CODEWITHSUNDEEP
cubuntugtkpango
user2062604
user2062604

Reputation: 285

Why is pango_layout_get_pixel_size slightly wrong on Ubuntu Linux in C

I'm experiencing a frustrating issue trying to draw text using Pango and Cairo libraries in C in a Gtk application running on Ubuntu Linux.

I'm creating a Pango layout and then drawing it at a given location which is determined by the size of the text as reported by pango_layout_get_pixel_size, but the size returned by that function is wrong in both width and height, especially in height. Here is my full code:

// Create a cairo context with which to draw
// Note that we already have a GtkDrawingArea widget m_pGtkDrawingArea
cairo_t *cr = gdk_cairo_create(m_pGtkDrawingArea->window);

// Text to draw
std::string szText("NO DATA AVAILABLE");

// Create the layout
PangoLayout *pLayout = gtk_widget_create_pango_layout(m_pGtkDrawingArea, szText.c_str());

// Set layout properties
pango_layout_set_alignment(pLayout, PANGO_ALIGN_LEFT);
pango_layout_set_width(pLayout, -1);

// The family to use
std::string szFontFamily("FreeSans");

// The font size to use
double dFontSize = 36.0;

// Format the font description string
char szFontDescription[32];
memset(&(szFontDescription[0]), 0, sizeof(szFontDescription));
snprintf(szFontDescription, sizeof(szFontDescription) - 1, "%s %.1f", szFontFamily.c_str(), dFontSize);

// Get a new pango font description
PangoFontDescription *pFontDescription = pango_font_description_from_string(szFontDescription);

// Set up the pango font description
pango_font_description_set_weight(pFontDescription, PANGO_WEIGHT_NORMAL);
pango_font_description_set_style(pFontDescription, PANGO_STYLE_NORMAL);
pango_font_description_set_variant(pFontDescription, PANGO_VARIANT_NORMAL);
pango_font_description_set_stretch(pFontDescription, PANGO_STRETCH_NORMAL);

// Set this as the pango font description on the layout
pango_layout_set_font_description(pLayout, pFontDescription);

// Use auto direction
pango_layout_set_auto_dir(pLayout, TRUE);

// Get the pixel size of this text - this reports a size of 481x54 pixels
int iPixelWidth = 0, iPixelHeight = 0;
pango_layout_get_pixel_size(pLayout, &iPixelWidth, &iPixelHeight);

// Calculate the text location based on iPixelWidth and iPixelHeight
double dTextLocX = ...;
double dTextLocY = ...;

// Set up the cairo context for drawing the text
cairo_set_source_rgba(cr, 1.0, 1.0, 1.0, 1.0);
cairo_set_antialias(cr, CAIRO_ANTIALIAS_BEST);

// Move into place
cairo_move_to(cr, dTextLocX, dTextLocY);

// Draw the layout
pango_cairo_show_layout(cr, pLayout);

//
// pango_layout_get_pixel_size() reported a size of 481x54 pixels,
// but the actual size when drawn is 478x37 pixels!
//

//
// Clean up...
//

So, as described at the bottom of the above code, the pango_layout_get_pixel_size function reports a size of 481x54 pixels, but the size of the text on the screen is actually 478x37 pixels.

What am I doing wrong here? How can I get the actual correct pixel size?

Thanks in advance for your help!

Upvotes: 2

Views: 1322

Answers (1)

rici
rici

Reputation: 241881

The text you are displaying ("NO DATA AVAILABLE") is all-caps, and consequently has no descenders (letters which are partly below the baseline, like j, p and q.) Normally, when you measure the extent of a text box, you include room for the descenders whether or not they are present; otherwise, you will see odd artifacts such as inconsistent line separation depending on whether or not a given line has a descender.

Pango provides APIs which return both the logical extent (which includes the full height of the font) and the ink extent (which is the bounding box of the inked part of the image). I suspect you are looking for the ink extent.

Upvotes: 3

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