GTK+ Forums

TriKri · Familiar Face Joined: 19 Mar 2008 Posts: 14

Hello, I have a question about pictures in cairo.

I have a cairo drawing context *cr, which I want to insert a picture into. The picture is going to be in a rectangle in *cr from point (x1, y1) to (x2, y2). The picture that's imported has the width w and the height h, and I want the picture to cover the whole rectangle, hence it's scaled with a factor f = MAX((x2-x1)/w, (y2-y1)/h) when a part of it is copied into *cr. For example if I want a rectangle in the picture that shows which part of the picture that's going to be shown, that rectangle would be a scale 1/f in size compared to the rectangle in *cr.

Now I know that there are functions cairo_scale and cairo_set_source_surface, that you can use to specify the relative position and scale between *cr and the picture, but I don't know how to use them. Then there are the function cairo_rectangle(cr, ...) which I'm going to use for specifying the part of *cr that's going to contain the part of the picture that's going to be shown. There is also a function cairo_transform, which you could use instead of cairo_scale, if you want to make a movement and a scaling, but I don't know if I should use that function or if I just have to make the scaling.

How shall I use these functions and in which order? I know that cairo works with transformation matrices, and that *cr contains such a matrix. I know how to use transformation matrices myself, but I don't know how cairo makes use of them and when. Please help!

Thanks in advance!

TriKri · Familiar Face Joined: 19 Mar 2008 Posts: 14

I have found one way to do it, but that way seems to scale the drawing context...

Anyway, here's the code:

Code: (Plaintext)

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cairo_surface_t *picture;
double w, h;
double f;
picture = cairo_image_surface_create_from_png("Images/Peyto_Lake-Banff_NP-Canada.png");
w = cairo_image_surface_get_width(picture);
h = cairo_image_surface_get_height(picture);
f = MAX((x2-x1)/w, (y2-y1)/h);

cairo_scale(cr, f, f);
cairo_set_source_surface(cr, picture, ((x1+x2)/f-w)/2, ((y1+y2)/f-h)/2);
cairo_rectangle(cr, x1/f, y1/f, (x2-x1)/f, (y2-y1)/f);
cairo_fill(cr);

I wanted the middle of the rectangle (x1, y1) to (x2, y2) in *cr to match the point (w/2, h/2) in the picture, as you can see in the call of cairo_set_source_surface. The only thing is that *cr gets scaled. Is there some other way to do this so *cr doesn't get scaled? Also, is there support for jpg images in cairo? There is cairo_image_surface_create_from_png but there isn't cairo_image_surface_create_from_jpg :(

subaren · Familiar Face Joined: 20 Aug 2007 Posts: 8

GTK+ Tutorial

Look at this link for the cairo tutorial
http://www.zetcode.com/tutorials/cairographicstutorial/cairoimages/

Subair

TriKri · Familiar Face Joined: 19 Mar 2008 Posts: 14

Thanks