本文作者html5tricks，转载请注明出处
截图在我们平常电脑应用中非常的广泛，包括开发者和一般的使用用户。今天介绍的这款HTML5截图应用可以帮助你在上传头像前截取自己的头像，你只需要拖拽移动图片即可选中要截取的部分，HTML5会自动将选取的图片自动生成一张新的图片来保存，从而完成截图的功能。另外，该HTML5截图应用还支持按住shift键实现图片同比例缩放。
下面我们来看看实现这款HTML5截图功能应用的过程和源代码，代码主要由HTML、CSS以及jQuery组成。
HTML代码：
<div class="component"> <div class="overlay"> <div class="overlay-inner"></div> </div> <img class="resize-image" src="img/image.jpg" alt="image for resizing"> <button class="btn-crop js-crop">截图<img class="icon-crop" src="img/crop.svg"></button> </div>
很简单的HTML布局，图片时加载了一张svg图。
CSS代码：
.resize-container {
position: relative;
display: inline-block;
cursor: move;
margin: 0 auto;
}
.resize-container img {
display: block
}
.resize-container:hover img,
.resize-container:active img {
outline: 2px dashed rgba(222,60,80,.9);
}
.resize-handle-ne,
.resize-handle-se,
.resize-handle-nw,
.resize-handle-sw {
position: absolute;
display: block;
width: 10px;
height: 10px;
background: rgba(222,60,80,.9);
z-index: 999;
}
.resize-handle-nw {
top: -5px;
left: -5px;
cursor: nw-resize;
}
.resize-handle-sw {
bottom: -5px;
left: -5px;
cursor: sw-resize;
}
.resize-handle-ne {
top: -5px;
right: -5px;
cursor: ne-resize;
}
.resize-handle-se {
bottom: -5px;
right: -5px;
cursor: se-resize;
}
.overlay {
position: absolute;
left: 50%;
top: 50%;
margin-left: -100px;
margin-top: -100px;
z-index: 999;
width: 200px;
height: 200px;
border: solid 2px rgba(222,60,80,.9);
box-sizing: content-box;
pointer-events: none;
}
.overlay:after,
.overlay:before {
content: '';
position: absolute;
display: block;
width: 204px;
height: 40px;
border-left: dashed 2px rgba(222,60,80,.9);
border-right: dashed 2px rgba(222,60,80,.9);
}
.overlay:before {
top: 0;
margin-left: -2px;
margin-top: -40px;
}
.overlay:after {
bottom: 0;
margin-left: -2px;
margin-bottom: -40px;
}
.overlay-inner:after,
.overlay-inner:before {
content: '';
position: absolute;
display: block;
width: 40px;
height: 204px;
border-top: dashed 2px rgba(222,60,80,.9);
border-bottom: dashed 2px rgba(222,60,80,.9);
}
.overlay-inner:before {
left: 0;
margin-left: -40px;
margin-top: -2px;
}
.overlay-inner:after{
right: 0;
margin-right: -40px;
margin-top: -2px;
}
.btn-crop {
position: absolute;
vertical-align: bottom;
right: 5px;
bottom: 5px;
padding: 6px 10px;
z-index: 999;
background-color: rgb(222,60,80);
border: none;
border-radius: 5px;
color: #FFF;
}
.btn-crop img {
vertical-align: middle;
margin-left: 8px;
}
以上的CSS代码主要定义了截图工具的图片拖拽和缩放手柄，以及画布的格子背景，也比较简单。
jQuery代码：
/*if(window.location.protocol == 'file:'){
alert('To test this demo properly please use a local server such as XAMPP or WAMP. See README.md for more details.');
}*/
var resizeableImage = function(image_target) {
// Some variable and settings
var $container,
orig_src = new Image(),
image_target = $(image_target).get(0),
event_state = {},
constrain = false,
min_width = 60, // Change as required
min_height = 60,
max_width = 800, // Change as required
max_height = 900,
resize_canvas = document.createElement('canvas');
init = function(){
// When resizing, we will always use this copy of the original as the base
orig_src.src=image_target.src;
// Wrap the image with the container and add resize handles
$(image_target).wrap('<div class="resize-container"></div>')
.before('<span class="resize-handle resize-handle-nw"></span>')
.before('<span class="resize-handle resize-handle-ne"></span>')
.after('<span class="resize-handle resize-handle-se"></span>')
.after('<span class="resize-handle resize-handle-sw"></span>');
// Assign the container to a variable
$container = $(image_target).parent('.resize-container');
// Add events
$container.on('mousedown touchstart', '.resize-handle', startResize);
$container.on('mousedown touchstart', 'img', startMoving);
$('.js-crop').on('click', crop);
};
startResize = function(e){
e.preventDefault();
e.stopPropagation();
saveEventState(e);
$(document).on('mousemove touchmove', resizing);
$(document).on('mouseup touchend', endResize);
};
endResize = function(e){
e.preventDefault();
$(document).off('mouseup touchend', endResize);
$(document).off('mousemove touchmove', resizing);
};
saveEventState = function(e){
// Save the initial event details and container state
event_state.container_width = $container.width();
event_state.container_height = $container.height();
event_state.container_left = $container.offset().left;
event_state.container_top = $container.offset().top;
event_state.mouse_x = (e.clientX || e.pageX || e.originalEvent.touches[0].clientX) + $(window).scrollLeft();
event_state.mouse_y = (e.clientY || e.pageY || e.originalEvent.touches[0].clientY) + $(window).scrollTop();
// This is a fix for mobile safari
// For some reason it does not allow a direct copy of the touches property
if(typeof e.originalEvent.touches !== 'undefined'){
event_state.touches = [];
$.each(e.originalEvent.touches, function(i, ob){
event_state.touches[i] = {};
event_state.touches[i].clientX = 0+ob.clientX;
event_state.touches[i].clientY = 0+ob.clientY;
});
}
event_state.evnt = e;
};
resizing = function(e){
var mouse={},width,height,left,top,offset=$container.offset();
mouse.x = (e.clientX || e.pageX || e.originalEvent.touches[0].clientX) + $(window).scrollLeft();
mouse.y = (e.clientY || e.pageY || e.originalEvent.touches[0].clientY) + $(window).scrollTop();
// Position image differently depending on the corner dragged and constraints
if( $(event_state.evnt.target).hasClass('resize-handle-se') ){
width = mouse.x - event_state.container_left;
height = mouse.y - event_state.container_top;
left = event_state.container_left;
top = event_state.container_top;
} else if($(event_state.evnt.target).hasClass('resize-handle-sw') ){
width = event_state.container_width - (mouse.x - event_state.container_left);
height = mouse.y - event_state.container_top;
left = mouse.x;
top = event_state.container_top;
} else if($(event_state.evnt.target).hasClass('resize-handle-nw') ){
width = event_state.container_width - (mouse.x - event_state.container_left);
height = event_state.container_height - (mouse.y - event_state.container_top);
left = mouse.x;
top = mouse.y;
if(constrain || e.shiftKey){
top = mouse.y - ((width / orig_src.width * orig_src.height) - height);
}
} else if($(event_state.evnt.target).hasClass('resize-handle-ne') ){
width = mouse.x - event_state.container_left;
height = event_state.container_height - (mouse.y - event_state.container_top);
left = event_state.container_left;
top = mouse.y;
if(constrain || e.shiftKey){
top = mouse.y - ((width / orig_src.width * orig_src.height) - height);
}
}
// Optionally maintain aspect ratio
if(constrain || e.shiftKey){
height = width / orig_src.width * orig_src.height;
}
if(width > min_width && height > min_height && width < max_width && height < max_height){
// To improve performance you might limit how often resizeImage() is called
resizeImage(width, height);
// Without this Firefox will not re-calculate the the image dimensions until drag end
$container.offset({'left': left, 'top': top});
}
}
resizeImage = function(width, height){
resize_canvas.width = width;
resize_canvas.height = height;
resize_canvas.getContext('2d').drawImage(orig_src, 0, 0, width, height);
$(image_target).attr('src', resize_canvas.toDataURL("image/png"));
};
startMoving = function(e){
e.preventDefault();
e.stopPropagation();
saveEventState(e);
$(document).on('mousemove touchmove', moving);
$(document).on('mouseup touchend', endMoving);
};
endMoving = function(e){
e.preventDefault();
$(document).off('mouseup touchend', endMoving);
$(document).off('mousemove touchmove', moving);
};
moving = function(e){
var mouse={}, touches;
e.preventDefault();
e.stopPropagation();
touches = e.originalEvent.touches;
mouse.x = (e.clientX || e.pageX || touches[0].clientX) + $(window).scrollLeft();
mouse.y = (e.clientY || e.pageY || touches[0].clientY) + $(window).scrollTop();
$container.offset({
'left': mouse.x - ( event_state.mouse_x - event_state.container_left ),
'top': mouse.y - ( event_state.mouse_y - event_state.container_top )
});
// Watch for pinch zoom gesture while moving
if(event_state.touches && event_state.touches.length > 1 && touches.length > 1){
var width = event_state.container_width, height = event_state.container_height;
var a = event_state.touches[0].clientX - event_state.touches[1].clientX;
a = a * a;
var b = event_state.touches[0].clientY - event_state.touches[1].clientY;
b = b * b;
var dist1 = Math.sqrt( a + b );
a = e.originalEvent.touches[0].clientX - touches[1].clientX;
a = a * a;
b = e.originalEvent.touches[0].clientY - touches[1].clientY;
b = b * b;
var dist2 = Math.sqrt( a + b );
var ratio = dist2 /dist1;
width = width * ratio;
height = height * ratio;
// To improve performance you might limit how often resizeImage() is called
resizeImage(width, height);
}
};
crop = function(){
//Find the part of the image that is inside the crop box
var crop_canvas,
left = $('.overlay').offset().left - $container.offset().left,
top = $('.overlay').offset().top - $container.offset().top,
width = $('.overlay').width(),
height = $('.overlay').height();
crop_canvas = document.createElement('canvas');
crop_canvas.width = width;
crop_canvas.height = height;
crop_canvas.getContext('2d').drawImage(image_target, left, top, width, height, 0, 0, width, height);
window.open(crop_canvas.toDataURL("image/png"));
}
init();
};
// Kick everything off with the target image
resizeableImage($('.resize-image'));
这里主要完成了一系列截图逻辑，其实内部是canvas操作，我们之前也对canvas的灵活性有一些介绍。
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