如何检测透明画布上的形状? [英] How to detect shape on a transparent canvas?
问题描述
我正在寻找一种检测透明PNG中的形状的方法. 例如，我将创建一个940x680的透明画布，然后在该画布中的某个位置放置一个完全不透明的对象.
我希望能够检测到该对象的大小(w，h)以及顶部和左侧的位置.
以下是原始图片的示例:
以下是我要实现的示例(边框重叠，带有顶部和左侧边距数据):
我找到了可以进行透明性检测的资源，但是我不确定如何将这样的内容扩展到所需的内容.
var imgData,
width = 200,
height = 200;
$('#mask').bind('mousemove', function(ev){
if(!imgData){ initCanvas(); }
var imgPos = $(this).offset(),
mousePos = {x : ev.pageX - imgPos.left, y : ev.pageY - imgPos.top},
pixelPos = 4*(mousePos.x + height*mousePos.y),
alpha = imgData.data[pixelPos+3];
$('#opacity').text('Opacity = ' + ((100*alpha/255) << 0) + '%');
});
function initCanvas(){
var canvas = $('<canvas width="'+width+'" height="'+height+'" />')[0],
ctx = canvas.getContext('2d');
ctx.drawImage($('#mask')[0], 0, 0);
imgData = ctx.getImageData(0, 0, width, height);
}
您需要做什么:
- 获取缓冲区
- 获取该缓冲区的32位引用(如果其他像素是透明的，则可以使用Uint32Array缓冲区进行迭代).
- 扫描0-宽度以找到x1边缘
- 扫描宽度-0以找到x2边缘
- 扫描0-高度以找到y1边缘
- 扫描高度-0以找到y2边缘
这些扫描可以合并，但为简单起见，我将分别显示每个步骤.
结果:
在加载图像时将其绘制(如果图像较小，则本示例的其余部分将很浪费，因为您会在绘制图像时知道坐标-假设此处绘制的图像较大，并且其中的图像很小).
(注意:为简单起见，这是未经优化的版本)
ctx.drawImage(this, 0, 0, w, h);
var idata = ctx.getImageData(0, 0, w, h), // get image data for canvas
buffer = idata.data, // get buffer (unnes. step)
buffer32 = new Uint32Array(buffer.buffer), // get a 32-bit representation
x, y, // iterators
x1 = w, y1 = h, x2 = 0, y2 = 0; // min/max values
然后扫描每个边缘.对于左边缘，您从0到每行的宽度进行扫描(未优化):
// get left edge
for(y = 0; y < h; y++) { // line by line
for(x = 0; x < w; x++) { // 0 to width
if (buffer32[x + y * w] > 0) { // non-transparent pixel?
if (x < x1) x1 = x; // if less than current min update
}
}
}
对于右边，您只需反转x迭代器:
// get right edge
for(y = 0; y < h; y++) { // line by line
for(x = w; x >= 0; x--) { // from width to 0
if (buffer32[x + y * w] > 0) {
if (x > x2) x2 = x;
}
}
}
顶部和底部的边缘也一样，只是迭代器是相反的:
// get top edge
for(x = 0; x < w; x++) {
for(y = 0; y < h; y++) {
if (buffer32[x + y * w] > 0) {
if (y < y1) y1 = y;
}
}
}
// get bottom edge
for(x = 0; x < w; x++) {
for(y = h; y >= 0; y--) {
if (buffer32[x + y * w] > 0) {
if (y > y2) y2 = y;
}
}
}
然后得到的区域是:
ctx.strokeRect(x1, y1, x2-x1, y2-y1);
您可以实现各种优化，但是它们完全取决于场景，例如，如果您知道大概的位置，则不必迭代所有行/列.
您可以跳过x个像素，以蛮力猜测其位置，当您发现一个不透明的像素时，可以基于该像素进行最大搜索，依此类推，但这不在本文的讨论范围之内.
希望这会有所帮助！
I'm looking for a method of detecting a shape in a transparent PNG. For example, I will create a transparent canvas of 940x680, then place a fully opaque object somewhere in that canvas.
I want to be able to detect the size (w, h), and top + left location of that object.
Here is an example of the original image:
Here is an example of what I would like to achieve (Bounding box overlay, with top + left margin data):
I've found a resource that does some transparency detection, but I'm not sure how I scale something like this to what I'm looking for.
var imgData,
width = 200,
height = 200;
$('#mask').bind('mousemove', function(ev){
if(!imgData){ initCanvas(); }
var imgPos = $(this).offset(),
mousePos = {x : ev.pageX - imgPos.left, y : ev.pageY - imgPos.top},
pixelPos = 4*(mousePos.x + height*mousePos.y),
alpha = imgData.data[pixelPos+3];
$('#opacity').text('Opacity = ' + ((100*alpha/255) << 0) + '%');
});
function initCanvas(){
var canvas = $('<canvas width="'+width+'" height="'+height+'" />')[0],
ctx = canvas.getContext('2d');
ctx.drawImage($('#mask')[0], 0, 0);
imgData = ctx.getImageData(0, 0, width, height);
}
What you need to do:
- Get the buffer
- Get a 32-bits reference of that buffer (If your other pixels are transparent then you can use a Uint32Array buffer to iterate).
- Scan 0 - width to find x1 edge
- Scan width - 0 to find x2 edge
- Scan 0 - height to find y1 edge
- Scan height - 0 to find y2 edge
These scans can be combined but for simplicity I'll show each step separately.
Online demo of this can be found here.
Result:
When image is loaded draw it in (if the image is small then the rest of this example would be waste as you would know the coordinates when drawing it - assuming here the image you draw is large with a small image inside it)
(note: this is a non-optimized version for the sake of simplicity)
ctx.drawImage(this, 0, 0, w, h);
var idata = ctx.getImageData(0, 0, w, h), // get image data for canvas
buffer = idata.data, // get buffer (unnes. step)
buffer32 = new Uint32Array(buffer.buffer), // get a 32-bit representation
x, y, // iterators
x1 = w, y1 = h, x2 = 0, y2 = 0; // min/max values
Then scan each edge. For left edge you scan from 0 to width for each line (non optimized):
// get left edge
for(y = 0; y < h; y++) { // line by line
for(x = 0; x < w; x++) { // 0 to width
if (buffer32[x + y * w] > 0) { // non-transparent pixel?
if (x < x1) x1 = x; // if less than current min update
}
}
}
For the right edge you just reverse x iterator:
// get right edge
for(y = 0; y < h; y++) { // line by line
for(x = w; x >= 0; x--) { // from width to 0
if (buffer32[x + y * w] > 0) {
if (x > x2) x2 = x;
}
}
}
And the same is for top and bottom edges just that the iterators are reversed:
// get top edge
for(x = 0; x < w; x++) {
for(y = 0; y < h; y++) {
if (buffer32[x + y * w] > 0) {
if (y < y1) y1 = y;
}
}
}
// get bottom edge
for(x = 0; x < w; x++) {
for(y = h; y >= 0; y--) {
if (buffer32[x + y * w] > 0) {
if (y > y2) y2 = y;
}
}
}
The resulting region is then:
ctx.strokeRect(x1, y1, x2-x1, y2-y1);
There are various optimizations you could implement but they depend entirely on the scenario such as if you know approximate placement then you don't have to iterate all lines/columns.
You could do a brute force guess of he placement by skipping x number of pixels and when you found a non-transparent pixel you could make a max search area based on that and so forth, but that is out of scope here.
Hope this helps!
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