Sobel算子边缘检测

2017-01-17

用Sobel算子来进行边缘检测
应该知道除了Sobel算子以外还有几种常用的算子。

Roberts

$$\left[\begin{matrix}
-1 & 0 \
0 & 1
\end{matrix}\right]$$$$\left[\begin{matrix}
0 & -1 \
1 & 0
\end{matrix}\right]$$

Prewitt

$$\left[\begin{matrix}
-1 & -1 & -1 \
0 & 0 & 0 \
1 & 1 & 1
\end{matrix}\right]$$$$\left[\begin{matrix}
-1 & 0 & 1 \
-1 & 0 & 1 \
-1 & 0 & 1
\end{matrix}\right]$$

Sobel

$$\left[\begin{matrix}
-1 & -2 & -1 \
0 & 0 & 0 \
1 & 2 & 1
\end{matrix}\right]$$$$\left[\begin{matrix}
-1 & 0 & 1 \
-2 & 0 & 2 \
-1 & 0 & 1
\end{matrix}\right]$$

我们取得一个纹素的大小，即1/texelSize。然后根据现在的纹理坐标前后左右偏移一个像素，得到9个方向的纹理坐标。
然后用Sobel函数算得该纹理坐标的卷积值

_EdgeOnly = 0时显示原图像和边缘的叠加图像，即withEdgeColor

_EdgeOnly = 1时只显示边缘，其他部分为背景色，即只显示onlyEdgeColor

v2f vert(appdata_img v)
          {
              v2f o;
              o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
              half2 uv = v.texcoord;

              o.uv[0] = uv + _MainTex_TexelSize.xy * half2(-1, -1);
              o.uv[1] = uv + _MainTex_TexelSize.xy * half2(0, -1);
              o.uv[2] = uv + _MainTex_TexelSize.xy * half2(1, -1);
              o.uv[3] = uv + _MainTex_TexelSize.xy * half2(-1, 0);
              o.uv[4] = uv + _MainTex_TexelSize.xy * half2(0, 0);
              o.uv[5] = uv + _MainTex_TexelSize.xy * half2(1, 0);
              o.uv[6] = uv + _MainTex_TexelSize.xy * half2(-1, 1);
              o.uv[7] = uv + _MainTex_TexelSize.xy * half2(0, 1);
              o.uv[8] = uv + _MainTex_TexelSize.xy * half2(1, 1);

              return o;
          }

          fixed luminance(fixed4 color)
          {
              return 0.2125 * color.r + 0.7154 * color.g + 0.0721 * color.b;
          }

          half Sobel(v2f i)
          {
              const half Gx[9] = { -1, -2, -1,
                  0,  0,  0,
                  1,  2,  1 };
              const half Gy[9] = { -1,  0,  1,
                  -2,  0,  2,
                  -1,  0,  1 };
              half texColor;
              half edgeX = 0;
              half edgeY = 0;
              for (int it = 0; it < 9; it++)
              {
                  texColor = luminance(tex2D(_MainTex, i.uv[it]));
                  edgeX += texColor * Gx[it];
                  edgeY += texColor * Gy[it];
              }
              half edge = 1 - abs(edgeX) - abs(edgeY);
              return edge;
          }

          fixed4 frag(v2f i) : SV_Target
          {
              half edge = Sobel(i);
              fixed4 withEdgeColor = lerp(_EdgeColor, tex2D(_MainTex, i.uv[4]), edge);
              fixed4 onlyEdgeColor = lerp(_EdgeColor, _BackgroundColor, edge);
              return lerp(withEdgeColor, onlyEdgeColor, _EdgeOnly);
          }