[cairo] transforming text

[Behdad Esfahbod]

On 08/05/2009 04:31 PM, Matt Bartolome wrote:
> On Wed, Aug 5, 2009 at 11:08 AM, Behdad Esfahbod<behdad at behdad.org>  wrote:
>> On 08/05/2009 01:47 PM, Matt Bartolome wrote:
>>> Hi,
>>> I'm drawing in geographic units so I applied a transform and translate
>>> to draw correctly in pixel space. This works fine. When I draw the
>>> text using the same transformation it appears vertically reflected
>>> (this is what I would expect it to do). How would you go about drawing
>>> the text so it doesn't appear flipped, but still in the correct x,y
>>> position and rotation? Attached is a clip of the image I'm getting,
>>> obviously the text is wrong and is what I'm trying to correct.
>> Hi Matt,
>>
>> Use a font matrix that reflects it back to normal.  cairo_set_font_matrix()
>> for example.  It normally is just a scale the size of the font size.
>
> Thanks for your help behdad,
>
> Hmm, I'm still lost with how to use set_font_matrix. When I set it (I
> must be setting it wrong) the output image isn't changing.

cairo_set_font_size() overrides the font matrix.  What you can do is 
get_font_matrix() *after* calling set_font_size(), do a 
cairo_matrix_scale(1,-1) on it, and set it back.  Or instead of calling 
cairo_set_font_size, do a cairo_matrix_scale(fontsize,-fontsize) manually.

behdad


> # scale
> ctx.scale(scale,scale)
>
> # flip y
> matrix = cairo.Matrix(1, 0, 0, -1, 0, 0)
> ctx.transform(matrix)
>
> # output from 0,0
> ctx.translate(xmin, -ymax)
>
> # draw 2d profile line components
> ctx = horizontal_center(ctx, sticks)
> ctx = stick_head(ctx, sticks)
> ctx = surface_elevations(ctx, sticks)
>
> # now draw text
> # I would think this would be where I would flip the text somehow in
> the current matrix so it draws in the same place but is reflected to
> the normal position, but not sure how to do it.
> ctx.set_font_matrix(cairo.Matrix(1, 0, 0, -1, 0, 0))
>
> ctx.new_path()
> ctx.select_font_face('DejaVu Sans ExtraLight')
> ctx.set_font_size(2)
> ctx.set_source_rgb(0, 0, 0)
> ctx.move_to(x, y)
> ctx.rotate(math.atan(slope))
> ctx.show_text("test")
> ctx.restore()
>
> Thanks for your help!
>
>> Cheers,
>> behdad
>>
>>
>>> Hints, pointers appreciated!
>>> Thanks,
>>> Matt
>>>
>>> #bounds in geographic units
>>> xmin, ymin, xmax, ymax = max_bounds(sticks)
>>>
>>> #scale factor
>>> scale_x = width / (xmax-xmin)
>>> scale_y = height / (ymax-ymin)
>>> scale = min([scale_x, scale_y])
>>>
>>> ctx.save()
>>> ctx.scale(scale,scale)
>>> #flip y
>>> matrix = cairo.Matrix(1, 0, 0, -1, 0, 0)
>>> ctx.transform(matrix)
>>> #output from 0,0
>>> ctx.translate(xmin, -ymax)
>>>
>>> #draw 2d profile components
>>> ctx = horizontal_center(ctx, sticks)
>>> ctx = stick_head(ctx, sticks)
>>> ctx = surface_elevations(ctx, sticks)
>>> ctx.restore()
>>>
>>> #at this point I would like to draw the text somehow with the proper
>>> transform, rotation etc...
>>> ctx = draw_text(ctx,sticks)
>>>
>>> surface.finish()
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>>>
>