[tom]

but I thought that 8 GB o ram is much…

Absolutely, it's not something weak! But, the size of MXI for color ml is xRes * yRes * ( 256 * numEmitters + 8 ) bytes which is something serious.

[christoph]

Thank you Tom. As I needed the high resolution quickly, I divided (rendered) the picture in two halves and put them together in Photoshop.