Precision xcb_render_composite_glyphs_8/16/32

[Uli Schlachter]

Hi,

Am 13.06.25 um 05:18 schrieb Max McDougle:
> Could you please provide an explanation as to what glyphcmds is supposed to
> be?

Well, not really helpful, but: A list of glyph commands. As [1] puts it:

> 	glyphcmds:	LISTofGLYPHITEM8	CompositeGlyphs8
> 	glyphcmds:	LISTofGLYPHITEM16	CompositeGlyphs16
> 	glyphcmds:	LISTofGLYPHITEM32	CompositeGlyphs32

and later:

> GLYPHITEM8	GLYPHELT8 or GLYPHABLE
> GLYPHELT16	[
> 			dx, dy:		INT16
> 			glyphs:		LISTofCARD16
> 		]

Perhaps a bit more helpful is an example. Here is some piece of code in 
cairo [2].

I will look at xcb_render_composite_glyphs_8. The first piece of data is 
a structure that cairo calls x_glyph_elt_t [3] and thathas a length 
field, some padding, and some delta. The delta is applied to the current 
point (whatever that is initially...) and then there come [len] glyphs. 
Each glyph is a uint8_t containing the glyph index. Afterwards, suitable 
padding is inserted and then a new x_glyph_elt_t follows.

Does that make a tiny bit of sense?

[1]: https://www.x.org/releases/current/doc/renderproto/renderproto.txt
[2]: 
https://gitlab.freedesktop.org/cairo/cairo/-/blob/3ffb6d5c07087676fabf7bd4b75c1ed9eb7e624d/src/cairo-xcb-surface-render.c?page=5#L4610
[3]: 
https://gitlab.freedesktop.org/cairo/cairo/-/blob/3ffb6d5c07087676fabf7bd4b75c1ed9eb7e624d/src/cairo-xcb-surface-render.c?page=5#L4078

Perhaps a tiny example helps?
Let's suppose we have glyphs 0, 1, and 2. Each glyph has an y offset of 
0 and an x offset of 10. We want to draw a run of five glyphs with this 
natural offset, then a gap of 5 and then some more glyphs.

The data argument would begin with a x_glyph_elt_t with len=5. Dunno 
what the deltas should be here, perhaps the position of the first glyph? 
Afterwards comes five bytes of data, each one being one glyph index. 
Then comes another x_glyph_elt_t describing the second run of glyphs and 
having a deltax=5.

In (pseudo) code:

size_t size_of_glyph_elt_t = 8;
uint8_t first_run[5] = {0, 0, 1, 1, 2};
uint8_t second_run[2] = {2, 1};

uint8_t buffer[size_of_glyph_elt_t*2 + sizeof(first_run) + 
sizeof(second_run)];
size_t position = 0;

x_glyph_elt_t* elt = &buffer[position];
elt->len = sizeof(first_run);
elt->deltax = dunno;
elt->deltay = dunno;
position += size_of_glyph_elt_t;

memcpy(&buffer[position], first_run, sizeof(first_run));
position += sizeof(first_run);

elt = &buffer[position];
elt->len = sizeof(second_run);
elt->deltax = 5;
elt->deltay = 0;
position += size_of_glyph_elt_t;

memcpy(&buffer[position], second_run, sizeof(second_run));
position += sizeof(second_run);

I hope this helps a bit.
Cheers,
Uli
-- 
A normal person is just someone you don't know well enough yet.
  - Nettie Wiebe