drawTextRSXform should not apply matrix to shader

drawTextRSXform should draw the same as drawTextOnPath (as regards shaders)

Bug: b/69904791
Change-Id: I393dd8fd7a5bdc6a018b1ca33592b208c7141868
Reviewed-on: https://skia-review.googlesource.com/114468
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>

docs/SkCanvas_Reference.bmh

@@ -5064,7 +5064,8 @@ void draw(SkCanvas* canvas) {
 #In Draw
 #Line # draws null terminated string at (x, y) using font advance ##
 Draw null terminated string, with origin at (x, y), using Clip, Matrix, and
-Paint paint.
+Paint paint. Note that this per-glyph xform does not affect the shader (if present)
+on the paint, just the glyph's geometry.
 
 string meaning depends on Paint_Text_Encoding; by default, strings are encoded
 as UTF-8. Other values of Paint_Text_Encoding are unlikely to produce the desired

gm/drawatlas.cpp

@@ -150,12 +150,20 @@ static void draw_text_on_path(SkCanvas* canvas, const void* text, size_t length,
     }
 }
 
+#include "SkGradientShader.h"
+static sk_sp<SkShader> make_shader() {
+    SkPoint pts[2] = {{0, 0}, {220, 0}};
+    SkColor colors[2] = {SK_ColorRED, SK_ColorBLUE};
+    return SkGradientShader::MakeLinear(pts, colors, nullptr, 2, SkShader::kMirror_TileMode);
+}
+
 static void drawTextPath(SkCanvas* canvas, bool useRSX, bool doStroke) {
     const char text0[] = "ABCDFGHJKLMNOPQRSTUVWXYZ";
     const int N = sizeof(text0) - 1;
     SkPoint pos[N];
 
     SkPaint paint;
+    paint.setShader(make_shader());
     paint.setAntiAlias(true);
     paint.setTextSize(100);
     if (doStroke) {

include/core/SkCanvas.h

@@ -2055,7 +2055,8 @@ public:
         using clip, SkMatrix, and SkPaint paint.
 
         SkRSXform array specifies a separate square scale, rotation, and translation for
-        each glyph.
+        each glyph. Note that this per-glyph xform does not affect the shader (if present)
+        on the paint, just the glyph's geometry.
 
         Optional SkRect cullRect is a conservative bounds of text, taking into account
         SkRSXform and paint. If cullRect is outside of clip, canvas can skip drawing.

src/core/SkDevice.cpp

@@ -14,6 +14,7 @@
 #include "SkImagePriv.h"
 #include "SkImage_Base.h"
 #include "SkLatticeIter.h"
+#include "SkLocalMatrixShader.h"
 #include "SkMatrixPriv.h"
 #include "SkPatchUtils.h"
 #include "SkPathMeasure.h"
@@ -495,6 +496,9 @@ void SkBaseDevice::drawTextRSXform(const void* text, size_t len,
             break;
     }
 
+    SkPaint localPaint(paint);
+    SkShader* shader = paint.getShader();
+
     SkMatrix localM, currM;
     const void* stopText = (const char*)text + len;
     while ((const char*)text < (const char*)stopText) {
@@ -502,8 +506,17 @@ void SkBaseDevice::drawTextRSXform(const void* text, size_t len,
         currM.setConcat(this->ctm(), localM);
         SkAutoDeviceCTMRestore adc(this, currM);
 
+        // We want to rotate each glyph by the rsxform, but we don't want to rotate "space"
+        // (i.e. the shader that cares about the ctm) so we have to undo our little ctm trick
+        // with a localmatrixshader so that the shader draws as if there was no change to the ctm.
+        if (shader) {
+            SkMatrix inverse;
+            SkAssertResult(localM.invert(&inverse));
+            localPaint.setShader(shader->makeWithLocalMatrix(inverse));
+        }
+
         int subLen = proc((const char*)text);
-        this->drawText(text, subLen, 0, 0, paint);
+        this->drawText(text, subLen, 0, 0, localPaint);
         text = (const char*)text + subLen;
     }
 }