I change the api to support an arbitrary affine transform. The current api is
def draw_image(self, gc, x, y, im, dx=None, dy=None, transform=None):
dx, dy is the width and height of the image, i.e.,
The 4 edge points of the image will be
(x, y), (x+dx, y), (x+dx, y+dy), (x, y+dy) after transformed by *transform*.
And if transform involves rotation or skew, the image should also be
rotated and skewed.
Attached is a small example to utilized this functionality.
Note that it only works with the ps-backend.
I personally don't have much use case for rotated or skewed images.
So, it would be great if others can suggest some user-space api.
What I have in my mind is to extend the "extent" keyword of the imshow
and make it optionally take a tuple of 6 numbers, which are (x1,
x_lrc, x2, y1, y_lrc, y2).
x1, x2, y1, y2 are same as the original "extent", and the (x_lrc,
y_lrc) represent the coordinate of the lower-right corner of the
image.
So, the imshow in the attached example will become somthing like
im = plt.imshow(Z, interpolation='nearest', cmap=cm.jet,
origin='lower', extent=[-2, 3, 4, -3, -2, 2])
And, the transformation will be calculated internally by the Image class.
Any thought?
-JJ
On Wed, Dec 16, 2009 at 3:41 PM, Andrew Straw <straw...@astraw.com> wrote:
> Jae-Joon Lee wrote:
>>
>> On Wed, Dec 16, 2009 at 12:59 PM, Jouni K. Seppänen <j...@iki.fi> wrote:
>>
>>>
>>> While the backend API is being changed, would it be similarly easy to
>>> support arbitrary affine transformations? It would make the API more
>>> symmetric, since many other draw_* methods take an affine
>>> transformation, and I guess at least the PS and PDF backends could
>>> easily support this.
>>>
>>>
>>
>> Yes, supporting arbitrary affine transform would not be difficult. On
>> the other hand, I'm not sure if that feature is practically useful,
>> assuming that other rasterizing backend won't implement it. I'll think
>> about it.
>
> I second the notion that affine transformations on images would be useful.
> There are a lot of things one can do with this, such as make
> pseudo-perspective projections of texture-mapped rectangles. This is exactly
> one of the things that I routinely do in Inkscape.
>
> I'm reasonably sure Cairo supports arbitrary affine transformations when
> drawing image data -- I don't know about Agg.
>
> -Andrew
>
#!/usr/bin/env python
import numpy as np
import matplotlib.cm as cm
import matplotlib.mlab as mlab
import matplotlib.pyplot as plt
import matplotlib.transforms as mtransforms
delta = 0.25
x = y = np.arange(-3.0, 3.0, delta)
X, Y = np.meshgrid(x, y)
Z1 = mlab.bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)
Z2 = mlab.bivariate_normal(X, Y, 1.5, 0.5, 1, 1)
Z = Z2-Z1 # difference of Gaussians
im = plt.imshow(Z, interpolation='nearest', cmap=cm.jet,
origin='lower', extent=[-2, 4, -3, 2])
#plt.savefig("test.eps")
def get_rotate_and_skew_transform(im, x3, y3):
x1, x2, y1, y2 = im.get_extent()
tr1 = mtransforms.Affine2D()
tr1.translate(-x1, -y1)
x2a, y2a = tr1.transform_point((x2, y2))
x3a, y3a = tr1.transform_point((x3, y3))
inv_mat = 1./(x2a*y3a-y2a*x3a) * np.mat([[y3a, -y2a],[-x3a, x2a]])
a, b = (inv_mat * np.mat([[x2a], [x2a]])).flat
c, d = (inv_mat * np.mat([[y2a], [0]])).flat
tr2 = mtransforms.Affine2D.from_values(a, c, b, d, 0, 0)
tr = (tr1 + tr2 + mtransforms.Affine2D().translate(x1, y1)).inverted().get_affine()
return tr
x1, x2, y1, y2 = im.get_extent()
x3, y3 = 3, -2
tr = get_rotate_and_skew_transform(im, x3, y3)
# anchor points in circle
plt.plot([x1, x2, x3], [y1, y2, y3], "o")
# original extent in black
plt.plot([x1, x2, x2, x1, x1], [y1, y1, y2, y2, y1], "k-")
trans2 = tr + plt.gca().transData.get_affine()
im.set_transform(trans2)
# transformed extent in red
plt.plot([x1, x2, x2, x1, x1], [y1, y1, y2, y2, y1], "r-", transform=trans2)
plt.savefig("a.eps")
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