在缩放画布绘制文本 [英] Drawing text on a scaled Canvas
问题描述
我有已经迈上了一个画布上的一个问题绘制文本。我要缩放画布以便绘制尺寸和坐标总是在范围画布的宽度和高度的0.0到1.0，即独立的。 （如果这是不良的做法，请随意评论给出的原因是这样的）。虽然我可以缩放画布上绘制线条和圆弧正确，我有很大的困难绘画文字和这个问题似乎只在Android出现4.2
I'm having a problem drawing text on a canvas that has been scaled. I want to scale the canvas so that drawing dimensions and co-ordinates are always in the range 0.0 to 1.0 i.e. independent of Canvas width and height. (If this is poor practice please feel free to comment giving a reason why this is so.) While I can draw lines and arcs on a scaled canvas correctly, I'm having great difficulty painting text and this problem only seems to occur on Android 4.2
作为一个例子，我创建了基于这个网页。
As an example, I have created some code based upon this page here. Not however I have stripped out a lot of the code for clarity.
首先，code正常工作，并作为上面的链接上（虽然剥离下来）：
First, this code works correctly and is as on the above link (albeit stripped down):
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.DashPathEffect;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.Rect;
import android.os.Bundle;
import android.view.View;
public class DrawDemo extends Activity {
DemoView demoview;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
demoview = new DemoView(this);
setContentView(demoview);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
private class DemoView extends View{
private int width = 0;
private int height = 0;
public DemoView(Context context){
super(context);
}
@Override
protected void onSizeChanged (int w, int h, int oldw, int oldh){
width = w;
height = h;
}
@Override protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
// custom drawing code here
// remember: y increases from top to bottom
// x increases from left to right
int x = 0;
int y = 0;
Paint paint = new Paint();
paint.setStyle(Paint.Style.FILL);
// make the entire canvas white
paint.setColor(Color.WHITE);
canvas.drawPaint(paint);
// draw some text using STROKE style
paint.setStyle(Paint.Style.STROKE);
paint.setStrokeWidth(1);
paint.setColor(Color.MAGENTA);
paint.setTextSize(30);
canvas.drawText("Style.STROKE", 75, 75, paint);
}
}
}
接下来，我更换了绝对像素大小与标准化（0-1.0）的价值观和缩放画布：
Next I replaced the absolute pixel sizes with normalised ( 0 to 1.0) values and scaled the canvas:
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.DashPathEffect;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.Rect;
import android.os.Bundle;
import android.view.View;
public class DrawDemo extends Activity {
DemoView demoview;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
demoview = new DemoView(this);
setContentView(demoview);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
private class DemoView extends View{
private int width = 0;
private int height = 0;
public DemoView(Context context){
super(context);
}
@Override
protected void onSizeChanged (int w, int h, int oldw, int oldh){
width = w;
height = h;
}
@Override protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
// custom drawing code here
// remember: y increases from top to bottom
// x increases from left to right
int x = 0;
int y = 0;
Paint paint = new Paint();
paint.setStyle(Paint.Style.FILL);
// make the entire canvas white
paint.setColor(Color.WHITE);
canvas.drawPaint(paint);
// draw some text using STROKE style
canvas.scale(width, height);
paint.setStyle(Paint.Style.STROKE);
//paint.setStrokeWidth(1);
paint.setColor(Color.MAGENTA);
paint.setTextSize(0.2f);
canvas.drawText("Style.STROKE", 0.5f, 0.5f, paint);
}
}
}
其结果是出现在屏幕上什么都没有。可能有人建议我在做什么错？请记住，在后者绘制文本已经为我工作在Android＆LT; 4.2。为code样品的格式道歉，S我仍然不能得到与Stackoverflows code格式交手。
The result is nothing appears on the screen. Could someone suggest what I'm doing wrong? Bear in mind that drawing text in the latter has worked for me on Android < 4.2. Apologies for the formatting of the code sample,s I still cannot get to grips with Stackoverflows code formatting.
推荐答案
在硬件加速上，文本渲染成你上了漆指定字体大小纹理。这意味着，在你的情况，文本是在0.2F像素栅格化。然后，该纹理绘制在时间，这就是为什么你没有看到任何东西放大。
When hardware acceleration is on, text is rendered into a texture at the font size you specify on the paint. This means that in your case, text is rasterized at 0.2f pixels. That texture is then scaled up at drawing time, which is why you're not seeing anything.
虽然这个问题已经在Android中的未来版本中得到解决，我会强烈建议你不用0..1值。你不仅可能碰到precision问题，但你也将迫使渲染管线（软件和硬件）来绕过非常有用的优化。例如，带刻度呈现的文本中的软件变换是使用路径而不是位图的blit呈现
While this issue has been addressed in a future version of Android, I would strongly recommend you didn't use 0..1 values. You might not only run into precision issues but you will also force the rendering pipelines (software and hardware) to bypass very useful optimizations. For instance, text rendered with a scale transform in software is rendered using paths instead of bitmaps blits.
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