[cairo] Image will not rotate

Evan White talguy385 at gmail.com
Fri Apr 3 11:51:05 PDT 2009

I look at the C programing samples posted on the Cairo site and saw that 
set_source is usually called after the rotation function.  So before I 
posted my first post I tried it and all I got was a black screen.  No 
image was painted to the window. Not the background or any of the gauge 


Gerdus van Zyl said the following On 4/3/2009 12:49 PM:
> You need to apply your rotation on the pattern (the image) matrix;
> from the api docs:
> "Note: The pattern's transformation matrix will be locked to the user
> space in effect at the time of cairo_set_source(). This means that
> further modifications of the current transformation matrix will not
> affect the source pattern."
> So the easiest would be to set the rotation before calling set_source
> The longer way you need to get the matrix of the pattern
> (cairo_get_source), then use cairo_pattern_get_matrix
> ,cairo_matrix_rotate and cairo_pattern_set_matrix (Note C api c++ api
> should have similar things as object methods)
> On Fri, Apr 3, 2009 at 5:50 PM, Evan White <talguy385 at gmail.com> wrote:
>> I am current building an app that will be a replacement for your vehicles
>> instrument cluster as my senior design project.  This app allows the user to
>> load custom skins based off PNG images.  Each gauge is stored in a list.
>>  When updating the interface the system searches the list and fines the
>> gauge fine.  It generated the correct angle given input value.  The problem
>> is that it does not rotate the image at all.  The dashboard class is the
>> controller for exposing the window and the analog class is where the actual
>> image is being processed and rotated.  I am not a computer science major I
>> am an electrical engineering one so please mind the poor programming
>> practices that are used in the code.  If you want to make suggestions that
>> would be great, but I have no clue why my image is not rotating.
>> Evan
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>> cairo at cairographics.org
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