following expressions:
for colour: case when RAT_LngtCrv_Mean > 0 then color_rgb(min(255,255*5*RAT_LngtCrv_Mean),0,0) else color_rgb(0,0,min(255,-255*5*RAT_LngtCrv_Mean)) end
for linewidth (using map units - metres): case when RAT_Slope_deg_Mean > 2.25 then max(20*((RAT_Slope_deg_Mean - 2.25)/ 90),0.4) else 0.0001 end
for size of the SVG arrow: case when "RAT_Slope_deg_Mean" > 2.25 then max(75*sqrt((RAT_Slope_deg_Mean - 2.25)/ 90),5) else 0.0001 end
for angle (working): 90 - RAT_Aspect_deg_mean
I have also raised this as an issue.
A fairly minimal example qgis project file reproducing this issue can be found here on my Dropbox