How I Built a Production Android Document Scanner in Kotlin — The Hard Parts Nobody Talks About

I spent months building a complete document scanner app in Kotlin with Jetpack Compose. 110 files. 21,000+ lines of code. Along the way I hit problems that no tutorial prepared me for. Here are the hard parts and how I solved them. 1. CameraX Frame Stability Detection The "auto-capture" feature sounds simple: detect when the document is steady and snap. In reality, you need frame-to-frame stability analysis. My approach: calculate an RMS difference between consecutive preview frames. If the RMS stays below a threshold for N consecutive frames, the document is stable. The key insight: sample every 10th pixel. Processing every pixel kills frame rate. Sampling gives you 95% accuracy at 10% of the cost. 2. Invisible OCR Text Layer in PDFs ML Kit gives you OCR text, but positioning it correctly inside a PDF so it is selectable but invisible? That is where tutorials stop and real engineering begins. ML Kit returns bounding boxes for each text block. You map those coordinates from image space