PPI stands for pixels per inch and measures the pixel density of a digital display. A standard desktop monitor at 1920x1080 spread over 24 diagonal inches has roughly 91 PPI; a modern high-resolution laptop screen runs 200-250 PPI; phone screens are routinely 400-500 PPI. Higher PPI screens display the same image with smaller individual pixels, making the image look sharper and more 'photographic'.
PPI matters for architectural rendering because high-PPI screens reveal flaws that low-PPI screens hide. A render that looks crisp on a 90 PPI office monitor may show visible noise or aliasing on a 250 PPI Retina laptop. Conversely, a render optimized for high-PPI display may look unnecessarily soft when viewed on an older monitor.
The relationship between DPI (print) and PPI (screen) trips people up because the terms are often used interchangeably in casual speech. The technical distinction: DPI describes ink dots from a printer; PPI describes pixels on a display. For digital deliverables, focus on pixel dimensions — a 3840x2160 render will look crisp on any modern display whether the display is 91 PPI or 500 PPI. For print deliverables, focus on DPI relative to the printed physical size.
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