I tried experimenting with using OpenCl with my AMD Radeon HD 7900 Series before, but it never worked. When the Blender 2.71 build came out, I tried again and the kernel would start to compile but then Blender would crash. I was deterred, but I downloaded the OpenCl 1.2 beta driver and the latest beta AMD Catalyst Control Center, and I was able to compile the kernel.
I started the cmd and set the environment variable CYCLES_OPENCL_TEST=all and then ran blender, and blender first crashed on me but I tried again and the kernel compiled successfully, and did not take that much time.
So I started to play around with it, and I have only used Blender with an Intel i5 and i7 processor as well as on gpu with a Quadro K1000M on a Lenovo laptop, and this thing was flying. Once in a while I would render and my display driver would crash and recover, but it was much faster than my CPU or that Nvidia GPU. Sometimes I get weird fireflies out of nowhere, but I can remove them with the despeckle node. I rendered the following in about 26 minutes (1000 samples) with film set to transparent and the black background added in in compositing.
The only things that I noticed would go wrong is that whenever I would tick multiple importance sample black lines would appear over the render and the environment world texture would change color when switching from CPU to GPU so I had to try to fix it with the color nodes.
The following is an image with a sun lamp with multiple importance sample enabled. There is no compositing here.
Also something nice about OpenCL is that when I do compositing in the performance section I can tick the OpenCL box and the defocus node, glare node, as well as other nodes that blur compute a lot faster than on CPU. For the render passes, I render a quick scene on my CPU without any shaders for the z and object id passes, and use both the render information to create the final image.
Neat. Maybe OpenCl will be a contender one day, I hope. Looking at GPU prices and comparisons, it seems like you get a much bigger bang for you buck with AMD.
I never heared about I may need to download and install the OpenCL Driver seperatly. Is this something different from the OpenCL Driver included within the Catalyst Driver Package ?
But I also never get my HD6850 to render on OpenCL successfully by now. No matter on what driver version or Blender version I tried, it always crashes Blender to the Desktop while compiling the render kernel. So I may give this a try.
I recently gave this a go too, the beta drivers work very well for blender and yes, I was getting the weird lines too, (on a Radeon R9 M275X) but I think I enabled something in kernel_types.h and that fixed it, haven’t tried with MIS though.