2b:null 2c:["$","title",null,{"children":"Thermal Printer (Print to cheap cat printer with BLE from observable notebook) | Alok Pepakayala | Observable"}] 31:[] 2f:{"id":"f3d45417d149c4c9","slug":null,"title":"Thermal Printer (Print to cheap cat printer with BLE from observable notebook)","thumbnail":"a5111942d4b0605d9435d8a47b4b1d65ab66f3e34bc65197b2425d5642e200cc","access_level":"public","create_time":"$D2025-12-21T05:24:25.037Z","update_time":"$D2025-12-21T05:33:30.525Z","view_time":null,"first_public_time":"$D2025-12-21T05:24:25.037Z","has_unpublished_edits":null,"fork":false,"stars":0,"starred":false,"owner":{"type":"team","id":"ebdd8a722ae381e2","login":"a10k","name":"Alok Pepakayala","avatar_url":"https://avatars-next.observableusercontent.com/f28a7b5f4fb16a63caaace8a87a531e60bea383d6e9d50e126570c77c0814ba9","tier":"starter_2024"},"version":48,"publish_version":null,"role":null,"authored":false,"authors":[{"type":"individual","id":"e594cd7b50bd5352","login":"a10k","name":"Alok Pepakayala","avatar_url":"https://avatars-next.observableusercontent.com/a79accbaf1cb39d92bac556179588a03429b7ed84c64707b7ae8d86c9ffacfc7","tier":null}],"members":[{"type":"individual","id":"e594cd7b50bd5352","login":"a10k","name":"Alok Pepakayala","avatar_url":"https://avatars-next.observableusercontent.com/a79accbaf1cb39d92bac556179588a03429b7ed84c64707b7ae8d86c9ffacfc7","tier":null,"role":"editor"},{"type":"team","id":"ebdd8a722ae381e2","login":"a10k","name":"Alok Pepakayala","avatar_url":"https://avatars-next.observableusercontent.com/f28a7b5f4fb16a63caaace8a87a531e60bea383d6e9d50e126570c77c0814ba9","tier":"starter_2024","role":"editor"}],"comments":[],"custom_thumbnail":false,"trashed":false,"lapsed":null,"subscribed":null,"forks":0,"collection":null,"type":"notebook","body":{"titling":"auto","title":"Thermal Printer (Print to cheap cat printer with BLE from observable notebook)","cells":[{"id":0,"value":"# Thermal Printer (Print to cheap cat printer with BLE from observable notebook)","pinned":false,"hidden":false,"mode":"md","author":"$undefined","summary":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"","database":"$undefined"},{"id":41,"value":"renderBW(image)","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":38,"value":"function renderBW(imageData) {\n const canvas = document.createElement(\"canvas\");\n canvas.width = imageData.width;\n canvas.height = imageData.height;\n const ctx = canvas.getContext(\"2d\");\n ctx.putImageData(imageData, 0, 0);\n return canvas;\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":5,"value":"viewof file = {\n const input = document.createElement(\"input\");\n input.type = \"file\";\n input.accept = \"image/*\";\n return input;\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":9,"value":"image = {\n const img = new Image();\n img.src = URL.createObjectURL(file);\n return img;\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":20,"value":"function resizeImage(img, targetWidth = 384) {\n const scale = targetWidth / img.width;\n const canvas = document.createElement(\"canvas\");\n canvas.width = targetWidth;\n canvas.height = Math.round(img.height * scale);\n\n const ctx = canvas.getContext(\"2d\");\n ctx.drawImage(img, 0, 0, canvas.width, canvas.height);\n return canvas;\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":23,"value":"function toGrayscale(imageData) {\n const d = imageData.data;\n for (let i = 0; i < d.length; i += 4) {\n const g = 0.299 * d[i] + 0.587 * d[i + 1] + 0.114 * d[i + 2];\n d[i] = d[i + 1] = d[i + 2] = g;\n }\n return imageData;\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":26,"value":"function floydSteinberg(imageData) {\n const w = imageData.width;\n const h = imageData.height;\n const d = imageData.data;\n\n for (let y = 0; y < h; y++) {\n for (let x = 0; x < w; x++) {\n const i = (y*w + x)*4;\n const old = d[i];\n const newVal = old < 128 ? 0 : 255;\n const err = old - newVal;\n d[i] = d[i+1] = d[i+2] = newVal;\n\n const distribute = (dx, dy, factor) => {\n const ni = ((y+dy)*w + (x+dx))*4;\n if (ni >= 0 && ni < d.length) {\n d[ni] += err * factor;\n }\n };\n\n distribute(1, 0, 7/16);\n distribute(-1, 1, 3/16);\n distribute(0, 1, 5/16);\n distribute(1, 1, 1/16);\n }\n }\n return imageData;\n}\n","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":28,"value":"function bitmapToEscPos(imageData) {\n const { width, height, data } = imageData;\n const bytes = [];\n\n // ESC * m nL nH\n for (let y = 0; y < height; y += 8) {\n bytes.push(0x1B, 0x2A, 0x00, width & 0xff, (width >> 8) & 0xff);\n\n for (let x = 0; x < width; x++) {\n let byte = 0;\n for (let bit = 0; bit < 8; bit++) {\n const yy = y + bit;\n if (yy >= height) continue;\n const i = (yy * width + x) * 4;\n const pixel = data[i]; // 0 or 255\n if (pixel === 0) byte |= (1 << (7 - bit));\n }\n bytes.push(byte);\n }\n bytes.push(0x0A); // newline\n }\n\n return new Uint8Array(bytes);\n}\n","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":30,"value":"async function connectPrinter() {\n const device = await navigator.bluetooth.requestDevice({\n filters: [{ services: [0xFFE0] }]\n });\n\n const server = await device.gatt.connect();\n const service = await server.getPrimaryService(0xFFE0);\n const characteristic = await service.getCharacteristic(0xFFE1);\n return characteristic;\n}\n","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"},{"id":32,"value":"async function writeChunks(characteristic, data, chunkSize = 20) {\n for (let i = 0; i < data.length; i += chunkSize) {\n const chunk = data.slice(i, i + chunkSize);\n await characteristic.writeValue(chunk);\n await new Promise(r => setTimeout(r, 10)); // printer buffer safety\n }\n}\n","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","providerMetadata":"$undefined","attachments":"$undefined","output":"$undefined","database":"$undefined"}],"files":"$W31","stdlib":"1"}} 30:"$2f:body"