starter procs for blending with pm4f

gm/xfer4f.cpp

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm.h"
+#include "SkCanvas.h"
+#include "SkImageInfo.h"
+#include "SkXfer4f.h"
+
+static void draw_rect(SkCanvas* canvas, const SkRect& r, SkColor c, SkColorProfileType profile) {
+    const SkIRect ir = r.round();
+
+    SkBitmap bm;
+    bm.allocN32Pixels(ir.width(), ir.height());
+    bm.eraseColor(0xFFFFFFFF);
+    SkPixmap pm;
+    bm.peekPixels(&pm);
+
+    uint32_t flags = 0;
+    if (SkColorGetA(c) == 0xFF) {
+        flags |= kSrcIsOpaque_SkXfer4fFlag;
+    }
+    if (kSRGB_SkColorProfileType == profile) {
+        flags |= kDstIsSRGB_SkXfer4fFlag;
+    }
+
+    const SkPM4f src = SkPM4f::FromPMColor(SkPreMultiplyColor(c));

[mtklein, 2016/01/29 15:23:00]

Just had a thought that we might want to generally do this in the opposite order
(-> float, then premul), so that the premultiply step does not lose precision.

[reed1, 2016/01/29 22:35:27]

Totally agree, was just being lazy since I already have a helper function to
premul the SkColor. Will change.

+    auto proc1 = SkPM4fXfer1ProcFactory(SkXfermode::kSrcOver_Mode, flags);
+    for (int y = 0; y < ir.height()/2; ++y) {
+        proc1(pm.writable_addr32(0, y), src, ir.width());
+    }
+
+    SkPM4f srcRow[1000];
+    for (int i = 0; i < ir.width(); ++i) {
+        srcRow[i] = src;
+    }
+    auto procN = SkPM4fXferNProcFactory(SkXfermode::kSrcOver_Mode, flags);
+    // +1 to skip a row, so we can see the boundary between proc1 and procN
+    for (int y = ir.height()/2 + 1; y < ir.height(); ++y) {
+        procN(pm.writable_addr32(0, y), srcRow, ir.width());
+    }
+
+    canvas->drawBitmap(bm, r.left(), r.top(), nullptr);
+}
+
+/*
+ *  Test SkXfer4fProcs directly for src-over, comparing them to current SkColor blits.
+ */
+DEF_SIMPLE_GM(xfer4f_srcover, canvas, 580, 380) {
+    const SkScalar W = 50;
+    const SkScalar H = 100;
+
+    const int profiles[] = {
+        -1,
+        kLinear_SkColorProfileType,
+        kSRGB_SkColorProfileType,
+    };
+    const SkColor colors[] = {
+        SK_ColorBLACK, SK_ColorRED, SK_ColorGREEN, SK_ColorBLUE,
+        0x88000000, 0x88FF0000, 0x8800FF00, 0x880000FF
+    };
+    canvas->translate(20, 20);
+
+    const SkRect r = SkRect::MakeWH(W, H);
+    for (auto profile : profiles) {
+        canvas->save();
+        for (SkColor c : colors) {
+            if (profile < 0) {
+                SkPaint p;
+                p.setColor(c);
+                canvas->drawRect(r, p);
+            } else {
+                draw_rect(canvas, r, c, (SkColorProfileType)profile);
+            }
+            canvas->translate(W + 20, 0);
+        }
+        canvas->restore();
+        canvas->translate(0, H + 20);
+    }
+}