Wednesday, December 19, 2012

gnocl::cairo instropection

The gnocl::cairo command is not intended to be some form of canvas object, but a means of drawing onto images for display on screen used perhaps in the creation of widgets or images. Bearing in mind the relatively small number of actions required for this task, it's reasonable to assume that the overhead of making multiple calls to the gnocl::cairo command is a worthwhile compromise.

In response to the problem of how to get the extents of rendered text back from Cairo I came up with the solution highlighted in the following code:


set pb(1) [gnocl::pixBuf new -width 256 -height 256]
set myString "Peter Piper picked a peck of pickled peppers."
set actions "
    select_font_face {Sans normal bold}
    set_font_size 15.0
    text_extents $myString"  
array set extents [gnocl::cairo $pb(1) -actions $actions]


In the example above, the following values were obtained.

extents(height)    = 14.0
extents(width)     = 320.0
extents(x_advance) = 322.0
extents(x_bearing) = 1.0
extents(y_advance) = 0.0
extents(y_bearing) = -11.0



Tuesday, December 18, 2012

New Pacakge gnocl::cairo

The process of tidying up the development code prior to a release increment has led me to weeding out some unused or difficult to maintain functions. One of these was the cairo.c module. This was experimental block of code which was an offshoot of developing the gnocl::splashscreen widget.  But, with the transition to Gtk3 on the horizon, all drawing operations will be performed by Cairo and so I thought that it was a good opportunity to make use of some of that old code. Hence, a new package. There's still way to go before it's finished and I'm still working on drawing to pixbufs but things are looking good. Here's the syntax for the command:

gnocl::cairo <drawable-id> -actions <script>

Where the script is simply a list of cairo operations.  Here's a snippet from my test script.

set actions "
# {sample set-line-join}
set_line_width 40.96
move_to {76.8 84.48}
rel_line_to {51.2 -51.2}
rel_line_to {51.2 51.2}
set_line_join miter
stroke

move_to {76.8 161.28}
rel_line_to {51.2 -51.2}
rel_line_to {51.2 51.2}
set_line_join bevel
stroke

move_to {76.8 238.08}
rel_line_to {51.2 -51.2}
rel_line_to {51.2 51.2}
set_line_join round
stroke
"
set pb(1) [gnocl::pixBuf new -width 512 -height 512]
gnocl::cairo $pb(1) -actions "$actions #"


This gives:



There will be some limitations. It's not possible to create several surfaces in advance and then to use them as required. There is only one surface and will be necessary to change its contents. I don't envisage implementing any introspection either, keeping track of what's taking place on the Cairo side of things will need to be handled Tcl-side. One change that I did make was to allow for angles to be specified in degrees rather than radians. I could make this optional with a -useDegree switch but that can be dealt with later.

Here's a list of the Cairo operations implemented so far. The names are derived from the Cairo api for which these are wraps.

        "image_surface_create_from_png",
        "paint",
        "line_to",
        "set_line_cap",
        "set_line_join",
        "rel_line_to",
        "rectangle",
        "arc",
        "arc_negative",
        "curve_to",
        "rel_curve_to",
        "stroke",
        "stroke_preserve",
        "set_source_rgb",
        "set_source_rgba",
        "fill",
        "fill_preserve",
        "set_dash",
        "set_line_width",
        "move_to",
        "pattern_create_linear",
        "pattern_add_color_stop_rgb",
        "pattern_add_color_stop_rgba",
        "set_source",
        "set_source_surface",
        "surface_destroy",
        "pattern_destroy",
        "select_font_face",
        "font_size",
        "show_text",
        "new_path",
        "close_path",
        "clip",
        "new_sub_path",
        "set_fill_rule",
        "scale",
        "rotate",
        "translate",
        "#"