PIL：生成垂直渐变图像

Question

在 Android 中，我使用以下代码生成了我需要的渐变背景：

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

背景从上到下由浅变暗。我想知道如何在 Python 中使用 PIL 做同样的事情，因为我需要对另一个用 Python 编写的程序产生相同的效果。

Answer 1

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，但也可以使用其他颜色空间——例如比较我对问题Range values to pseudocolor 的回答的结果。

这可以很容易地扩展为生成 RGBA (RGB+Alpha) 模式的图像。

Answer 2

如果你只需要两种颜色，这可以很简单地完成：

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

这会为每种颜色创建一个图层，然后创建一个具有根据y 位置变化的透明度的蒙版。您可以将第 10 行中的 y / height 替换为 x / width 以获得水平渐变，或者将 x 和 y 的任何函数替换为另一个渐变。

Answer 3

这里是详细说明的技术。你需要 2 层相互叠加，每种颜色一个。然后，您使顶层的透明度增加，底层的透明度降低。对于额外的作业，您可以将透明度更改为递增的对数刻度而不是线性。玩得开心。

Answer 4

对@martineau 的代码进行一些修改，该函数以度为单位处理渐变方向（不仅是垂直或水平）：

from PIL import Image
import math


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

    def rot_x(self, degrees):
        radians = math.radians(degrees)
        return self.x * math.cos(radians) + self.y * math.sin(radians)


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)

    def min_max_rot_x(self, degrees):
        first = True
        for x in [self.min.x, self.max.x]:
            for y in [self.min.y, self.max.y]:
                p = Point(x, y)
                rot_d = p.rot_x(degrees)
                if first:
                    min_d = rot_d
                    max_d = rot_d
                else:
                    min_d = min(min_d, rot_d)
                    max_d = max(max_d, rot_d)
                first = False
        return min_d, max_d

    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 degrees_gradient(im, rect, color_func, color_palette, degrees):
    minval, maxval = 1, len(color_palette)
    delta = maxval - minval
    min_d, max_d = rect.min_max_rot_x(degrees)
    range_d = max_d - min_d
    for x in range(rect.min.x, rect.max.x + 1):
        for y in range(rect.min.y, rect.max.y+1):
            p = Point(x, y)
            f = (p.rot_x(degrees) - min_d) / range_d
            val = minval + f * delta
            color = color_func(minval, maxval, val, color_palette)
            im.putpixel((x, y), color)


def gradient_image(color_palette, degrees):
    region = Rect(0, 0, 600, 400)
    width, height = region.max.x+1, region.max.y+1
    image = Image.new("RGB", (width, height), WHITE)
    degrees_gradient(image, region, gradient_color, color_palette, -degrees)
    return image

这种灵活性的代价是必须逐像素设置颜色，而不是使用线条。