Align SkLumaColorFilter with the spec.
The spec requires the resulting RGB channels for LuminanceToAlpha to be
0 (and not just scaled by the luminance value</facepalm>):
| R' | | 0 0 0 0 0 | | R |
| G' | | 0 0 0 0 0 | | G |
| B' | = | 0 0 0 0 0 | * | B |
| A' | | 0.2125 0.7154 0.0721 0 0 | | A |
| 1 | | 0 0 0 0 1 | | 1 |
(http://www.w3.org/TR/2011/REC-SVG11-20110816/filters.html#feColorMatrixElement)
This doesn't affect luminance masking (which depends only on the
resulting alpha channel), but other color filter users may care
about all color components.
R=bsalomon@google.com, reed@google.com, robertphillips@google.com
Author: fmalita@chromium.org
Review URL: https://codereview.chromium.org/26467003
git-svn-id: http://skia.googlecode.com/svn/trunk@11713 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/lumafilter.cpp b/gm/lumafilter.cpp
index 69a4f07..c12bae0 100644
--- a/gm/lumafilter.cpp
+++ b/gm/lumafilter.cpp
@@ -34,6 +34,7 @@
c = bounds;
c.fRight = bounds.centerX();
+ paint.setARGB(0x20, 0, 0, 0xff);
canvas->drawRect(bounds, paint);
canvas->saveLayer(&bounds, NULL);
diff --git a/include/effects/SkLumaColorFilter.h b/include/effects/SkLumaColorFilter.h
index fef06ab..a0c08bb 100644
--- a/include/effects/SkLumaColorFilter.h
+++ b/include/effects/SkLumaColorFilter.h
@@ -15,9 +15,10 @@
* http://www.w3.org/TR/SVG/masking.html#Masking
* http://www.w3.org/TR/css-masking/#MaskValues
*
- * Each color is scaled by the (unpremultiplied) luminance value:
+ * The resulting color is black with transparency equal to the
+ * luminance value modulated by alpha:
*
- * C' = [Lum * a, Lum * r, Lum * g, Lum * b]
+ * C' = [ Lum * a, 0, 0, 0 ]
*
*/
class SK_API SkLumaColorFilter : public SkColorFilter {
diff --git a/src/effects/SkLumaColorFilter.cpp b/src/effects/SkLumaColorFilter.cpp
index eff1645..fd4826b 100644
--- a/src/effects/SkLumaColorFilter.cpp
+++ b/src/effects/SkLumaColorFilter.cpp
@@ -9,7 +9,6 @@
#include "SkColorPriv.h"
#include "SkString.h"
-#include "SkUnPreMultiply.h"
#if SK_SUPPORT_GPU
#include "gl/GrGLEffect.h"
@@ -19,44 +18,21 @@
void SkLumaColorFilter::filterSpan(const SkPMColor src[], int count,
SkPMColor dst[]) const {
- const SkUnPreMultiply::Scale* table = SkUnPreMultiply::GetScaleTable();
-
for (int i = 0; i < count; ++i) {
SkPMColor c = src[i];
- unsigned r = SkGetPackedR32(c);
- unsigned g = SkGetPackedG32(c);
- unsigned b = SkGetPackedB32(c);
- unsigned a = SkGetPackedA32(c);
-
- // No need to do anything for white (luminance == 1.0)
- if (a != r || a != g || a != b) {
- /*
- * To avoid un-premultiplying multiple components, we can start
- * with the luminance computed in PM space:
- *
- * Lum = i * (r / a) + j * (g / a) + k * (b / a)
- * Lum = (i * r + j * g + k * b) / a
- * Lum = Lum'(PM) / a
- *
- * Then the filter function is:
- *
- * C' = [ Lum * a, Lum * r, Lum * g, Lum * b ]
- *
- * which is equivalent to:
- *
- * C' = [ Lum'(PM), Lum * r, Lum * g, Lum * b ]
- */
- unsigned pm_lum = SkComputeLuminance(r, g, b);
- unsigned lum = SkUnPreMultiply::ApplyScale(table[a], pm_lum);
-
- c = SkPackARGB32(pm_lum,
- SkMulDiv255Round(r, lum),
- SkMulDiv255Round(g, lum),
- SkMulDiv255Round(b, lum));
- }
-
- dst[i] = c;
+ /*
+ * While LuminanceToAlpha is defined to operate on un-premultiplied
+ * inputs, due to the final alpha scaling it can be computed based on
+ * premultipled components:
+ *
+ * LumA = (k1 * r / a + k2 * g / a + k3 * b / a) * a
+ * LumA = (k1 * r + k2 * g + k3 * b)
+ */
+ unsigned luma = SkComputeLuminance(SkGetPackedR32(c),
+ SkGetPackedG32(c),
+ SkGetPackedB32(c));
+ dst[i] = SkPackARGB32(luma, 0, 0, 0);
}
}
@@ -97,7 +73,9 @@
virtual void getConstantColorComponents(GrColor* color,
uint32_t* validFlags) const SK_OVERRIDE {
- *validFlags = 0;
+ // The output is always black.
+ *color = GrColorPackRGBA(0, 0, 0, GrColorUnpackA(*color));
+ *validFlags = kRGB_GrColorComponentFlags;
}
class GLEffect : public GrGLEffect {
@@ -123,16 +101,13 @@
inputColor = "vec4(1)";
}
- // The max() is to guard against 0 / 0 during unpremul when the incoming color is
- // transparent black.
- builder->fsCodeAppendf("\tfloat nonZeroAlpha = max(%s.a, 0.00001);\n", inputColor);
builder->fsCodeAppendf("\tfloat luma = dot(vec3(%f, %f, %f), %s.rgb);\n",
SK_ITU_BT709_LUM_COEFF_R,
SK_ITU_BT709_LUM_COEFF_G,
SK_ITU_BT709_LUM_COEFF_B,
inputColor);
- builder->fsCodeAppendf("\t%s = vec4(%s.rgb * luma / nonZeroAlpha, luma);\n",
- outputColor, inputColor);
+ builder->fsCodeAppendf("\t%s = vec4(0, 0, 0, luma);\n",
+ outputColor);
}
diff --git a/tests/ColorFilterTest.cpp b/tests/ColorFilterTest.cpp
index b333ad6..8c96d75 100644
--- a/tests/ColorFilterTest.cpp
+++ b/tests/ColorFilterTest.cpp
@@ -101,11 +101,14 @@
SkPMColor in, out;
SkAutoTUnref<SkColorFilter> lf(SkLumaColorFilter::Create());
- // Applying luma to white is a nop (luminance(white) == 1.0)
+ // Applying luma to white produces black with the same transparency.
for (unsigned i = 0; i < 256; ++i) {
in = SkPackARGB32(i, i, i, i);
lf->filterSpan(&in, 1, &out);
- REPORTER_ASSERT(reporter, out == in);
+ REPORTER_ASSERT(reporter, SkGetPackedA32(out) == i);
+ REPORTER_ASSERT(reporter, SkGetPackedR32(out) == 0);
+ REPORTER_ASSERT(reporter, SkGetPackedG32(out) == 0);
+ REPORTER_ASSERT(reporter, SkGetPackedB32(out) == 0);
}
// Applying luma to black yields transparent black (luminance(black) == 0)
@@ -115,15 +118,15 @@
REPORTER_ASSERT(reporter, out == SK_ColorTRANSPARENT);
}
- // For general colors, a luma filter has an attenuating effect.
+ // For general colors, a luma filter generates black with an attenuated alpha channel.
for (unsigned i = 1; i < 256; ++i) {
in = SkPackARGB32(i, i, i / 2, i / 3);
lf->filterSpan(&in, 1, &out);
REPORTER_ASSERT(reporter, out != in);
REPORTER_ASSERT(reporter, SkGetPackedA32(out) <= i);
- REPORTER_ASSERT(reporter, SkGetPackedR32(out) <= i);
- REPORTER_ASSERT(reporter, SkGetPackedG32(out) <= i / 2);
- REPORTER_ASSERT(reporter, SkGetPackedB32(out) <= i / 3);
+ REPORTER_ASSERT(reporter, SkGetPackedR32(out) == 0);
+ REPORTER_ASSERT(reporter, SkGetPackedG32(out) == 0);
+ REPORTER_ASSERT(reporter, SkGetPackedB32(out) == 0);
}
}