PencilSheep is a free professional image editor with full GPU acceleration, interactive filter layers, and HDR support. PencilSheep provides all the features you'd expect to find in a professional image editor, such as layers, filters, layer styles, adjustment layers, masks, selections, undo/redo, and various different tools. In addition to that, PencilSheep adds many new features, such as interactive filter layers, full HDR support, and full GPU acceleration. PencilSheep executes all performance-critical operations (blending, filters, paint tools, etc.) on the GPU. Unlike traditional image editors (which either don't use GPU computing at all, or only offer partial GPU acceleration), PencilSheep has been specifically designed from the ground up to fully utilize the GPU for all image manipulations. For many tasks, PencilSheep is orders of magnitude faster than traditional image editors.
In addition to applying filters destructively like in traditional image editors, PencilSheep also allows you to place filters as interactive filter layers anywhere in the filter stack. Filter layers are automatically updated when the underlying layers are changed. Immediate recomputation is made possible by PencilSheep's fully GPU accelerated renderer. PencilSheep supports various different color formats with different ranges and bit depths (up to 32 bit per channel / 256 bit per pixel HDR). Every tool and filter fully supports all available formats.