gpu与image的来回转换

src\third_party\skia\docs\examples

在 gpu 上绘制，然后用surface的snapshot获取 图片； 

skimage也可以直接从texture

// Copyright 2019 Google LLC.
// Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
#include "tools/fiddle/examples.h"
// HASH=06aeb3cf63ffccf7b49fe556e5def351
REG_FIDDLE(Image_MakeBackendTextureFromSkImage, 256, 64, false, 0) {
static sk_sp<SkImage> create_gpu_image(GrRecordingContext* rContext) {
    // rContext 是个gpu绘制上下文
    const SkImageInfo info = SkImageInfo::MakeN32(20, 20, kOpaque_SkAlphaType);
    auto surface(SkSurface::MakeRenderTarget(rContext, SkBudgeted::kNo, info));
    
    //通过canvas绘制，绘制在gpu进行
    SkCanvas* canvas = surface->getCanvas();
    canvas->clear(SK_ColorWHITE);
    SkPaint paint;
    paint.setColor(SK_ColorBLACK);
    canvas->drawRect(SkRect::MakeXYWH(5, 5, 10, 10), paint);
    //获取刚才绘制的图片
    return surface->makeImageSnapshot();
}

void draw(SkCanvas* canvas) {
    auto dContext = GrAsDirectContext(canvas->recordingContext());
    if (!dContext) {
        return;
    }
    sk_sp<SkImage> backEndImage = create_gpu_image(dContext);
    canvas->drawImage(backEndImage, 0, 0);
    
    //将图片绘制到gpu的texture上
    GrBackendTexture texture;
    SkImage::BackendTextureReleaseProc proc;
    if (!SkImage::MakeBackendTextureFromSkImage(dContext, std::move(backEndImage),
                                                &texture, &proc)) {
        return;
    }
    //从texture绘制获取成skimage图片。
    sk_sp<SkImage> i2 = SkImage::MakeFromTexture(dContext, texture, kTopLeft_GrSurfaceOrigin,
                                                 kN32_SkColorType, kOpaque_SkAlphaType, nullptr);
    canvas->drawImage(i2, 30, 30);
}
}  // END FIDDLE