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 class="lazy" data-src, Rect dst, Paint paint)
将传入的源图bitmap指定的矩形区域class="lazy" data-src绘制到目标矩形区域dst中，如果矩形区域class="lazy" data-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class="lazy" data-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 class="lazy" data-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.class="lazy" data-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 class="lazy" data-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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