Bug #53154 [Ana->Csd]: Zero-height rectangle has whiskers

Edit report at https://bugs.php.net/bug.php?id=53154&edit=1 ID: 53154 Updated by: cmb@php.net Reported by: lbayuk at users dot sourceforge dot net Summary: Zero-height rectangle has whiskers -Status: Analyzed +Status: Closed Type: Bug Package: GD related Operating System: * PHP Version: 5.6.10 Assigned To: cmb Block user comment: N Private report: N New Comment: Automatic comment on behalf of cmb Revision: http://git.php.net/?p=php-src.git;a=commit;h=094decc3c0b62a72bacebaf85c6269049bf12885 Log: Fix #53154: Zero-height rectangle has whiskers Previous Comments: ------------------------------------------------------------------------ [2015-06-29 20:15:03] lbayuk at users dot sourceforge dot net I rebuilt the PHP GD extension with your patch, for PHP-5.6.10. I can confirm it fixed the bug and I saw no regressions. ------------------------------------------------------------------------ [2015-06-29 11:35:44] cmb@php.net No need to apologize. IMHO it is fine to remind that a bug is still there in recent versions after nearly 5 years. Anyhow, thanks for your analysis; the behavior to draw the corner pixels only once makes sense, and should be kept. In the same manner, there probably should be separate cases for rectangles with zero widht or height (drawing a single line in this case would be sufficient), as done in the "zero-rectangle" patch. ------------------------------------------------------------------------ [2015-06-29 11:34:56] cmb@php.net The following patch has been added/updated: Patch Name: zero-rectangle Revision: 1435577696 URL: https://bugs.php.net/patch-display.php?bug=53154&patch=zero-rectangle&revision=1435577696 ------------------------------------------------------------------------ [2015-06-28 22:28:23] lbayuk at users dot sourceforge dot net (Sorry for the 'still there' comment... I needed to change my email on the report, and it doesn't seem to work unless a new comment is added. (Should I open a bug report on the bug system?)) By "two assignments", do you mean y1v=y1h+1 and y2v=y2h-1 ? Or in the libgd code in your second link, the in-line equivalents in the 3rd and 4th calls to gdImageLine() ? These seem to be there to prevent the corner pixels from being drawn twice. That is, the vertical line segments that make up the rectangle are 2 pixels shorter than the sides of the rectangle. When the rectangle height is > 0 this prevents the corner pixels from being drawn twice, once as part of the horizontal segments and once as part of the vertical segments. But in the edge case of rectangle height = 0, this code causes the "whiskers" on the left and right ends. Why would you want to prevent the corner pixels from being drawn twice? The only reason I can think of where it matters is when alpha blending is on (as it is by default with truecolor images). In this case, drawing over the same pixel with the same color more than once will darken it each time. This will be true when the color is partially transparent (alpha > 0). The effect is subtle, but can be seen with a screen magnifier. That's my guess, anyway. ------------------------------------------------------------------------ [2015-06-28 20:15:09] cmb@php.net > Verified still there in PHP-5.6.10 Thanks for the reminder. :) The issue is caused by two strange assignments[1] in the bundled libgd. Interestingly this code has changed in libgd already two years ago[2]. I can't find any regression when removing these two assignments, but I'm puzzled why they're there in the first place. [1] <http://lxr.php.net/xref/PHP_TRUNK/ext/gd/libgd/gd.c#2096> [2] <https://github.com/libgd/libgd/commit/a3016ad01b762aa81534ed99ee2af976d45b800a> ------------------------------------------------------------------------ The remainder of the comments for this report are too long. To view the rest of the comments, please view the bug report online at https://bugs.php.net/bug.php?id=53154 -- Edit this bug report at https://bugs.php.net/bug.php?id=53154&edit=1