資料層回應 GeoTIFF 檔案。您可以使用自己的 這項工具可讓你取得所需資料舉例來說 顯示特定區域溫度值的 GeoTIFF 圖片。使用 TypeScript 能將低溫對應到藍色,並將高溫對應到紅色來創造出 彩色圖片,無論是將溫度視覺化,都能立即理解 例如模式
這個 TypeScript 程式碼旨在擷取名為 GeoTIFF 的特殊圖片檔 使用 HTML 畫布 (例如數位相框) 在網站上顯示圖像。 程式碼使用下列元件:
- GeoTIFF 圖片:GeoTIFF 可儲存多層圖像資料, 非常適合用於地圖或科學分析
- RGB 圖片:這些是我們熟悉的圖片類型 (例如 相片)。每個像素都有紅、綠和藍色值 顏色。
- 區塊面板:就像彩繪組合。其中包含 可用來為圖片著色
本頁說明如何取得像素資料值 (儲存在 數位圖片的個別像素,包括色彩值和其他 屬性),並計算 GeoTIFF 的經緯度, 將其儲存在 TypeScript 物件中
以下程式碼片段示範了我們儲存以下項目的資料類型: 一些興趣欄位和資料類型是 「type」 。在這個具體範例中,我們選擇允許進行型別檢查 減少型別錯誤,增加程式碼的可靠性,讓 維護。定義用於儲存該項資料的類型,以便傳回多個值 例如像素值和經緯度定界框
export interface GeoTiff { width: number; height: number; rasters: Array<number>[]; bounds: Bounds; }
核心函式
程式碼有多個可以搭配運作的函式:
renderRGB
:拍攝 RGB GeoTIFF 圖片和選擇性遮罩 (例如 透明度) 會產生網站畫布元素,這個元素會循環顯示 然後為畫布上的對應像素套用顏色。renderPalette
:採用單一資料層與顏色的 GeoTIFF。 區塊面板,會將 GeoTIFF 資料值對應至調色盤中的顏色,然後建立 使用調色盤顏色的新 RGB 圖片,並呼叫renderRGB
來顯示 自訂畫布中的圖片
/** * Renders an RGB GeoTiff image into an HTML canvas. * * The GeoTiff image must include 3 rasters (bands) which * correspond to [Red, Green, Blue] in that order. * * @param {GeoTiff} rgb GeoTiff with RGB values of the image. * @param {GeoTiff} mask Optional mask for transparency, defaults to opaque. * @return {HTMLCanvasElement} Canvas element with the rendered image. */ export function renderRGB(rgb: GeoTiff, mask?: GeoTiff): HTMLCanvasElement { // Create an HTML canvas to draw the image. // https://www.w3schools.com/tags/canvas_createimagedata.asp const canvas = document.createElement('canvas'); // Set the canvas size to the mask size if it's available, // otherwise set it to the RGB data layer size. canvas.width = mask ? mask.width : rgb.width; canvas.height = mask ? mask.height : rgb.height; // Since the mask size can be different than the RGB data layer size, // we calculate the "delta" between the RGB layer size and the canvas/mask // size. For example, if the RGB layer size is the same as the canvas size, // the delta is 1. If the RGB layer size is smaller than the canvas size, // the delta would be greater than 1. // This is used to translate the index from the canvas to the RGB layer. const dw = rgb.width / canvas.width; const dh = rgb.height / canvas.height; // Get the canvas image data buffer. const ctx = canvas.getContext('2d')!; const img = ctx.getImageData(0, 0, canvas.width, canvas.height); // Fill in every pixel in the canvas with the corresponding RGB layer value. // Since Javascript doesn't support multidimensional arrays or tensors, // everything is stored in flat arrays and we have to keep track of the // indices for each row and column ourselves. for (let y = 0; y < canvas.height; y++) { for (let x = 0; x < canvas.width; x++) { // RGB index keeps track of the RGB layer position. // This is multiplied by the deltas since it might be a different // size than the image size. const rgbIdx = Math.floor(y * dh) * rgb.width + Math.floor(x * dw); // Mask index keeps track of the mask layer position. const maskIdx = y * canvas.width + x; // Image index keeps track of the canvas image position. // HTML canvas expects a flat array with consecutive RGBA values. // Each value in the image buffer must be between 0 and 255. // The Alpha value is the transparency of that pixel, // if a mask was not provided, we default to 255 which is opaque. const imgIdx = y * canvas.width * 4 + x * 4; img.data[imgIdx + 0] = rgb.rasters[0][rgbIdx]; // Red img.data[imgIdx + 1] = rgb.rasters[1][rgbIdx]; // Green img.data[imgIdx + 2] = rgb.rasters[2][rgbIdx]; // Blue img.data[imgIdx + 3] = mask // Alpha ? mask.rasters[0][maskIdx] * 255 : 255; } } // Draw the image data buffer into the canvas context. ctx.putImageData(img, 0, 0); return canvas; }
輔助函式
程式碼也包含幾個輔助函式,可啟用額外的 功能:
createPalette
:建立用於為圖片著色的顏色清單 以十六進位顏色代碼清單為基礎。colorToRGB
:轉換顏色代碼,例如「#FF00FF」轉換成紅色、綠色 藍色元件normalize
、lerp
、clamp
:適用於圖片的數學輔助函式 和資料處理之間
/** * Renders a single value GeoTiff image into an HTML canvas. * * The GeoTiff image must include 1 raster (band) which contains * the values we want to display. * * @param {GeoTiff} data GeoTiff with the values of interest. * @param {GeoTiff} mask Optional mask for transparency, defaults to opaque. * @param {string[]} colors Hex color palette, defaults to ['000000', 'ffffff']. * @param {number} min Minimum value of the data range, defaults to 0. * @param {number} max Maximum value of the data range, defaults to 1. * @param {number} index Raster index for the data, defaults to 0. * @return {HTMLCanvasElement} Canvas element with the rendered image. */ export function renderPalette({ data, mask, colors, min, max, index, }: { data: GeoTiff; mask?: GeoTiff; colors?: string[]; min?: number; max?: number; index?: number; }): HTMLCanvasElement { // First create a palette from a list of hex colors. const palette = createPalette(colors ?? ['000000', 'ffffff']); // Normalize each value of our raster/band of interest into indices, // such that they always map into a value within the palette. const indices = data.rasters[index ?? 0] .map((x) => normalize(x, max ?? 1, min ?? 0)) .map((x) => Math.round(x * (palette.length - 1))); return renderRGB( { ...data, // Map each index into the corresponding RGB values. rasters: [ indices.map((i: number) => palette[i].r), indices.map((i: number) => palette[i].g), indices.map((i: number) => palette[i].b), ], }, mask, ); } /** * Creates an {r, g, b} color palette from a hex list of colors. * * Each {r, g, b} value is a number between 0 and 255. * The created palette is always of size 256, regardless of the number of * hex colors passed in. Inbetween values are interpolated. * * @param {string[]} hexColors List of hex colors for the palette. * @return {{r, g, b}[]} RGB values for the color palette. */ export function createPalette(hexColors: string[]): { r: number; g: number; b: number }[] { // Map each hex color into an RGB value. const rgb = hexColors.map(colorToRGB); // Create a palette with 256 colors derived from our rgb colors. const size = 256; const step = (rgb.length - 1) / (size - 1); return Array(size) .fill(0) .map((_, i) => { // Get the lower and upper indices for each color. const index = i * step; const lower = Math.floor(index); const upper = Math.ceil(index); // Interpolate between the colors to get the shades. return { r: lerp(rgb[lower].r, rgb[upper].r, index - lower), g: lerp(rgb[lower].g, rgb[upper].g, index - lower), b: lerp(rgb[lower].b, rgb[upper].b, index - lower), }; }); } /** * Convert a hex color into an {r, g, b} color. * * @param {string} color Hex color like 0099FF or #0099FF. * @return {{r, g, b}} RGB values for that color. */ export function colorToRGB(color: string): { r: number; g: number; b: number } { const hex = color.startsWith('#') ? color.slice(1) : color; return { r: parseInt(hex.substring(0, 2), 16), g: parseInt(hex.substring(2, 4), 16), b: parseInt(hex.substring(4, 6), 16), }; } /** * Normalizes a number to a given data range. * * @param {number} x Value of interest. * @param {number} max Maximum value in data range, defaults to 1. * @param {number} min Minimum value in data range, defaults to 0. * @return {number} Normalized value. */ export function normalize(x: number, max: number = 1, min: number = 0): number { const y = (x - min) / (max - min); return clamp(y, 0, 1); } /** * Calculates the linear interpolation for a value within a range. * * @param {number} x Lower value in the range, when `t` is 0. * @param {number} y Upper value in the range, when `t` is 1. * @param {number} t "Time" between 0 and 1. * @return {number} Inbetween value for that "time". */ export function lerp(x: number, y: number, t: number): number { return x + t * (y - x); } /** * Clamps a value to always be within a range. * * @param {number} x Value to clamp. * @param {number} min Minimum value in the range. * @param {number} max Maximum value in the range. * @return {number} Clamped value. */ export function clamp(x: number, min: number, max: number): number { return Math.min(Math.max(x, min), max); }