But we know another vector that is also perpendicular to the level set of g. That is the gradient of g.
但我们知道有另外一个向量也是和g的等值面垂直的，这个向量就是g的梯度。
That means that the gradient of f is perpendicular to the level set of g.
也就是说f的梯度向量,应该与g的水平集垂直。
Set your Foreground color to #00315C and Background to #001B32 and choose the Gradient Tool (G) with a gradient type option set to Radial Gradient.
设置前景色为 #00315C ，背景色为 #001B32 ，然后选择渐变工具（G），渐变类型为径向渐变。
Then grab the gradient tool (shorcut ‘G’ on the keyboard) and setup a white to gray to white gradient like the one referenced below.
选择渐变工具（快捷键“G”），设置一个白-灰-白的渐变，如下图。
Click on the gradient Tool (g) and using a Black to 0% opacity gradient click and drag the gradient from the top of the photo to about where the mountains start.
选中渐变工具(G)，使用从黑色到0%不透明度的渐变，在图片上部点击并拖拉到山顶位置。
Click on the gradient Tool (g) and, using a Black to 0%opacity gradient, click and drag the gradient from the top of the photo to about where the mountains start.
选中渐变工具(G)，使用从黑色到0%不透明度的渐变，在图片上部点击并拖拉到山顶位置。
Choose the gradient Tool (g) and with a black, white linear gradient drag from down to top.
选择渐变工具，选择黑-白线性渐变，从下往上拖动(下图中红线所示，从下往上拖动)。
Switch to the Gradient Tool (G).
切换到渐变工具(G)。
Use the Gradient Tool (G) to squish and reposition the gradients.
应用渐变工具（G）调整渐变的形态和位置。
Use the Gradient Tool (G) to squish and reposition the gradients.
应用渐变工具（G）调整渐变的形态和位置。
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