While searching for a simpler way to generate simple images from a Python snakelet, I hit upon this pure Ruby Sparklines generator and decided to port the canvas bits to Python.
As is, the canvas class lets you draw points, anti-aliased lines and polylines, as well as saving the result to a buffer in PNG format (which you can just spit out to the browser). I will eventually port the Sparklines bits as well, but this may be of immediate use to somebody.
After a few simple optimizations this turned out to be surprisingly fast, but you should nevertheless try to cache the output and use it for consecutive HTTP requests.
I will be working on adding a few more graphics primitives, and this will be updated piecemeal as they're properly tested.
Mini-FAQ:
- Why not use PIL or GD? - because either of them is overkill for most purposes. This can be deployed just about anywhere, even on hosting accounts without shell access: No installation, no native bindings, no dependencies.
- How fast is it? - Surprisingly fast, actually. Gradients and file operations take a while, but line primitives and block copies are very fast.
- What about text? - I'm working on it. If you're in a hurry, generate a PNG image of your font and use copyRect from an auxiliary canvas containing it - that's what I'm doing...
- What about line styles? - Not necessary for what I'm doing, so they're highly unlikely to crop up here.
- It blows up loading large images! - yes, it does. Usually on zlib, which is not a big concern - I'm using it to manipulate 256x256 tiles with success. It can create large images though (1024x768 at least).
Revision History:
- 0.7 - Near-complete PNG file decoding (not fully tested yet, but loads most images correctly), lots of miscellaneous tweaks.
- 0.6 - dramatically faster file save (thanks to reading this page)
- 0.5 - first stab at loading PNG images from files (limited to identically-formatted PNG files). This is a small step towards supporting arbitrary-width bitmapped fonts (which I intend to load from a linear, 1-character-height PNG file).
- 0.3 - added gradient primitives
- 0.2 - offset fixes
- 0.1 - initial port
#!/usr/bin/env python
"""Simple PNG Canvas for Python"""
__version__ = "0.7"
__author__ = "Rui Carmo (http://the.taoofmac.com)"
__copyright__ = "CC Attribution-NonCommercial-NoDerivs 2.0 Rui Carmo"
__contributors__ = "http://collaboa.weed.rbse.com/repository/file/branches/pgsql/lib/spark_pr.rb"
import zlib, struct
signature = struct.pack("8B", 137, 80, 78, 71, 13, 10, 26, 10)
# alpha blends two colors, using the alpha given by c2
def blend(c1, c2):
return [c1[i]*(0xFF-c2[3]) + c2[i]*c2[3] >> 8 for i in range(3)]
# calculate a new alpha given a 0-0xFF intensity
def intensity(c,i):
return [c[0],c[1],c[2],(c[3]*i) >> 8]
# calculate perceptive grayscale value
def grayscale(c):
return int(c[0]*0.3 + c[1]*0.59 + c[2]*0.11)
# calculate gradient colors
def gradientList(start,end,steps):
delta = [end[i] - start[i] for i in range(4)]
grad = []
for i in range(steps+1):
grad.append([start[j] + (delta[j]*i)/steps for j in range(4)])
return grad
class PNGCanvas:
def __init__(self, width, height,bgcolor=[0xff,0xff,0xff,0xff],color=[0,0,0,0xff]):
self.canvas = []
self.width = width
self.height = height
self.color = color #rgba
bgcolor = bgcolor[0:3] # we don't need alpha for background
for i in range(height):
self.canvas.append([bgcolor] * width)
def point(self,x,y,color=None):
if x<0 or y<0 or x>self.width-1 or y>self.height-1: return
if color == None: color = self.color
self.canvas[y][x] = blend(self.canvas[y][x],color)
def _rectHelper(self,x0,y0,x1,y1):
x0, y0, x1, y1 = int(x0), int(y0), int(x1), int(y1)
if x0 > x1: x0, x1 = x1, x0
if y0 > y1: y0, y1 = y1, y0
return [x0,y0,x1,y1]
def verticalGradient(self,x0,y0,x1,y1,start,end):
x0, y0, x1, y1 = self._rectHelper(x0,y0,x1,y1)
grad = gradientList(start,end,y1-y0)
for x in range(x0, x1+1):
for y in range(y0, y1+1):
self.point(x,y,grad[y-y0])
def rectangle(self,x0,y0,x1,y1):
x0, y0, x1, y1 = self._rectHelper(x0,y0,x1,y1)
self.polyline([[x0,y0],[x1,y0],[x1,y1],[x0,y1],[x0,y0]])
def filledRectangle(self,x0,y0,x1,y1):
x0, y0, x1, y1 = self._rectHelper(x0,y0,x1,y1)
for x in range(x0, x1+1):
for y in range(y0, y1+1):
self.point(x,y,self.color)
def copyRect(self,x0,y0,x1,y1,dx,dy,destination):
x0, y0, x1, y1 = self._rectHelper(x0,y0,x1,y1)
for x in range(x0, x1+1):
for y in range(y0, y1+1):
destination.canvas[dy+y-y0][dx+x-x0] = self.canvas[y][x]
def blendRect(self,x0,y0,x1,y1,dx,dy,destination,alpha=0xff):
x0, y0, x1, y1 = self._rectHelper(x0,y0,x1,y1)
for x in range(x0, x1+1):
for y in range(y0, y1+1):
rgba = self.canvas[y][x] + [alpha]
destination.point(dx+x-x0,dy+y-y0,rgba)
# draw a line using Xiaolin Wu's antialiasing technique
def line(self,x0, y0, x1, y1):
# clean params
x0, y0, x1, y1 = int(x0), int(y0), int(x1), int(y1)
if y0>y1:
y0, y1, x0, x1 = y1, y0, x1, x0
dx = x1-x0
if dx < 0:
sx = -1
else:
sx = 1
dx *= sx
dy = y1-y0
# 'easy' cases
if dy == 0:
for x in range(x0,x1,sx):
self.point(x, y0)
return
if dx == 0:
for y in range(y0,y1):
self.point(x0, y)
self.point(x1, y1)
return
if dx == dy:
for x in range(x0,x1,sx):
self.point(x, y0)
y0 = y0 + 1
return
# main loop
self.point(x0, y0)
e_acc = 0
if dy > dx: # vertical displacement
e = (dx << 16) / dy
for i in range(y0,y1-1):
e_acc_temp, e_acc = e_acc, (e_acc + e) & 0xFFFF
if (e_acc <= e_acc_temp):
x0 = x0 + sx
w = 0xFF-(e_acc >> 8)
self.point(x0, y0, intensity(self.color,(w)))
y0 = y0 + 1
self.point(x0 + sx, y0, intensity(self.color,(0xFF-w)))
self.point(x1, y1)
return
# horizontal displacement
e = (dy << 16) / dx
for i in range(x0,x1-sx,sx):
e_acc_temp, e_acc = e_acc, (e_acc + e) & 0xFFFF
if (e_acc <= e_acc_temp):
y0 = y0 + 1
w = 0xFF-(e_acc >> 8)
self.point(x0, y0, intensity(self.color,(w)))
x0 = x0 + sx
self.point(x0, y0 + 1, intensity(self.color,(0xFF-w)))
self.point(x1, y1)
def polyline(self,arr):
for i in range(0,len(arr)-1):
self.line(arr[i][0],arr[i][1],arr[i+1][0], arr[i+1][1])
def dump(self):
raw_list = []
for y in range(self.height):
raw_list.append(chr(0)) # filter type 0 (None)
for x in range(self.width):
raw_list.append(struct.pack("!3B",*self.canvas[y][x]))
raw_data = ''.join(raw_list)
# 8-bit image represented as RGB tuples
# simple transparency, alpha is pure white
return signature + \
self.pack_chunk('IHDR', struct.pack("!2I5B",self.width,self.height,8,2,0,0,0)) + \
self.pack_chunk('tRNS', struct.pack("!6B",0xFF,0xFF,0xFF,0xFF,0xFF,0xFF)) + \
self.pack_chunk('IDAT', zlib.compress(raw_data,9)) + \
self.pack_chunk('IEND', '')
def pack_chunk(self,tag,data):
to_check = tag + data
return struct.pack("!I",len(data)) + to_check + struct.pack("!I",zlib.crc32(to_check))
def load(self,f):
assert f.read(8) == signature
self.canvas=[]
for tag, data in self.chunks(f):
if tag == "IHDR":
( width,
height,
bitdepth,
colortype,
compression, filter, interlace ) = struct.unpack("!2I5B",data)
self.width = width
self.height = height
if (bitdepth,colortype,compression, filter, interlace) != (8,2,0,0,0):
raise TypeError('Unsupported PNG format')
# we ignore tRNS because we use pure white as alpha anyway
elif tag == 'IDAT':
raw_data = zlib.decompress(data)
rows = []
i = 0
for y in range(height):
filtertype = ord(raw_data[i])
i = i + 1
cur = [ord(x) for x in raw_data[i:i+width*3]]
if y == 0:
rgb = self.defilter(cur,None,filtertype)
else:
rgb = self.defilter(cur,prev,filtertype)
prev = cur
i = i+width*3
row = []
j = 0
for x in range(width):
pixel = rgb[j:j+3]
row.append(pixel)
j = j + 3
self.canvas.append(row)
def defilter(self,cur,prev,filtertype,bpp=3):
if filtertype == 0: # No filter
return cur
elif filtertype == 1: # Sub
xp = 0
for xc in range(bpp,len(cur)):
cur[xc] = (cur[xc] + cur[xp]) % 256
xp = xp + 1
elif filtertype == 2: # Up
for xc in range(len(cur)):
cur[xc] = (cur[xc] + prev[xc]) % 256
elif filtertype == 3: # Average
xp = 0
for xc in range(len(cur)):
cur[xc] = (cur[xc] + (cur[xp] + prev[xc])/2) % 256
xp = xp + 1
elif filtertype == 4: # Paeth
xp = 0
for i in range(bpp):
cur[i] = (cur[i] + prev[i]) % 256
for xc in range(bpp,len(cur)):
a = cur[xp]
b = prev[xc]
c = prev[xp]
p = a + b - c
pa = abs(p - a)
pb = abs(p - b)
pc = abs(p - c)
if pa <= pb and pa <= pc:
value = a
elif pb <= pc:
value = b
else:
value = c
cur[xc] = (cur[xc] + value) % 256
xp = xp + 1
else:
raise TypeError('Unrecognized scanline filter type')
return cur
def chunks(self,f):
while 1:
try:
length = struct.unpack("!I",f.read(4))[0]
tag = f.read(4)
data = f.read(length)
crc = struct.unpack("!i",f.read(4))[0]
except:
return
if zlib.crc32(tag + data) != crc:
raise IOError
yield [tag,data]
if __name__ == '__main__':
width = 128
height = 64
print "Creating Canvas..."
c = PNGCanvas(width,height)
c.color = [0xff,0,0,0xff]
c.rectangle(0,0,width-1,height-1)
print "Generating Gradient..."
c.verticalGradient(1,1,width-2, height-2,[0xff,0,0,0xff],[0x20,0,0xff,0x80])
print "Drawing Lines..."
c.color = [0,0,0,0xff]
c.line(0,0,width-1,height-1)
c.line(0,0,width/2,height-1)
c.line(0,0,width-1,height/2)
# Copy Rect to Self
print "Copy Rect"
c.copyRect(1,1,width/2-1,height/2-1,0,height/2,c)
# Blend Rect to Self
print "Blend Rect"
c.blendRect(1,1,width/2-1,height/2-1,width/2,0,c)
# Write test
print "Writing to file..."
f = open("test.png", "wb")
f.write(c.dump())
f.close()
# Read test
print "Reading from file..."
f = open("test.png", "rb")
c.load(f)
f.close()
# Write back
print "Writing to new file..."
f = open("recycle.png","wb")
f.write(c.dump())
f.close()