[FFmpeg-devel] av_parse_color() improvement

[Michael Niedermayer]

On Fri, Nov 12, 2010 at 11:53:10PM +0100, Aurelien Jacobs wrote:
> On Fri, Nov 12, 2010 at 07:38:40PM +0100, Michael Niedermayer wrote:
> > On Thu, Nov 11, 2010 at 02:29:27AM +0100, Aurelien Jacobs wrote:
> > > Hi,
> > > 
> > > Attached patch makes the av_parse_color() a bit more versatile.
> > > It allows parsing of html formated hex color (ie. starting with a #
> > > instead of 0x), including its loose form where the # is missing.
> > > This makes quite some sens as the function is already able to parse
> > > html named colors.
> > > This is required to be able easily use this function to parse colors
> > > out of SubRip files.
> > > 
> > > Aurel
> > >  parseutils.c |   22 +++++++++++++++++-----
> > >  parseutils.h |    2 +-
> > >  2 files changed, 18 insertions(+), 6 deletions(-)
> > > 4ae48624cebdcb2a693371dd00f51f6cff086711  parse_color.diff
> > > Index: libavfilter/parseutils.h
> > > ===================================================================
> > > --- libavfilter/parseutils.h	(revision 25719)
> > > +++ libavfilter/parseutils.h	(working copy)
> > > @@ -31,7 +31,7 @@
> > >   * Put the RGBA values that correspond to color_string in rgba_color.
> > >   *
> > >   * @param color_string a string specifying a color. It can be the name of
> > > - * a color (case insensitive match) or a 0xRRGGBB[AA] sequence,
> > > + * a color (case insensitive match) or a [0x|#]RRGGBB[AA] sequence,
> > >   * possibly followed by "@" and a string representing the alpha
> > >   * component.
> > >   * The alpha component may be a string composed by "0x" followed by an
> > > Index: libavfilter/parseutils.c
> > > ===================================================================
> > > --- libavfilter/parseutils.c	(revision 25719)
> > > +++ libavfilter/parseutils.c	(working copy)
> > > @@ -187,9 +187,17 @@
> > >  {
> > >      char *tail, color_string2[128];
> > >      const ColorEntry *entry;
> > > -    av_strlcpy(color_string2, color_string, sizeof(color_string2));
> > > +    int len, hex_offset = 0;
> > > +
> > > +    if (color_string[0] == '#')
> > > +        hex_offset = 1;
> > > +    else if (!strncmp(color_string, "0x", 2))
> > > +        hex_offset = 2;
> > > +
> > > +    av_strlcpy(color_string2, color_string + hex_offset, sizeof(color_string2));
> > >      if ((tail = strchr(color_string2, ALPHA_SEP)))
> > >          *tail++ = 0;
> > > +    len = strlen(color_string2);
> > >      rgba_color[3] = 255;
> > >  
> > >      if (!strcasecmp(color_string2, "random") || !strcasecmp(color_string2, "bikeshed")) {
> > > @@ -198,16 +206,16 @@
> > >          rgba_color[1] = rgba >> 16;
> > >          rgba_color[2] = rgba >> 8;
> > >          rgba_color[3] = rgba;
> > > -    } else if (!strncmp(color_string2, "0x", 2)) {
> > > +    } else if (hex_offset ||
> > 
> > > +               strspn(color_string2, "0123456789ABCDEFabcdef") == len) {
> > 
> > why is that needed?
> > i thought things failing this would fail some other tests too
> 
> yes and no...
> The code has 3 branches looking like this:
>     if("random") {
>         color = random();
>     } else if(has_hex_prefix()) {
>         color = parse_as_hex_number();
>     } else {
>         color = parse_as_color_name();
>     }
> 
> Right now, the has_hex_prefix() branch will only be entered if the
> string start with 0x.
> I need this function to also be able to parse hex numbers without a
> prefix. Right now, such a string fall back into the
> parse_as_color_name() case and just fail.
> 
> I see to solution to solve this:
> 1) Always try to parse as a color name first, and if it fail, try to
> parse as a hex number. But this would slow down parsing hex numbers by
> first trying to match them with the entries of the color name table.

have you checked that is slower than strspn() ?
also you could try to parse as hex first and if it fails fallback to the names

its also not a real issue if you leave the strspn() there if you prefer

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