NanoVG 优化笔记：性能提高5倍的秘密

NanoVG 优化笔记

nanovg正如其名称所示的那样，是一个非常小巧的矢量绘图函数库。相比cairo和skia的数十万行代码，nanovg不足5000行的C语言代码，称为nano也是名副其实了。nanovg的设计、接口和代码质量都堪称典范，唯一美中不足的就是性能不太理想。特别是在Android的低端机型和大屏幕的机型上，一个简单的界面每秒只能画十几帧。最近我把AWTK移植到Android上时，就碰到了这个尴尬的问题。

经过优化之后，AWTK在低端机型上，整体渲染性能有了3到5倍的提升。这里做个笔记，供有需要的朋友参考。

nanovg的性能瓶颈在于片段着色器（fragment shader），片段着色器可以认为是为GPU提供的一个回调函数，该回调函数在处理每个像素时被调用，在每一帧绘制时都会执行数百万次，可见该函数的对性能的影响是很大的。

我们先看看nanovg的片段着色器（fragment shader）代码：

	static const char* fillFragShader =
		"#ifdef GL_ES
"
		"#if defined(GL_FRAGMENT_PRECISION_HIGH) || defined(NANOVG_GL3)
"
		" precision highp float;
"
		"#else
"
		" precision mediump float;
"
		"#endif
"
		"#endif
"
		"#ifdef NANOVG_GL3
"
		"#ifdef USE_UNIFORMBUFFER
"
		"	layout(std140) uniform frag {
"
		"		mat3 scissorMat;
"
		"		mat3 paintMat;
"
		"		vec4 innerCol;
"
		"		vec4 outerCol;
"
		"		vec2 scissorExt;
"
		"		vec2 scissorScale;
"
		"		vec2 extent;
"
		"		float radius;
"
		"		float feather;
"
		"		float strokeMult;
"
		"		float strokeThr;
"
		"		int texType;
"
		"		int type;
"
		"	};
"
		"#else
" // NANOVG_GL3 && !USE_UNIFORMBUFFER
		"	uniform vec4 frag[UNIFORMARRAY_SIZE];
"
		"#endif
"
		"	uniform sampler2D tex;
"
		"	in vec2 ftcoord;
"
		"	in vec2 fpos;
"
		"	out vec4 outColor;
"
		"#else
" // !NANOVG_GL3
		"	uniform vec4 frag[UNIFORMARRAY_SIZE];
"
		"	uniform sampler2D tex;
"
		"	varying vec2 ftcoord;
"
		"	varying vec2 fpos;
"
		"#endif
"
		"#ifndef USE_UNIFORMBUFFER
"
		"	#define scissorMat mat3(frag[0].xyz, frag[1].xyz, frag[2].xyz)
"
		"	#define paintMat mat3(frag[3].xyz, frag[4].xyz, frag[5].xyz)
"
		"	#define innerCol frag[6]
"
		"	#define outerCol frag[7]
"
		"	#define scissorExt frag[8].xy
"
		"	#define scissorScale frag[8].zw
"
		"	#define extent frag[9].xy
"
		"	#define radius frag[9].z
"
		"	#define feather frag[9].w
"
		"	#define strokeMult frag[10].x
"
		"	#define strokeThr frag[10].y
"
		"	#define texType int(frag[10].z)
"
		"	#define type int(frag[10].w)
"
		"#endif
"
		"
"
		"float sdroundrect(vec2 pt, vec2 ext, float rad) {
"
		"	vec2 ext2 = ext - vec2(rad,rad);
"
		"	vec2 d = abs(pt) - ext2;
"
		"	return min(max(d.x,d.y),0.0) + length(max(d,0.0)) - rad;
"
		"}
"
		"
"
		"// Scissoring
"
		"float scissorMask(vec2 p) {
"
		"	vec2 sc = (abs((scissorMat * vec3(p,1.0)).xy) - scissorExt);
"
		"	sc = vec2(0.5,0.5) - sc * scissorScale;
"
		"	return clamp(sc.x,0.0,1.0) * clamp(sc.y,0.0,1.0);
"
		"}
"
		"#ifdef EDGE_AA
"
		"// Stroke - from [0..1] to clipped pyramid, where the slope is 1px.
"
		"float strokeMask() {
"
		"	return min(1.0, (1.0-abs(ftcoord.x*2.0-1.0))*strokeMult) * min(1.0, ftcoord.y);
"
		"}
"
		"#endif
"
		"
"
		"void main(void) {
"
		"   vec4 result;
"
		"	float scissor = scissorMask(fpos);
"
		"#ifdef EDGE_AA
"
		"	float strokeAlpha = strokeMask();
"
		"	if (strokeAlpha < strokeThr) discard;
"
		"#else
"
		"	float strokeAlpha = 1.0;
"
		"#endif
"
		"	if (type == 0) {			// Gradient
"
		"		// Calculate gradient color using box gradient
"
		"		vec2 pt = (paintMat * vec3(fpos,1.0)).xy;
"
		"		float d = clamp((sdroundrect(pt, extent, radius) + feather*0.5) / feather, 0.0, 1.0);
"
		"		vec4 color = mix(innerCol,outerCol,d);
"
		"		// Combine alpha
"
		"		color *= strokeAlpha * scissor;
"
		"		result = color;
"
		"	} else if (type == 1) {		// Image
"
		"		// Calculate color fron texture
"
		"		vec2 pt = (paintMat * vec3(fpos,1.0)).xy / extent;
"
		"#ifdef NANOVG_GL3
"
		"		vec4 color = texture(tex, pt);
"
		"#else
"
		"		vec4 color = texture2D(tex, pt);
"
		"#endif
"
		"		if (texType == 1) color = vec4(color.xyz*color.w,color.w);"
		"		if (texType == 2) color = vec4(color.x);"
		"		// Apply color tint and alpha.
"
		"		color *= innerCol;
"
		"		// Combine alpha
"
		"		color *= strokeAlpha * scissor;
"
		"		result = color;
"
		"	} else if (type == 2) {		// Stencil fill
"
		"		result = vec4(1,1,1,1);
"
		"	} else if (type == 3) {		// Textured tris
"
		"#ifdef NANOVG_GL3
"
		"		vec4 color = texture(tex, ftcoord);
"
		"#else
"
		"		vec4 color = texture2D(tex, ftcoord);
"
		"#endif
"
		"		if (texType == 1) color = vec4(color.xyz*color.w,color.w);"
		"		if (texType == 2) color = vec4(color.x);"
		"		color *= scissor;
"
		"		result = color * innerCol;
"
		"	}
"
		"#ifdef NANOVG_GL3
"
		"	outColor = result;
"
		"#else
"
		"	gl_FragColor = result;
"
		"#endif
"
		"}
";

它的功能很完整也很复杂，裁剪和反走样都做了处理。仔细分析之后，我发现了几个性能问题：

一、颜色填充的问题

简单颜色填充和渐变颜色填充使用了相同的代码：

		"	if (type == 0) {			// Gradient
"
		"		// Calculate gradient color using box gradient
"
		"		vec2 pt = (paintMat * vec3(fpos,1.0)).xy;
"
		"		float d = clamp((sdroundrect(pt, extent, radius) + feather*0.5) / feather, 0.0, 1.0);
"
		"		vec4 color = mix(innerCol,outerCol,d);
"
		"		// Combine alpha
"
		"		color *= strokeAlpha * scissor;
"
		"		result = color;
"

问题

简单颜色填充只需一条指令，而渐变颜色填充则需要数十条指令。这两种情况重用一段代码，会让简单颜色填充慢10倍以上。

方案

把颜色填充分成以下几种情况，分别进行优化：

• 矩形简单颜色填充。

对于无需裁剪的矩形(这是最常见的情况)，直接赋值即可，性能提高20倍以上。

      " if (type == 5) {    //fast fill color
"
      "   result = innerCol;
"

• 通用多边形简单颜色填充。

去掉渐变的采样函数，性能会提高一倍以上：

    " } else if(type == 7) {      // fill color
"
      "   strokeAlpha = strokeMask();
"
      "   if (strokeAlpha < strokeThr) discard;
"
      "   float scissor = scissorMask(fpos);
"
      "   vec4 color = innerCol;
"
      "   color *= strokeAlpha * scissor;
"
      "   result = color;
"

• 渐变颜色填充(只占极小的部分)。

这种情况非常少见，还是使用之前的代码。

效果：

平均情况，填充性能提高10倍以上！

二、字体的问题

对于文字而言，需要显示的像素和不显示的像素，平均算下来在1:1左右。

		"	} else if (type == 3) {		// Textured tris
"
		"#ifdef NANOVG_GL3
"
		"		vec4 color = texture(tex, ftcoord);
"
		"#else
"
		"		vec4 color = texture2D(tex, ftcoord);
"
		"#endif
"
		"		if (texType == 1) color = vec4(color.xyz*color.w,color.w);"
		"		if (texType == 2) color = vec4(color.x);"
		"		color *= scissor;
"
		"		result = color * innerCol;
"
		"	}
"

问题：

如果显示的像素和不显示的像素都走完整的流程，会浪费调一半的时间。

方案：

• 当color.x < 0.02时直接跳过。
• 裁剪和反走样放到判断语句之后。

      " } else if (type == 3) {   // Textured tris
"
      "#ifdef NANOVG_GL3
"
      "   vec4 color = texture(tex, ftcoord);
"
      "#else
"
      "   vec4 color = texture2D(tex, ftcoord);
"
      "#endif
"
      "   if(color.x < 0.02) discard;
"
      "   strokeAlpha = strokeMask();
"
      "   if (strokeAlpha < strokeThr) discard;
"
      "   float scissor = scissorMask(fpos);
"
      "   color = vec4(color.x);"
      "   color *= scissor;
"
      "   result = color * innerCol;
"
      " }
"

效果：

字体渲染性能提高一倍！

三、反走样的问题

反走样的实现函数如下(其实我也不懂):

		"float strokeMask() {
"
		"	return min(1.0, (1.0-abs(ftcoord.x*2.0-1.0))*strokeMult) * min(1.0, ftcoord.y);
"
		"}
"

问题：

与简单的赋值操作相比，加上反走样功能，性能会下降5-10倍。但是不加反走样功能，绘制多边形时边缘效果比较差。不加不好看，加了又太慢，看起来是个两难的选择。

方案：

矩形填充是可以不用反走样功能的。而90%以上的情况都是矩形填充。矩形填充单独处理，一条指令搞定，性能提高20倍以上：

      " if (type == 5) {    //fast fill color
"
      "   result = innerCol;
"

效果：

配合裁剪和矩形的优化，性能提高10倍以上。

四、裁剪的问题

裁剪放到Shader中虽然合理，但是性能就要大大折扣了。

		"// Scissoring
"
		"float scissorMask(vec2 p) {
"
		"	vec2 sc = (abs((scissorMat * vec3(p,1.0)).xy) - scissorExt);
"
		"	sc = vec2(0.5,0.5) - sc * scissorScale;
"
		"	return clamp(sc.x,0.0,1.0) * clamp(sc.y,0.0,1.0);
"
		"}
"

问题：

与简单的赋值操作相比，加上裁剪功能，性能会下降10以上倍。但是不加裁剪功能，像滚动视图这样的控件就没法实现，这看起来也是个两难的选择。

方案：

而90%以上的填充都是在裁剪区域的内部的，没有必要每个像素都去判断，放在Shader之外进行判断即可。

static int glnvg__pathInScissor(const NVGpath* path, NVGscissor* scissor) {
  int32_t i = 0;
  float cx = scissor->xform[4];
  float cy = scissor->xform[5];
  float hw = scissor->extent[0];
  float hh = scissor->extent[1];

  float l = cx - hw;
  float t = cy - hh;
  float r = l + 2 * hw - 1;
  float b = t + 2 * hh - 1;

  const NVGvertex* verts = path->fill;
  for (i = 0; i < path->nfill; i++) {
    const NVGvertex* iter = verts + i;
    int x = iter->x;
    int y = iter->y;
    if (x < l || x > r || y < t || y > b) {
      return 0;
    }
  }

  return 1;
}

效果：

配合裁剪和矩形的优化，性能提高10倍以上。

五、综合

综合裁剪、反走样和矩形，新增3个类型，进行特殊处理：

• 快速填充无需裁剪的矩形：NSVG_SHADER_FAST_FILLCOLOR
• 快速填充无需裁剪的图片：NSVG_SHADER_FAST_FILLIMG
• 快速用简单颜色填充多边形：NSVG_SHADER_FILLCOLOR

裁剪、反走样和矩形可以组合更多类型，进行更精细的优化。但即使只作这三种情况处理，AWTK在Android平台的整体性能已经有了3-5倍的提高，demoui在我们测试的机型上，都稳稳的保持在60FPS，没有必要为了性能增加它的复杂度了。

详细情况和完整代码请参考AWTK