javascript 相关问题

我正在尝试测试驱动 jsfeat.js 但只看到黑色画布输出。我究竟做错了什么？图像{ 边框：1px实心红色； </desc> <question vote="0"> <p>我正在尝试测试驱动<a href="https://github.com/inspirit/jsfeat" rel="nofollow noreferrer">jsfeat.js</a>，但只看到黑色画布输出。我做错了什么？</p> <pre><code><html> <head> <style> img { border: solid 1px red; width: 20vw; } canvas { border: solid 1px blue; width: 80vw; } </style> <script src="https://cdnjs.cloudflare.com/ajax/libs/jsfeat/0.0.8/jsfeat-min.js"></script> </head> <body> <img src='http://localhost/.../images/test-image.jpg'> <script> function processImage(canvas, img, radius, low_threshold, high_threshold) { let w=img.naturalWidth; let h=img.naturalHeight; canvas.width = w; canvas.height = h; const ctx = canvas.getContext('2d'); let img_u8 = new jsfeat.matrix_t(w, h, jsfeat.U8_t | jsfeat.C1_t); var imageData = ctx.getImageData(0, 0, w, h); jsfeat.imgproc.grayscale(imageData.data, w, h, img_u8); // render result back to canvas var data_u32 = new Uint32Array(imageData.data.buffer); var alpha = (0xff << 24); var i = img_u8.cols*img_u8.rows, pix = 0; while(--i >= 0) { pix = img_u8.data[i]; data_u32[i] = alpha | (pix << 16) | (pix << 8) | pix; } ctx.putImageData(imageData, 0, 0); } let canvas = document.createElement('canvas'); document.body.appendChild(canvas); const img = document.querySelector('img'); let radius=3; let low_threshold=127; let high_threshold=127; if (img.complete) { console.log('img already loaded'); processImage(canvas, img,radius, low_threshold, high_threshold); } else { img.onload = function () { console.log('img loaded'); processImage(canvas, img,radius, low_threshold, high_threshold); }; img.onerror = function () { console.error('Failed to load the image.'); }; } </script> </body> </html> </code></pre> </question> <answer tick="false" vote="0"> <p>在处理图像数据之前，您需要将 <pre><code>img</code></pre> 绘制到画布中，因为 <pre><code>ctx.getImageData(0, 0, w, h)</code></pre> 正在尝试从画布中获取图像数据，而画布在脚本的这个阶段是空的。</p> <pre><code>function processImage(canvas, img, radius, low_threshold, high_threshold) { const ctx = canvas.getContext('2d'); canvas.width = img.naturalWidth; canvas.height = img.naturalHeight; ctx.drawImage(img, 0, 0, img.naturalWidth, img.naturalHeight); let img_u8 = new jsfeat.matrix_t(canvas.width, canvas.height, jsfeat.U8_t | jsfeat.C1_t); let imageData = ctx.getImageData(0, 0, canvas.width, canvas.height); jsfeat.imgproc.grayscale(imageData.data, canvas.width, canvas.height, img_u8); // render result back to canvas var data_u32 = new Uint32Array(imageData.data.buffer); var alpha = (0xff << 24); var i = img_u8.cols*img_u8.rows, pix = 0; while(--i >= 0) { pix = img_u8.data[i]; data_u32[i] = alpha | (pix << 16) | (pix << 8) | pix; } ctx.putImageData(imageData, 0, 0); } </code></pre> </answer> </body></html>

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