Android, Draw text using system font OpenGLES 2.0

1. Typeface

  you can get typeface info by [Typeface.createFromAsset] method. You don't call this method every frame. Because it happen memory leak problem, so I recommend to use Map class to Manage font data. Below show code using Typeface.createFromAsset.

Hashtable<String, Typeface> m_font_type_face = new Hashtable<String, Typeface>();
	
private ResourceManager()
{
	m_font_type_face.put("CORBEL.TTF", Typeface.createFromAsset(MainActivity.context.getAssets(), "CORBEL.TTF"));
};
	
public Typeface getTypeface(String key)
{
	if(m_font_type_face.containsKey(key))
		return m_font_type_face.get(key);
	return null;
}

※ Use ttf font. Some device don't  support ttc or other fonts.

2. Make Bitmap

  In Android, you can draw a string on Canvas using font predefined. Through Paint class, you can set size, color, etc. I use below code to use in NDK OpenGLES code.

public static void decodeStringTexture(String str,String str_key, String font_name, float fontSize, int r, int g, int b, int a )
{
	Log.i("AndroidOpenGL", "getStringTextureData start, str : "+ str+"  size : "+ fontSize);
	try
	{
		Rect rect 	= new Rect();
		Paint paint 	= new Paint(Paint.FILTER_BITMAP_FLAG);	
		
		paint.setTypeface(ResourceManager.instance.getTypeface(font_name));
		paint.setTextSize(fontSize);
		paint.setAntiAlias(true);
		paint.setFilterBitmap(true);
		paint.setDither(true);
		paint.setTextAlign(Align.LEFT);
		paint.setARGB(a, r, g, b);
		paint.getTextBounds(str,0, str.length(), rect);
		
		int width		= rect.right;
		int height		= rect.height();
		int tex_width	= 8;
		int tex_height	= 8;
		
		while(width > tex_width)
			tex_width *= 2;
		while(height > tex_height)
			tex_height *= 2;
		
		
		Bitmap bitmap 	= Bitmap.createBitmap(tex_width, tex_height, Config.ARGB_8888);
		Canvas canvas 	= new Canvas(bitmap);
		canvas.drawText(str, 0, 0-rect.top, paint);
		
		
		
		ByteBuffer bb 	= ByteBuffer.allocate(tex_width*tex_height*4);
		bitmap.copyPixelsToBuffer(bb);
		bitmap.recycle();
		Log.i("AndroidOpenGL", "getStringTextureData end");
		Log.i("AndroidOpenGL", "Allocated allocate buffer capacity:"+ bb.capacity());
		Log.i("AndroidOpenGL", "getStringTextureData end, bb width:"+ tex_width +"   height:"+tex_height);
		
		onDecodeStringTextureComplete(bb.array(), width, height, tex_width, tex_height, str_key);
	}
	catch(Exception e)
	{
		Log.e("AndroidOpenGL", "getStringTexture failed.\n"+e.toString());
	}
}

paint.setTypeface - Set system font predefined.

paint.antialias - make smooth font

paint.getTextBound - get string width, height

cavas.drawText(str, 0, 0-rect.top, paint)

glTexImage2D's width, height parameter is 2^n(exponent) so I set canvas size exponent of 2.

onDecodeStringTextureComplet is jni call. I send  size of string and size of canvas to calculate tex-coordinate value. For example

m_tex_coord[0] = 0.0f;
m_tex_coord[1] = 0.0f;

m_tex_coord[2] = 0.0f;
m_tex_coord[3] = string_height/canvas_height;

m_tex_coord[4] = string_width/canvas_whdith;
m_tex_coord[5] = 0.0f;

m_tex_coord[6] = string_width/canvas_whdith;
m_tex_coord[7] = string_height/canvas_height;

 lastly you can use send data by glTexImage2D.

3. Result Scene

<OpenGL Diary>