Pixel recalculation when scaling footage up or down — determines how sharp or soft your size change looks. Bicubic is your friend.
When scaling footage—whether up for DCI projection or down for the web—you're not just deciding on a new size. The computer has to invent millions of new pixels that never existed in the original. Resampling is the mathematical process that defines how this interpolation works. The quality of your scale-up depends on which algorithm you choose—and you'll immediately notice it in sharpness, aliasing, and edge stability.
In practice, you need to distinguish between three scenarios: Upsampling (enlarging) is the critical task. Here, the algorithm interpolates between existing pixels, trying to preserve sharpness without creating artifacts. Downsampling (reducing) is less dramatic but can also fail if you're too aggressive—then you lose detail and get moiré patterns. Nearest Neighbor is fast and brutal: it simply copies the closest pixel. Good for pixel art or for quick previews, but unusable for cinematic material—the stair-step artifacts are immediately visible. Linear Interpolation (Bilinear) smooths more gently, but with larger scaling, it appears messy, fuzzy. Bicubic is your standard for professional work: four neighboring pixels influence each new pixel, resulting in significantly sharper images and less color distortion. Some grading systems and VFX software use even higher-order methods—Lanczos, Mitchell, or custom kernels—but this costs render time.
Practically: In DaVinci Resolve, you set Bicubic in Project Settings; in Nuke, you explicitly select the resampling filter in the Transform node. With extreme upsampling (e.g., SD material to 4K), even Bicubic can reach its limits—then AI-based super-resolution tools can help, but that's another chapter. A common mistake: Avoid multiple scaling operations in a row. It's better to calculate directly to the target size once rather than scaling twice consecutively—each pass loses minimal quality.
Most importantly: Resampling always happens, whether you set it actively or not. If you don't set anything, the software uses defaults—usually Linear or Bicubic. Know your tool and check it with test patterns: checkerboards, sharp lines, fine details. This way, you'll immediately see which algorithm delivers what quality in your workflow.