Inverted text when rendering text in Cairo - c++

Question

I am writing a program using Cairo where I need to change the coordinates from screen to Cartesian using the following code:
cairo_translate( cr, x, height );
cairo_scale( cr, 1.0, -1.0 ); // FLIP the Y axis
But when I try to render text (cairo_show_text), the text is inverted also and, hence, unreadable.
The same happens when using a scaled font (cairo_scaled_font_t):
cairo_font_options_t *font_options;
cairo_matrix_t ctm, font_matrix;
cairo_scaled_font_t *scaled_font;
font_options = cairo_font_options_create();
cairo_get_matrix( cr, &ctm );
cairo_get_font_matrix( cr, &font_matrix );
font_matrix.xx = font_matrix.yy = 20.0; // font size
// font_face initialized elsewhere and is valid
scaled_font = cairo_scaled_font_create( font_face, &font_matrix, &ctm, font_options );
cairo_set_scaled_font( cr, scaled_font );
cairo_move_to( cr, 1, 50 );
cairo_show_text( cr, "some text" );
...
I think the matrix needs some adjustments, but I don't know how. Any help is appreciated. Thanks in advance.
EDIT:
The text is upside down. For example, 'L' looks like a Gamma, and 'W' looks like an 'M'.

Answer 1

Theres two ways to do this that I know:
cairo_matrix_t ms, mt, m;
cairo_matrix_init_scale(&ms, 1.0f, -1.0f);
cairo_matrix_init_translate(&mt, 0.0f, -m_height);
cairo_matrix_multiply(&m, &mt, &ms);
cairo_set_matrix(cr, &m);
or:
cairo_scale(cr, 1.0f, -1.0f);
cairo_translate(cr, 0.0f, -m_height);
Both solved the problem for me. Scale the cr by the -1 on y, but that means the text is off the surface, so translate by the -height of the surface and it should sit right.

Answer 2

Since you flip the coordinate system upside down, text is also shown upside down. The solution to this problem is to manually flip back text, by setting a font matrix with a negative entry for the vertical scale. You can do this, by modifying your code for setting the font size like this:
double font_size = 20.0;
font_matrix.xx = font_size;
font_matrix.yy = -font_size; // negative size to vertically flip text

Answer 3

Well, you are flipping the Y-axis, thus the text gets flipped, too. Try this:
cairo_save(cr);
cairo_reset_clip(cr);
cairo_show_text(cr, "Some Text");
cairo_restore(cr);
The above replaces the cairo_show_text() call in your example.