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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