python之PIL : Generating Vertical Gradient Image

在 Android 中，我使用以下代码生成我需要的渐变背景:

<gradient 
    android:angle="90" 
    android:startColor="#40000000" 
    android:endColor="#00000000" 
    android:type="linear" /> 

背景从上到下从亮到相对暗。我想知道如何使用 PIL 在 Python 中执行相同的操作，因为我需要在另一个用 Python 编写的程序上实现相同的效果。

请您参考如下方法:

这里展示了绘制多色矩形水平和垂直渐变的方法。

rom PIL import Image, ImageDraw 
 
BLACK, DARKGRAY, GRAY = ((0,0,0), (63,63,63), (127,127,127)) 
LIGHTGRAY, WHITE = ((191,191,191), (255,255,255)) 
BLUE, GREEN, RED = ((0, 0, 255), (0, 255, 0), (255, 0, 0)) 
 
 
class Point(object): 
    def __init__(self, x, y): 
        self.x, self.y = x, y 
 
class Rect(object): 
    def __init__(self, x1, y1, x2, y2): 
        minx, maxx = (x1,x2) if x1 < x2 else (x2,x1) 
        miny, maxy = (y1,y2) if y1 < y2 else (y2,y1) 
        self.min = Point(minx, miny) 
        self.max = Point(maxx, maxy) 
 
    width  = property(lambda self: self.max.x - self.min.x) 
    height = property(lambda self: self.max.y - self.min.y) 
 
 
def gradient_color(minval, maxval, val, color_palette): 
    """ Computes intermediate RGB color of a value in the range of minval 
        to maxval (inclusive) based on a color_palette representing the range. 
    """ 
    max_index = len(color_palette)-1 
    delta = maxval - minval 
    if delta == 0: 
        delta = 1 
    v = float(val-minval) / delta * max_index 
    i1, i2 = int(v), min(int(v)+1, max_index) 
    (r1, g1, b1), (r2, g2, b2) = color_palette[i1], color_palette[i2] 
    f = v - i1 
    return int(r1 + f*(r2-r1)), int(g1 + f*(g2-g1)), int(b1 + f*(b2-b1)) 
 
def horz_gradient(draw, rect, color_func, color_palette): 
    minval, maxval = 1, len(color_palette) 
    delta = maxval - minval 
    width = float(rect.width)  # Cache. 
    for x in range(rect.min.x, rect.max.x+1): 
        f = (x - rect.min.x) / width 
        val = minval + f * delta 
        color = color_func(minval, maxval, val, color_palette) 
        draw.line([(x, rect.min.y), (x, rect.max.y)], fill=color) 
 
def vert_gradient(draw, rect, color_func, color_palette): 
    minval, maxval = 1, len(color_palette) 
    delta = maxval - minval 
    height = float(rect.height)  # Cache. 
    for y in range(rect.min.y, rect.max.y+1): 
        f = (y - rect.min.y) / height 
        val = minval + f * delta 
        color = color_func(minval, maxval, val, color_palette) 
        draw.line([(rect.min.x, y), (rect.max.x, y)], fill=color) 
 
 
if __name__ == '__main__': 
    # Draw a three color vertical gradient. 
    color_palette = [BLUE, GREEN, RED] 
    region = Rect(0, 0, 730, 350) 
    width, height = region.max.x+1, region.max.y+1 
    image = Image.new("RGB", (width, height), WHITE) 
    draw = ImageDraw.Draw(image) 
    vert_gradient(draw, region, gradient_color, color_palette) 
    image.show() 
    #image.save("vert_gradient.png", "PNG") 
    #print('image saved') 
 

这是它生成并显示的图像:

这会计算 RGB color space 中的中间颜色, 但可以使用其他颜色空间——例如比较我对问题 Range values to pseudocolor 的回答结果.

这可以轻松扩展以生成 RGBA (RGB+Alpha) 模式图像。