fix zero-length tangent

fix zero-length tangent

If the end point and the control point are the same, computing
the tangent will result in (0, 0). In this case, use the prior
control point instead.

R=reed@google.com

BUG=skia:4191
Committed: https://skia.googlesource.com/skia/+/7544124fb8ee744f68f549a353f8a9163cd7432d

[caryclark, 2015-08-24 20:10:26]

caryclark@google.com changed reviewers:
+ reed@google.com

[caryclark, 2015-08-24 20:10:39]

The CQ bit was checked by caryclark@google.com

[commit-bot: I haz the power, 2015-08-24 20:10:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1311273002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1311273002/1

[commit-bot: I haz the power, 2015-08-24 20:10:47]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2015-08-25 02:10 UTC

[reed1, 2015-08-24 20:15:46]

I suggest both the check and the GM mention the condition they are trying to
correct for.

https://codereview.chromium.org/1311273002/diff/1/gm/strokes.cpp
File gm/strokes.cpp (right):

https://codereview.chromium.org/1311273002/diff/1/gm/strokes.cpp#newcode269
gm/strokes.cpp:269: class Strokes5GM : public skiagm::GM {
// We want to explicitly test stroking (which needs to compute the tangent to
the curve)
// for curves that can produce degenerate values when t is 0 or 1 (see bug ???)

https://codereview.chromium.org/1311273002/diff/1/src/core/SkGeometry.cpp
File src/core/SkGeometry.cpp (right):

https://codereview.chromium.org/1311273002/diff/1/src/core/SkGeometry.cpp#new...
src/core/SkGeometry.cpp:159: if (tangent) {
// The derivative equation is just 2(b - a +(a - 2b +c)t), and this can return a
zero tangent vector at
// these two T values if the control-point (b) is collinear with the end-point,
hence the need to test
// for those explicitly.

[commit-bot: I haz the power, 2015-08-25 02:10:09]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-25 02:10:09]

No LGTM from a valid reviewer yet. Please ask for an LGTM from a
full Skia committer from
https://skia.googlesource.com/skia/+/master/CQ_COMMITTERS

[caryclark, 2015-08-25 12:04:40]

The CQ bit was checked by caryclark@google.com

[commit-bot: I haz the power, 2015-08-25 12:04:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1311273002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1311273002/20001

[caryclark, 2015-08-25 12:04:48]

https://codereview.chromium.org/1311273002/diff/1/gm/strokes.cpp
File gm/strokes.cpp (right):

https://codereview.chromium.org/1311273002/diff/1/gm/strokes.cpp#newcode269
gm/strokes.cpp:269: class Strokes5GM : public skiagm::GM {
On 2015/08/24 20:15:46, reed1 wrote:
> // We want to explicitly test stroking (which needs to compute the tangent to
> the curve)
> // for curves that can produce degenerate values when t is 0 or 1 (see bug
???)

Done.

https://codereview.chromium.org/1311273002/diff/1/src/core/SkGeometry.cpp
File src/core/SkGeometry.cpp (right):

https://codereview.chromium.org/1311273002/diff/1/src/core/SkGeometry.cpp#new...
src/core/SkGeometry.cpp:159: if (tangent) {
On 2015/08/24 20:15:46, reed1 wrote:
> // The derivative equation is just 2(b - a +(a - 2b +c)t), and this can return
a
> zero tangent vector at
> // these two T values if the control-point (b) is collinear with the
end-point,
> hence the need to test
> // for those explicitly.

Done.

[commit-bot: I haz the power, 2015-08-25 12:04:48]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-25 12:04:49]

No LGTM from a valid reviewer yet. Please ask for an LGTM from a
full Skia committer from
https://skia.googlesource.com/skia/+/master/CQ_COMMITTERS

[reed1, 2015-08-25 14:51:00]

lgtm

[caryclark, 2015-08-25 14:57:19]

The CQ bit was checked by caryclark@google.com

[commit-bot: I haz the power, 2015-08-25 14:57:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1311273002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1311273002/20001

[commit-bot: I haz the power, 2015-08-25 15:03:05]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/7544124fb8ee744f68f549a353f8a9163cd7432d

[caryclark, 2015-08-25 17:02:33]

A revert of this CL (patchset #2 id:20001) has been created in
https://codereview.chromium.org/1312243002/ by caryclark@google.com.

The reason for reverting is: causes layout test to draw differently -- new
drawing is more correct. Reverting until layout test ignore is landed..