Bug 8550

Hi there,

you'll love this one.  Here's my compiler

nickgmini:~/unix/cairo-1.2.4 nickg$ gcc -v
Using built-in specs.
Target: i686-apple-darwin8
Configured with: /private/var/tmp/gcc/gcc-5363.obj~28/src/configure --disable-checking -enable-
werror --prefix=/usr --mandir=/share/man --enable-languages=c,objc,c++,obj-c++ --program-
transform-name=/^[cg][^.-]*$/s/$/-4.0/ --with-gxx-include-dir=/include/c++/4.0.0 --with-
slibdir=/usr/lib --build=powerpc-apple-darwin8 --with-arch=nocona --with-tune=generic --
program-prefix= --host=i686-apple-darwin8 --target=i686-apple-darwin8
Thread model: posix
gcc version 4.0.1 (Apple Computer, Inc. build 5363)

I can hear the groans already.  yes, it's mac os x.

This compiler, at least with intel processors has a major bug which makes cairo useless.

When compiling with -O3, the compiler miscasts the "alpha_short" value in _cario_color_t to a SIGNED 
short not a UNSIGNED short value causing color mayhem.

When using the set_color_rgb function, the alpha channel is set to 0.5 not 1.0 !!!

Here's proof:

I added a printf statement at the end of static void 
_cairo_color_compute_shorts (cairo_color_t *color) {
...
    color->alpha_short = (unsigned)(color->alpha * CAIRO_COLOR_ONE_MINUS_EPSILON);
    printf("compute shorts: alpha is %f %d %f\n", color->alpha, color->alpha_short,
           color->alpha * CAIRO_COLOR_ONE_MINUS_EPSILON);
}



--> compute shorts: alpha is 1.000000 32767 65535.999990

As you see the short value SHOULD be 65535.

This took about 6 hours to figure out.

The fix is to add a cast to a unsigned int.  casting to unsigned short does not work!

around line 115 in cairo-color.c

    /**                                                                                                                                 
     * The cast to (unsigned) is technically not needed, BUT the gcc                                                                    
     * 4.0.1 compiler (used by Mac OS X 10.4.X) has a bug.  Under -O3                                                                   
     * the double value gets converted to a SIGNED short (max 32673)                                                                    
     * instead of a unsigned short.  Explcity casting to (unsigned                                                                      
     * short) does not work.  First, casting to a normal unsigned int                                                                   
     * appears to work (and then automatically cast to unsigned short).                                                                 
     * This bug is not triggered under -O2.                                                                                             
     */
    color->alpha_short = (unsigned)(color->alpha * CAIRO_COLOR_ONE_MINUS_EPSILON);


Only the alpha channel is effected.  The other channels are ok.
This only occurs when compiling at -O3, -O2 is fine
This only occurs when using the set_rgb, not set_rgba functions
It does not matter if the image soruce is RGB24 or ARGB32

I have not checked to see if the same problem occurs with other cpus / os

Given that this is the default compiler for Mac OS ,  this is bummer.  If not a patch we should add a note 
in README not to use -O3 for this compiler.

let me know if you need any more details,

thanks,

--nickg


stupid sample program:

#include <cairo.h>
#include <cairo-pdf.h>
#define WIDTH 400
#define HEIGHT 400

#define COMPOSITE_OP CAIRO_OPERATOR_OVER

void paintit(cairo_surface_t* cs)
{
    cairo_t* ctx = cairo_create (cs);
    cairo_set_operator(ctx, COMPOSITE_OP);
    cairo_set_source_rgb(ctx, 0.0, 1.0, 1.0);
    cairo_paint(ctx);
    cairo_set_source_rgb(ctx, 1.0, 0.0, 0.0);
    cairo_set_line_width(ctx, 15);
    cairo_move_to(ctx,200,100);
    cairo_line_to(ctx,300, 300);
    cairo_rel_line_to(ctx,-200,0);
    cairo_close_path(ctx);
    cairo_stroke(ctx);
    cairo_rectangle(ctx, 0, 0, 250, 250);
    cairo_set_source_rgb(ctx, 0, 0, 0);
    cairo_fill(ctx);
    cairo_show_page(ctx);
    cairo_destroy (ctx);
}
int main() {
    cairo_surface_t *cs;                                           
    cs = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, WIDTH, HEIGHT);
    paintit(cs);
    cairo_surface_write_to_png (cs, "triangle_argb32.png");
    cairo_surface_destroy(cs);
    return 0;
}