Android Canvas和Bitmap结合绘图详解流程
rect/rectf
存储四个值的矩形类:左侧、顶部、右侧和底部。可用于直接在画布上绘制或仅用于存储要绘制的对象的大小。rect和rectf类之间的区别在于 rectf 存储浮点值,而rect类存储整数。
private static bitmap createdrawablebitmap(drawable drawable) { int width = drawable.getintrinsicwidth(); int height = drawable.getintrinsicheight(); if (width <= 0 || height <= 0) { return null; } float scale = math.min(1.0f, ((float) max_image_size) / ((float) (width * height))); if ((drawable instanceof bitmapdrawable) && scale == 1.0f) { return ((bitmapdrawable) drawable).getbitmap(); } int bitmapwidth = (int) (((float) width) * scale); int bitmapheight = (int) (((float) height) * scale); bitmap bitmap = bitmap.createbitmap(bitmapwidth, bitmapheight, config.argb_8888); canvas canvas = new canvas(bitmap); rect existingbounds = drawable.getbounds(); int left = existingbounds.left; int top = existingbounds.top; int right = existingbounds.right; int bottom = existingbounds.bottom; drawable.setbounds(0, 0, bitmapwidth, bitmapheight); drawable.draw(canvas); drawable.setbounds(left, top, right, bottom); return bitmap; }
matrix
一个3 x 3的矩阵,用于存储可用于转换画布的信息。矩阵可以存储以下类型的变换信息:缩放、倾斜、旋转、平移。而每种变换方式都对应着三种方法:set方法将用新值替换当前的matrix,不管之前matrix的值是什么。pre和post 方法将在当前matrix包含的任何内容之前或之后应用新的转换。
matrix m = new matrix(); m.setrotate(90); m.setscale(3f,1f); m.settranslate(200, 200);
只有平移,旋转值和缩放值被重置
matrix m = new matrix(); m.prescale(3f,1f); m.pretranslate(200f, 100f); m.postscale(0.5f, 1f); m.posttranslate(100f, 0f);
先进行平移(200f, 100f),然后进行缩放(3f, 1f),然后进行缩放(0.5f, 1f),最后进行平移(100f, 0f)
matrix m = new matrix(); m.posttranslate(200f, 0f); m.prescale(0.5f, 1f); m.setscale(1f, 1f); m.postscale(5f, 1f); m.pretranslate(200f, 100f);
先进行平移(200f, 100f),然后进行缩放(1f, 1f),最后进行缩放(5f, 1f)。因为用了set方法所以平移(200f, 0f)和缩放(0.5f, 1f)被覆盖,不起作用
假如先进行平移(x, y),再进行缩放(sx, sy),那么看到的平移效果等同于(x*sx, y*sy),因为缩放是将整个画布或者坐标系进行缩放的
canvas
canvas相当于android的画布,可以把画布想象成一块内存空间,也就是一个bitmap。canvas的api提供一整套在这个bitmap上进行绘图的操作方法。
- drawbitmap(bitmap bitmap, matrix matrix, paint paint)
使用指定的矩阵绘制位图,绘制的时候会使用矩阵进行变换,矩阵和画笔可以传入空值
- drawbitmap (bitmap bitmap, rect src, rect dst, paint paint)
将传入的源图bitmap指定的矩形区域src绘制到目标矩形区域dst中,如果矩形区域src传入空值,则表示绘制整个源图到目标矩形区域dst中,绘制的时候源图或子集自动缩放/平移以填充目标矩形。如果绘制对应的画笔通过方法setmaskfilter指定了超出原始位图宽/高的掩码过滤器(如blurmaskfilter),则会位图将继续被绘制,就像在具有clamp模式的着色器中一样。因此,原始宽/高之外的颜色将是复制的边缘颜色。因为源矩形区域src对应的坐标空间是相对于源图的,而目标矩形区域dst对应的坐标空间是绘制视图对应的坐标空间,因此要控制好对应的缩放因子。
- drawbitmap (bitmap bitmap, rect src, rectf dst, paint paint)
矩阵示例:
bitmap background = bitmap.createbitmap((int)width, (int)height, config.argb_8888); float originalwidth = originalimage.getwidth(); float originalheight = originalimage.getheight(); canvas canvas = new canvas(background); float scale = width / originalwidth; float xtranslation = 0.0f; float ytranslation = (height - originalheight * scale) / 2.0f; matrix transformation = new matrix(); transformation.posttranslate(xtranslation, ytranslation); transformation.prescale(scale, scale); paint paint = new paint(); paint.setfilterbitmap(true); canvas.drawbitmap(originalimage, transformation, paint);
矩形区域示例:
public bitmap cropcircle(bitmap bitmap) { bitmap output = bitmap.createbitmap(bitmap.getwidth(), bitmap.getheight(), config.argb_8888); canvas canvas = new canvas(output); final int color = 0xff424242; final paint paint = new paint(); final rect rect = new rect(0, 0, bitmap.getwidth(), bitmap.getheight()); paint.setantialias(true); canvas.drawargb(0, 0, 0, 0); paint.setcolor(color); canvas.drawcircle(bitmap.getwidth() / 2, bitmap.getheight() / 2, bitmap.getwidth()/2, paint); paint.setxfermode(new porterduffxfermode(mode.src_in)); canvas.drawbitmap(bitmap, rect, rect, paint); return output; }
bitmap
位图,点阵图,可以理解为int[] buffer,用来存储每个像素点的容器。
- bitmap.createbitmap(int width, int height, bitmap.config config)
- bitmap.createbitmap(bitmap src)
- bitmap.createbitmap(bitmap source, int x, int y, int width, int height)
- bitmap.createbitmap(bitmap source, int x, int y, int width, int height,matrix m, boolean filter)
- bitmapfactory.decodebytearray(byte[] data, int offset, int length, bitmapfactory.options opts)
- bitmapfactory.decodefile(string pathname, options opts)
- bitmapfactory.decodestream(inputstream is, rect outpadding,options opts)
createbitmap生成示例:
public bitmap transform(bitmap source) { int size = math.min(source.getwidth(), source.getheight()); int x = (source.getwidth() - size) / 2; int y = (source.getheight() - size) / 2; bitmap squaredbitmap = bitmap.createbitmap(source, x, y, size, size); if (squaredbitmap != source) { source.recycle(); } bitmap bitmap = bitmap.createbitmap(size, size, source.getconfig()); canvas canvas = new canvas(bitmap); paint avatarpaint = new paint(); bitmapshader shader = new bitmapshader(squaredbitmap, bitmapshader.tilemode.clamp, bitmapshader.tilemode.clamp); avatarpaint.setshader(shader); paint outlinepaint = new paint(); outlinepaint.setcolor(color.white); outlinepaint.setstyle(paint.style.stroke); outlinepaint.setstrokewidth(stroke_width); outlinepaint.setantialias(true); float r = size / 2f; canvas.drawcircle(r, r, r, avatarpaint); canvas.drawcircle(r, r, r - stroke_width / 2, outlinepaint); squaredbitmap.recycle(); return bitmap; }
bitmapfactory生成示例:
private static bitmap decodesampledbitmapfromurl(string url, int reqwidth, int reqheight) throws ioexception { // first decode with injustdecodebounds=true to check dimensions final options options = new options(); options.injustdecodebounds = true; inputstream stream = fetchstream(url); bitmapfactory.decodestream(stream, null, options); stream.close(); // calculate insamplesize options.insamplesize = calculateinsamplesize(options, reqwidth, reqheight); // decode bitmap with insamplesize set options.injustdecodebounds = false; stream = fetchstream(url); bitmap bitmap = bitmapfactory.decodestream(stream, null, options); stream.close(); return bitmap; } private static inputstream fetchstream(string urlstring) throws illegalstateexception, ioexception { defaulthttpclient httpclient = new defaulthttpclient(); httpget request = new httpget(urlstring); httpresponse response = httpclient.execute(request); return response.getentity().getcontent(); } private static int calculateinsamplesize(options options, int reqwidth, int reqheight) { // raw height and width of image final int height = options.outheight; final int width = options.outwidth; int insamplesize = 1; if (height > reqheight || width > reqwidth) { // calculate ratios of height and width to requested height and width final int heightratio = math.round((float) height / (float) reqheight); final int widthratio = math.round((float) width / (float) reqwidth); // choose the smallest ratio as insamplesize value, this will guarantee // a final image with both dimensions larger than or equal to the // requested height and width. insamplesize = heightratio < widthratio ? heightratio : widthratio; } return insamplesize; }
注意:通过bitmap.createbitmap生成的bitmap对象是可变对象,可以向bitmap上绘制内容,而通过bitmapfactory生成的bitmap对象必须指定bitmapfactory.options.inmutable = true,否则就是不可变对象,不能向上面绘制内容。
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