PIL：生成垂直渐变图像

Question

在Android中，我使用以下代码来生成所需的渐变背景：

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

背景从上到下从浅到暗。我想知道如何在带有PIL的Python中执行相同的操作，因为我需要对另一个用Python编写的程序具有相同的效果。

Answer 1

如果您只需要两种颜色，则可以非常简单地完成：

def generate_gradient(
        colour1: str, colour2: str, width: int, height: int) -> Image:
    """Generate a vertical gradient."""
    base = Image.new('RGB', (width, height), colour1)
    top = Image.new('RGB', (width, height), colour2)
    mask = Image.new('L', (width, height))
    mask_data = []
    for y in range(height):
        mask_data.extend([int(255 * (y / height))] * width)
    mask.putdata(mask_data)
    base.paste(top, (0, 0), mask)
    return base

这将创建每种颜色的图层，然后创建透明度根据y位置而变化的蒙版。您可以y / height在第 10 行中将x / width水平渐变替换为 ，或者将x和的任何函数替换y为另一个渐变。

Answer 2

这里显示了绘制多色矩形水平和垂直渐变的方法。

from 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

    @staticmethod
    def from_point(other):
        return Point(other.x, other.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)

    @staticmethod
    def from_points(p1, p2):
        return Rect(p1.x, p1.y, p2.x, p2.y)

    def __str__(self):
        return 'Rect({:d}, {:d}, {:d}, {:d})'.format(self.min.x, self.min.y,
                                                     self.max.x, self.max.x)
    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
    v = float(val-minval) / float(maxval-minval) * 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
    for x in range(rect.min.x, rect.max.x+1):
        f = (x - rect.min.x) / float(rect.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
    for y in range(rect.min.y, rect.max.y+1):
        f = (y - rect.min.y) / float(rect.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)

color_palette = [BLUE, GREEN, RED]
region = Rect(0, 0, 730, 350)
imgx, imgy = region.max.x+1, region.max.y+1
image = Image.new("RGB", (imgx, imgy), 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：

这会插值RGB色彩空间中的中间色彩，但可以使用其他色彩空间-例如，请参见我对问题Range values to pseudocolor的回答。

这可以轻松扩展以生成ARGB（Alpha + RGB）图像。