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How to lower transparency to line in Pillow?

How to lower opacity to line? I would like to lower opacity to one of line in example bellow.
from PIL import Image, ImageDraw
img = Image.new('RGB', (100, 100), (255, 255, 255))
draw = ImageDraw.Draw(img)
draw.line((100, 30, 0, 30), (0, 0, 0), 20)
draw.line((100, 70, 0, 70), (0, 0, 0), 20)
img.show()
I have seen in one example they created opacity like this...
TRANSPARENCY = .25 # Degree of transparency, 0-100%
OPACITY = int(255 * TRANSPARENCY)
But don't know how to apply to one of lines. Any ideas?
EDIT
I made some changes (based on answer of #Pedro Maia), it still doesn't work, just changes a color, it doesn't lower opacity to see background color.
from PIL import Image, ImageDraw
img = Image.new('RGBA', (500, 500), (255, 255, 255))
draw = ImageDraw.Draw(img)
TRANSPARENCY = .25 # Degree of transparency, 0-100%
draw.line((200, 0, 200, 600),(255, 0, 0), 60)
draw.line((500, 100, 0, 100), (0, 0, 0, int(255 * TRANSPARENCY)), 60)
draw.line((500, 400, 0, 400),(0, 0, 0), 60)
img
And I have to convert it to RGB to export it as 'jpg'

You would have to do something like this, which is similar to how the example code works, to do what (I think) you want to. I changed the code you added to your question in the EDIT slightly so it better demonstrates that lines of different amounts of transparency can be drawn.
from PIL import Image, ImageDraw
RED = (255, 0, 0)
BLACK = (0, 0, 0)
WHITE = (255, 255, 255)
# Calculate alpha given a 0-100% opacity value.
opacity = lambda transparency: (int(255 * (transparency/100.)),) # Returns a monuple.
def draw_transp_line(image, xy, color, width=1, joint=None):
""" Draw line with transparent color on the specified image. """
if len(color) < 4: # Missing alpha?
color += opacity(100) # Opaque since alpha wasn't specified.
# Make an overlay image the same size as the specified image, initialized to
# a fully transparent (0% opaque) version of the line color, then draw a
# semi-transparent line on it.
overlay = Image.new('RGBA', image.size, color[:3]+opacity(0))
draw = ImageDraw.Draw(overlay) # Create a context for drawing things on it.
draw.line(xy, color, width, joint)
# Alpha composite the overlay image onto the original.
image.alpha_composite(overlay)
# Create opaque white RGBA background image.
img = Image.new('RGBA', (500, 500), (255, 255, 255)+opacity(100))
draw_transp_line(img, ((200, 0), (200, 600)), RED+opacity(100), 60)
draw_transp_line(img, ((500, 100), (0, 100)), BLACK+opacity(25), 60)
draw_transp_line(img, ((150, 50), (600, 400)), BLACK+opacity(50), 60)
img = img.convert("RGB") # Remove alpha for saving in jpg format.
img.save('transparent_lines.jpg')
img.show()
JPG image created:

With draw.line you can pass as argument RGB or RGBA just pass the value of the transparency:
draw.line((100, 30, 0, 30), (0, 0, 0, int(255 * TRANSPARENCY)), 20)
Also when creating the image set it as RGBA:
img = Image.new('RGBA', (100, 100), (255, 255, 255))

Why does Pillow convert return colours outside the specified palette?

Using Pillow 5.4.1, Python 3.6.8
Given an image image.png with 9 distinct colours, and given a data palette with 5 distinct colours, one would expect that asking pillow to reduce the image to the described palette that the resulting image would contain colours from only that palette.
However, using the im.im.convert method returns an image with colours outside the specified palette; specifically they are always greyscale images (R==B==G values)
Sample Code, outputting the unique set of colours for the original image, palette, and converted image.
from PIL import Image
im = Image.open("image.png")
# create palette from raw data
# colours: Red, Green, Blue, Black, and White (5 total)
RGBBW = [(255,0,0), (0,255,0), (0,0,255), (0,0,0), (255,255,255)]
data = sum([list(x) for x in RGBBW], [])[:256]
pimg = Image.new("P",(16,16))
pimg.putpalette(data)
# Hack
im.convert("RGB")
cim_ = im.im.convert("P", 0, pimg.im)
cim = im._new(cim_).convert("RGB")
def colors(im):
cs = []
for x in range(im.width):
for y in range(im.height):
cs.append(im.getpixel((x,y)))
return list(set(cs))
print("Original: %s" % colors(im))
print("Palette: %s" % RGBBW)
print("Convert: %s" % colors(cim))
Input image: -> <- (3x3 pixel image, all pixels unique colours)
(Larger version, for visualisation only: )
Output:
Original: [(85, 85, 85, 255), (0, 0, 255, 255), (0, 0, 0, 255), (255, 0, 0, 255), (0, 255, 255, 255), (255, 255, 255, 255), (255, 255, 0, 255), (255, 0, 255, 255), (0, 255, 0, 255)]
Palette: [(255, 0, 0), (0, 255, 0), (0, 0, 255), (0, 0, 0), (255, 255, 255)]
Convert: [(252, 252, 252), (0, 0, 255), (255, 0, 0), (0, 0, 0), (170, 170, 170), (0, 255, 0), (84, 84, 84)]
(Note that the hack to prevent dither is a workaround, pending a fix I've contributed to master (yet to be cut into a new release))
The values [(170, 170, 170), (84, 84, 84), (252, 252, 252)] appear in the converted image, but were not specified in the original palette. They all happen to be greyscale.
I think there's something in src/libImaging/Palette.c that's effecting this, but I'm not sure if this is a bug of the code, or a 'feature' of libjpeg

Turns out this issue is both user error and an unexpected initialisation issue.
The initialisation issue: As pointed out in the comments, the palette for a new image is specifically initialised as greyscale.
If we replace the entire palette with our own, then we're fine. Except, I wasn't.
data = sum([list(x) for x in RGBBW], [])[:256]
This line is logically incorrect.
The palette expects a flattened list of up to 256 triples of RGB, that is, an array of max len 768. If the array is anything less than this, then the rest of the greyscale will still be in play.
The better way to re-initialise the palette is to ensure we repeat a value as to override the greyscale.
In this case:
data = (sum([list(x) for x in RGBBW], []) + (RGBBW[-1] * (256 - len(RGBBW))))[:256*3]
That is:
data = (
sum([list(x) for x in RGBBW], []) # flatten the nested array
+ (RGBBW[-1] * (256 - len(RGBBW))) # extend with the last value, to our required length, if needed
)[:256*3] # and trim back, if needed.
This will result in the palette always being 768 length.
Using the last value from our provided array is an arbitrary choice, as is only used as a valid padding value.

How to create overlapping semitransparent shapes on semitransparent background with PIL

I'm trying to create image with semitransparent shapes drawn on transparent background. For some reason instead of staying transparent, the shapes are completely covering those beneath them. My code:
from PIL import Image, ImageDraw
img = Image.new("RGBA", (256, 256), (255,0,0,127))
drawing = ImageDraw.Draw(img, "RGBA")
drawing.ellipse((127-79, 127 - 63, 127 + 47, 127 + 63), fill=(0, 255, 0, 63), outline=(0, 255, 0, 255))
drawing.ellipse((127-47, 127 - 63, 127 + 79, 127 + 63), fill=(0, 0, 255, 63), outline=(0, 0, 255, 255))
img.save("foo.png", "png")
I would expect the result to look something like (except for background not being transparent):but it looks like:
When I try to save it as GIF with img.save("foo.gif", "gif"), result is even worse. Circles are solid, no difference between outline and fill.

As I mentioned in a comment, ImageDraw.Draw doesn't do blending—whatever is drawn replaces whatever pixels that were there previously. To get the effect you want requires drawing things in a two-step process. The ellipse must first be drawn on a blank transparent background, and then that must be alpha-composited with current image (bg_img) to preserve transparency.
In the code below this has been implementing in re-usable function:
from PIL import Image, ImageDraw
def draw_transp_ellipse(img, xy, **kwargs):
""" Draws an ellipse inside the given bounding box onto given image.
Supports transparent colors
"""
transp = Image.new('RGBA', img.size, (0,0,0,0)) # Temp drawing image.
draw = ImageDraw.Draw(transp, "RGBA")
draw.ellipse(xy, **kwargs)
# Alpha composite two images together and replace first with result.
img.paste(Image.alpha_composite(img, transp))
bg_img = Image.new("RGBA", (256, 256), (255, 0, 0, 127)) # Semitransparent background.
draw_transp_ellipse(bg_img, (127-79, 127-63, 127+47, 127+63),
fill=(0, 255, 0, 63), outline=(0, 255, 0, 255))
draw_transp_ellipse(bg_img, (127-47, 127-63, 127+79, 127+63),
fill=(0, 0, 255, 63), outline=(0, 0, 255, 255))
bg_img.save("foo.png")
This is the image it created viewed in my image file editor app which renders semi-transparent portions of images with a checker-board pattern. As you can see the opaque outlines are the only part that isn't.

All 4th dimensions-transparency must be half. Also maybe problem is with drawing. Make mask for each ellipse and then sum up images. Last option- check pil function for bending images. That will propably be most certain and easiest solution.
The function for blending is:
PIL.Image. blend (im1, im2, alpha)

Merging background with transparent image in PIL

I have a png image as background and I want to add a transparent mesh to this background but this doesn't work as expected. The background image is converted to transparent on places where I apply transparent mesh.
I am doing:
from PIL import Image, ImageDraw
map_background = Image.open(MAP_BACKGROUND_FILE).convert('RGBA')
map_mesh = Image.new('RGBA', (width, height), (0, 0, 0, 0))
draw = ImageDraw.Draw(map_mesh)
# Create mesh using: draw.line([...], fill=(255, 255, 255, 50), width=1)
...
map_background.paste(map_mesh, (0, 0), map_mesh)
But the result is:
You can see a chessboard pattern if you look carefully (used in graphics programs as no background). Transparent lines makes the background layer transparent too in places where both layers met. But I only want the transparent line to be added on top of the background.
I can solve it with:
map_background.paste((255,255,255), (0, 0), map_mesh)
but as I use different colors for different lines, I would have to make for every color this process. If I had 100 colors, then I need 100 layers what is not very good solution.

What you are trying to do is to composite the grid onto the background, and for that you need to use Image.blend or Image.composite. Here's an example using the latter to composite red lines with random alpha values onto a white background:
import Image, ImageDraw, random
background = Image.new('RGB', (100, 100), (255, 255, 255))
foreground = Image.new('RGB', (100, 100), (255, 0, 0))
mask = Image.new('L', (100, 100), 0)
draw = ImageDraw.Draw(mask)
for i in range(5, 100, 10):
draw.line((i, 0, i, 100), fill=random.randrange(256))
draw.line((0, i, 100, i), fill=random.randrange(256))
result = Image.composite(background, foreground, mask)
From left to right:
[background] [mask]
[foreground]
[result]
(If you are happy to write the result back to the background image, then you can use one of the masked versions of Image.paste, as pointed out by Paulo Scardine in a deleted answer.)

I had trouble getting the above examples to work well. Instead, this worked for me:
import numpy as np
import Image
import ImageDraw
def add_craters(image, craterization=20.0, width=256, height=256):
foreground = Image.new('RGBA', (width, height), (0, 0, 0, 0))
draw = ImageDraw.Draw(foreground)
for c in range(0, craterization):
x = np.random.randint(10, width-10)
y = np.random.randint(10, height-10)
radius = np.random.randint(2, 10)
dark_color = (0, 0, 0, 128)
draw.ellipse((x-radius, y-radius, x+radius, y+radius), fill=dark_color)
image_new = Image.composite(foreground, image, foreground)
return image_new

Python `aggdraw` module bug (?): visible alpha channel

Imagine simple script:
from PIL import Image
from aggdraw import Draw, Brush
im = Image.new("RGBA", (600, 600), (0, 0, 0, 0))
draw = Draw(im)
brush = Brush("yellow")
draw.polygon(
(
50, 50,
550, 60,
550, 550,
60, 550,
),
None, brush
)
draw.flush()
im.save("2.png")
And the result:
(Sorry for big size but that is more clearly)
And the problem:
Can you see non-yellow and non-white edges? This is the alpha-channel or something.
When I trying to do this only with PIL's Draw object - it looks clearly and good but not anti-aliased.
But with aggdraw's Draw object it looks anti-aliased but with that ugly dirty edges.
I need exactly polygons with non-standard side angles. Simple box is not what I want.
Please, help me with some good optimistic answer how to solve this problem.

It's because your background is black, but transparent. If you set your image background to white, you don't get the visible edges. Either transparent or solid white works in my simple tests.
Try these values:
transBlack = (0, 0, 0, 0) # shows your example with visible edges
solidBlack = (0, 0, 0, 255) # shows shape on a black background
transWhite = (255, 255, 255, 0)
solidWhite = (255, 255, 255, 255)
im = Image.new("RGBA", (600, 600), solidWhite)