Create a semi-stable API for capturing the state of an SkCanvas and reconstructing that state acros…

src/utils/SkCanvasStateUtils.cpp

Index: src/utils/SkCanvasStateUtils.cpp
diff --git a/src/utils/SkCanvasStateUtils.cpp b/src/utils/SkCanvasStateUtils.cpp
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+++ b/src/utils/SkCanvasStateUtils.cpp
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+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkCanvasStateUtils.h"
+
+#include "SkBitmapDevice.h"
+#include "SkCanvas.h"
+#include "SkNWayCanvas.h"
+#include "SkWriter32.h"
+
+#define CANVAS_STATE_VERSION 1
+/*
+ * WARNING: The structs below are part of a stable ABI and as such we explicitly
+ * use unambigious primitives (e.g. int32_t instead of an enum).
+ *
+ * ANY CHANGES TO THE STRUCTS BELOW THAT IMPACT THE ABI SHOULD RESULT IN AN
+ * UPDATE OF THE CANVAS_STATE_VERSION. SUCH CHANGES SHOULD ONLY BE MADE IF
+ * ABSOLUTELY NECESSARY!
+ */
+enum RasterConfigs {
+  kUnknown_RasterConfig   = 0,
+  kRGB_565_RasterConfig   = 1,
+  kARGB_8888_RasterConfig = 2
+};
+typedef int32_t RasterConfig;
+
+enum CanvasBackends {
+    kUnknown_CanvasBackend = 0,
+    kRaster_CanvasBackend  = 1,
+    kGPU_CanvasBackend     = 2,
+    kPDF_CanvasBackend     = 3
+};
+typedef int32_t CanvasBackend;
+
+struct ClipRect {
+    int32_t left, top, right, bottom;
+};
+
+struct SkMCState {
+    float matrix[9];
+    // NOTE: this only works for non-antialiased clips
+    int32_t clipRectCount;
+    ClipRect* clipRects;
+};
+
+// NOTE: If you add more members, bump CanvasState::version.
+struct SkCanvasLayerState {
+    CanvasBackend type;
+    int32_t x, y;
+    int32_t width;
+    int32_t height;
+
+    SkMCState mcState;
+
+    union {
+        struct {
+            RasterConfig config; // pixel format: a value from RasterConfigs.
+            uint32_t rowBytes;   // Number of bytes from start of one line to next.
+            void* pixels;        // The pixels, all (height * rowBytes) of them.
+        } raster;
+        struct {
+            int32_t textureID;
+        } gpu;
+    };
+};
+
+class SkCanvasState {
+public:
+    SkCanvasState(SkCanvas* canvas) {
+        SkASSERT(canvas);
+        version = CANVAS_STATE_VERSION;
+        width = canvas->getDeviceSize().width();
+        height = canvas->getDeviceSize().height();
+        layerCount = 0;
+        layers = NULL;
+        originalCanvas = SkRef(canvas);
+
+        mcState.clipRectCount = 0;
+        mcState.clipRects = NULL;
+    }
+
+    ~SkCanvasState() {
+        // loop through the layers and free the data allocated to the clipRects
+        for (int i = 0; i < layerCount; ++i) {
+            sk_free(layers[i].mcState.clipRects);
+        }
+
+        sk_free(mcState.clipRects);
+        sk_free(layers);
+
+        // it is now safe to free the canvas since there should be no remaining
+        // references to the content that is referenced by this canvas (e.g. pixels)
+        originalCanvas->unref();
+    }
+
+    /**
+     * The version this struct was built with.  This field must always appear
+     * first in the struct so that when the versions don't match (and the
+     * remaining contents and size are potentially different) we can still
+     * compare the version numbers.
+     */
+    int32_t version;
+
+    int32_t width;
+    int32_t height;
+
+    SkMCState mcState;
+
+    int32_t layerCount;
+    SkCanvasLayerState* layers;
+
+private:
+    SkCanvas* originalCanvas;
+};
+
+////////////////////////////////////////////////////////////////////////////////
+
+class ClipValidator : public SkCanvas::ClipVisitor {
+public:
+    ClipValidator() : fFailed(false) {}
+    bool failed() { return fFailed; }
+
+    // ClipVisitor
+    virtual void clipRect(const SkRect& rect, SkRegion::Op op, bool antialias) SK_OVERRIDE {
+        fFailed |= antialias;
+    }
+
+    virtual void clipPath(const SkPath&, SkRegion::Op, bool antialias) SK_OVERRIDE {
+        fFailed |= antialias;
+    }
+
+private:
+    bool fFailed;
+};
+
+static void setup_MC_state(SkMCState* state, const SkMatrix& matrix, const SkRegion& clip) {
+    // initialize the struct
+    state->clipRectCount = 0;
+
+    // capture the matrix
+    for (int i = 0; i < 9; i++) {
+        state->matrix[i] = matrix.get(i);
+    }
+
+    /*
+     * capture the clip
+     *
+     * storage is allocated on the stack for the first 4 rects. This value was
+     * chosen somewhat arbitrarily, but does allow us to represent simple clips
+     * and some more common complex clips (e.g. a clipRect with a sub-rect
+     * clipped out of its interior) without needing to malloc any additional memory.
+     */
+    const int clipBufferSize = 4 * sizeof(ClipRect);
+    char clipBuffer[clipBufferSize];
+    SkWriter32 clipWriter(sizeof(ClipRect), clipBuffer, clipBufferSize);
+
+    if (!clip.isEmpty()) {
+        // only returns the b/w clip so aa clips fail
+        SkRegion::Iterator clip_iterator(clip);
+        for (; !clip_iterator.done(); clip_iterator.next()) {
+            // this assumes the SkIRect is stored in l,t,r,b ordering which
+            // matches the ordering of our ClipRect struct
+            clipWriter.writeIRect(clip_iterator.rect());
+            state->clipRectCount++;
+        }
+    }
+
+    // allocate memory for the clip then and copy them to the struct
+    state->clipRects = (ClipRect*) sk_malloc_throw(clipWriter.size());
+    clipWriter.flatten(state->clipRects);
+}
+
+
+
+SkCanvasState* SkCanvasStateUtils::CaptureCanvasState(SkCanvas* canvas) {
+    SkASSERT(canvas);
+
+    // Check the clip can be decomposed into rectangles (i.e. no soft clips).
+    ClipValidator validator;
+    canvas->replayClips(&validator);
+    if (validator.failed()) {
+        SkDEBUGF(("CaptureCanvasState does not support canvases with antialiased clips.\n"));
+        return NULL;
+    }
+
+    SkAutoTDelete<SkCanvasState> canvasState(SkNEW_ARGS(SkCanvasState, (canvas)));
+
+    // decompose the total matrix and clip
+    setup_MC_state(&canvasState->mcState, canvas->getTotalMatrix(), canvas->getTotalClip());
+
+    /*
+     * decompose the layers
+     *
+     * storage is allocated on the stack for the first 3 layers. It is common in
+     * some view systems (e.g. Android) that a few non-clipped layers are present
+     * and we will not need to malloc any additional memory in those cases.
+     */
+    const int layerBufferSize = 3 * sizeof(SkCanvasLayerState);
+    char layerBuffer[layerBufferSize];
+    SkWriter32 layerWriter(sizeof(SkCanvasLayerState), layerBuffer, layerBufferSize);
+    int layerCount = 0;
+    for (SkCanvas::LayerIter layer(canvas, true/*skipEmptyClips*/); !layer.done(); layer.next()) {
+
+        // we currently only work for bitmap backed devices
+        const SkBitmap& bitmap = layer.device()->accessBitmap(true/*changePixels*/);
+        if (bitmap.empty() || bitmap.isNull() || !bitmap.lockPixelsAreWritable()) {
+            return NULL;
+        }
+
+        SkCanvasLayerState* layerState =
+                (SkCanvasLayerState*) layerWriter.reserve(sizeof(SkCanvasLayerState));
+        layerState->type = kRaster_CanvasBackend;
+        layerState->x = layer.x();
+        layerState->y = layer.y();
+        layerState->width = bitmap.width();
+        layerState->height = bitmap.height();
+
+        switch (bitmap.config()) {
+            case SkBitmap::kARGB_8888_Config:
+                layerState->raster.config = kARGB_8888_RasterConfig;
+                break;
+            case SkBitmap::kRGB_565_Config:
+                layerState->raster.config = kRGB_565_RasterConfig;
+                break;
+            default:
+                return NULL;
+        }
+        layerState->raster.rowBytes = bitmap.rowBytes();
+        layerState->raster.pixels = bitmap.getPixels();
+
+        setup_MC_state(&layerState->mcState, layer.matrix(), layer.clip());
+        layerCount++;
+    }
+
+    // allocate memory for the layers and then and copy them to the struct
+    SkASSERT(layerWriter.size() == layerCount * sizeof(SkCanvasLayerState));
+    canvasState->layerCount = layerCount;
+    canvasState->layers = (SkCanvasLayerState*) sk_malloc_throw(layerWriter.size());
+    layerWriter.flatten(canvasState->layers);
+
+    // for now, just ignore any client supplied DrawFilter.
+    if (canvas->getDrawFilter()) {
+        SkDEBUGF(("CaptureCanvasState will ignore the canvases draw filter.\n"));
+    }
+
+    return canvasState.detach();
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+static void setup_canvas_from_MC_state(const SkMCState& state, SkCanvas* canvas) {
+    // reconstruct the matrix
+    SkMatrix matrix;
+    for (int i = 0; i < 9; i++) {
+        matrix.set(i, state.matrix[i]);
+    }
+
+    // reconstruct the clip
+    SkRegion clip;
+    for (int i = 0; i < state.clipRectCount; ++i) {
+        clip.op(SkIRect::MakeLTRB(state.clipRects[i].left,
+                                  state.clipRects[i].top,
+                                  state.clipRects[i].right,
+                                  state.clipRects[i].bottom),
+                SkRegion::kUnion_Op);
+    }
+
+    canvas->setMatrix(matrix);
+    canvas->setClipRegion(clip);
+}
+
+static SkCanvas* create_canvas_from_canvas_layer(const SkCanvasLayerState& layerState) {
+    SkASSERT(kRaster_CanvasBackend == layerState.type);
+
+    SkBitmap bitmap;
+    SkBitmap::Config config =
+        layerState.raster.config == kARGB_8888_RasterConfig ? SkBitmap::kARGB_8888_Config :
+        layerState.raster.config == kRGB_565_RasterConfig ? SkBitmap::kRGB_565_Config :
+        SkBitmap::kNo_Config;
+
+    if (config == SkBitmap::kNo_Config) {
+        return NULL;
+    }
+
+    bitmap.setConfig(config, layerState.width, layerState.height,
+                     layerState.raster.rowBytes);
+    bitmap.setPixels(layerState.raster.pixels);
+
+    SkASSERT(!bitmap.empty());
+    SkASSERT(!bitmap.isNull());
+
+    // create a device & canvas
+    SkAutoTUnref<SkBitmapDevice> device(SkNEW_ARGS(SkBitmapDevice, (bitmap)));
+    SkAutoTUnref<SkCanvas> canvas(SkNEW_ARGS(SkCanvas, (device.get())));
+
+    // setup the matrix and clip
+    setup_canvas_from_MC_state(layerState.mcState, canvas.get());
+
+    return canvas.detach();
+}
+
+SkCanvas* SkCanvasStateUtils::CreateFromCanvasState(const SkCanvasState* state) {
+    SkASSERT(state);
+
+    // check that the versions match
+    if (CANVAS_STATE_VERSION != state->version) {
+        SkDebugf("CreateFromCanvasState version does not match the one use to create the input");
+        return NULL;
+    }
+
+    if (state->layerCount < 1) {
+        return NULL;
+    }
+
+    SkAutoTUnref<SkNWayCanvas> canvas(SkNEW_ARGS(SkNWayCanvas, (state->width, state->height)));
+
+    // setup the matrix and clip on the n-way canvas
+    setup_canvas_from_MC_state(state->mcState, canvas);
+
+    // Iterate over the layers and add them to the n-way canvas
+    for (int i = 0; i < state->layerCount; ++i) {
+        SkAutoTUnref<SkCanvas> canvasLayer(create_canvas_from_canvas_layer(state->layers[i]));
+        if (!canvasLayer.get()) {
+            return NULL;
+        }
+        canvas->addCanvas(canvasLayer.get());

[joth, 2013/08/30 06:45:57]

I think there's a bug here that you never look at the restored layer's x & y
values. I believe this is the reason for the symptoms I'm seeing over in
https://b/10152369#ISSUE_HistoryHeader64 although I can't exactly explain the
link.

I tried translating canvasLayer by -x & -y prior to adding it, but that seemed
to have no effect what-so-ever. So I'm not quite sure. (Aside: as you have a
clip and matrix exported per layer, maybe the x & y are redundant in the
flattened state. Just need to work out how to pre-apply them into the other
fields on the export side).

[joth, 2013/08/30 07:05:04]

Hmm OK x & y maybe a red-herring.

I looked a bit more at how this is implemented using  SkNWayCanvas vs a stack of
devices (as I had in
https://codereview.chromium.org/22799011/diff/3001/android_webview/browser/in...)
and I see SkNWayCanvas::setMatrix is just forwarded verbatim to each underlying
canvas -- there's not way for each layer canvas to retain it's own "device"
layer translation indenpendent of how the user uses the public API translate /
matrix.
 
the cc software_renderer makes pretty extensive use of setMatrix which as I
understand it will blow away and translate  and result in each layer having
exact same matrix. This is why I see the top of the logical page appear in all
the layers - even the one that is positioned at the bottom of the page.

[djsollen, 2013/08/30 11:51:19]

Yes, the setMatrix will cause some havoc with this solution.  I'll work on
coding something up so that if someone overwrites the entire matrix like that we
preserve the original x,y position for every canvas that we insert into the
n-way canvas.

+    }
+
+    return canvas.detach();
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+void SkCanvasStateUtils::ReleaseCanvasState(SkCanvasState* state) {
+    SkDELETE(state);
+}