You can, of course, push the problem to the client by using CSS and transparency in the browser. But it's far nicer to do the processing server-side. My experience with OpenGL and GLSL has been very positive so far and I found the GLSL to be relatively readable and quite concise in formulating the process that I wanted to happen, as soon as I had a mental model of how the calculation I wanted to be done could be modelled as a single pass over an array.

Maybe in the next 2 to 5 years, GPUs will become stock hardware, even without the graphics output, because they are incredibly suited to problems that can be formulated in an easily parallelizable way that reads from one or more memory sections and only write to one memory destination. But until then, you will be easier off by using the Inline::C version, especially in such one-off situations where it's likely that the bandwidth needed for uploading/downloading the image outweighs the performance gain of the (relatively simple) operation.