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)。
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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