概述
Skia 是一款用 C++ 开发的、性能彪悍的 2D 图像绘制引擎，其前身是一个向量绘图软件。2005年被谷歌收购，Skia也成为了Android系统的2D渲染显示核心。Skia 在图形转换、文字渲染、位图渲染方面都表现卓越，并提供了开发者友好的 API。
Skia在Android中的地位
- 规定2D绘制API
- 规定图像数据结构
- 承担编解码调度和软件渲染职责
Skia初探学习
Android Java 2D作图主要通过JNI调用skia图形库完成。Canvas是个2D的概念，在skia中可以把这个canvas理解成系统提供给我们的一块内存区域，但实际上它只是一套绘图API，真正的内存是下面的Bitmap，skia 提供一个bitmap对象。
在surface上绘制UI，通过lock来获取surface对应的数据buffer，并当做skia bitmap的内存，unlockandpost来通知Surfaceflinger来输出显示。
SkBitmap用来设置像素。SkCanvas执行写入操作。SkPaint相当于画笔，用来设置颜色(color), 字体(typeface), 文字大小(textSize), 文字粗细(strokeWidth), 渐变(gradients, patterns)等。
int main(int argc, char **argv)
{
// create a client to surfaceflinger
sp<SurfaceComposerClient> client = new SurfaceComposerClient();
sp<IBinder> dtoken(SurfaceComposerClient::getBuiltInDisplay(
ISurfaceComposer::eDisplayIdMain));
DisplayInfo dinfo;
status_t status = SurfaceComposerClient::getDisplayInfo(dtoken, &dinfo);
printf("w=%d,h=%d,xdpi=%f,ydpi=%f,fps=%f,ds=%f\n",
dinfo.w, dinfo.h, dinfo.xdpi, dinfo.ydpi, dinfo.fps, dinfo.density);
sp<SurfaceControl> surfaceControl =
client->createSurface(String8("debugview"),
dinfo.w, dinfo.h,
PIXEL_FORMAT_RGBA_8888,
0/*ISurfaceComposerClient::eHidden*/);
surfaceControl;
SurfaceComposerClient::openGlobalTransaction();
surfaceControl->setLayer(100000);
surfaceControl->setPosition(20, 20);
//surfaceControl->setAlpha(0.3f);
//surfaceControl->setSize(800, 800);
//surfaceControl->setSize(1500, 1000);
//Rect rect(900,900,900,900);
//Region region(rect);
//surfaceControl->setTransparentRegionHint(region);
SurfaceComposerClient::closeGlobalTransaction();
surfaceControl->show();
sp<Surface> surface = surfaceControl->getSurface();
ANativeWindow_Buffer outBuffer;
//Surface::SurfaceoutBuffer outBuffer;
//surface->lock(&outBuffer,NULL);//
//ssize_t bpr = outBuffer.stride * bytesPerPixel(outBuffer.format);
//android_memset16((uint16_t*)outBuffer.bits, 0xF800, bpr*outBuffer.height);
//surface->unlockAndPost();
//sleep(1);
ssize_t bpr;
SkBitmap bitmap;
//SkImageInfo info = SkImageInfo::MakeN32Premul(512, 512)
//SkImageInfo info = SkImageInfo::Make(512, 512,
// kN32_SkColorType,
// kOpaque_SkAlphaType);
SkImageInfo info = SkImageInfo::Make(512, 512,
kN32_SkColorType,
kPremul_SkAlphaType);
surface->lock(&outBuffer, NULL);
bpr = outBuffer.stride * bytesPerPixel(outBuffer.format);
bitmap.installPixels(info, outBuffer.bits, bpr);
SkCanvas canvas(bitmap);
canvas.clear(SK_ColorTRANSPARENT);
SkPaint paint;
paint.setTextSize(28);
paint.setColor(SK_ColorBLACK);
//canvas.drawColor(SK_ColorRED);
const char *str = "aaaaaaaaaaaaaaaaaaE";
canvas.drawText(str, strlen(str), 8, 28, paint);
surface->unlockAndPost();
sleep(3);
#if 0
SurfaceComposerClient::openGlobalTransaction();
surfaceControl->setLayer(100000);
surfaceControl->setPosition(0, 0);
SurfaceComposerClient::closeGlobalTransaction();
surfaceControl->show();
sp<Surface> surface = surfaceControl->getSurface();
ANativeWindow_Buffer outBuffer;
surface->lock(&outBuffer,NULL);
ssize_t bpr = outBuffer.stride * bytesPerPixel(outBuffer.format);
android_memset16((uint16_t*)outBuffer.bits, 0xF800, bpr*outBuffer.height);
surface->unlockAndPost();
sleep(3);
SurfaceComposerClient::openGlobalTransaction();
surfaceControl->setSize(480, 272);
surfaceControl->setPosition(100, 100);
SurfaceComposerClient::closeGlobalTransaction();
surfaceControl->show();
FILE *fp = fopen("/tmp/rgb565.rgb","rb");
if(fp != NULL){
unsigned char *rgb565Data = new unsigned char[480*272*2];
memset(rgb565Data,0x00,480*272*2);
fread(rgb565Data,1,480*272*2,fp);
surface->lock(&outBuffer,NULL);
memcpy(outBuffer.bits,rgb565Data,480*272*2);
delete[] rgb565Data;
surface->unlockAndPost();
}
fclose(fp);
sleep(3);
SurfaceComposerClient::openGlobalTransaction();
surfaceControl->setSize(320, 420);
surfaceControl->setPosition(100, 100);
SurfaceComposerClient::closeGlobalTransaction();
surfaceControl->show();
SkPaint paint;
paint.setColor(SK_ColorBLUE);
Rect rect(0, 0, 320, 240);
Region dirtyRegion(rect);
surface->lock(&outBuffer, &rect);
bpr = outBuffer.stride * bytesPerPixel(outBuffer.format);
SkBitmap bitmap;
bitmap.setConfig(convertPixelFormat(outBuffer.format), 320, 240, bpr);
bitmap.setPixels(outBuffer.bits);
SkCanvas canvas;
SkRegion clipReg;
const Rect b(dirtyRegion.getBounds());
clipReg.setRect(b.left, b.top, b.right, b.bottom);
canvas.clipRegion(clipReg);
canvas.drawARGB(0, 0xFF, 0x00, 0xFF);
canvas.drawCircle(200, 200, 100, paint);
bitmap.notifyPixelsChanged();
surface->unlockAndPost();
sleep(3);
SkFILEStream stream("/tmp/test.jpg");
SkImageDecoder* codec = SkImageDecoder::Factory(&stream);
if(codec){
SkBitmap bmp;
stream.rewind();
codec->decode(&stream, &bmp,
SkBitmap::kRGB_565_Config,
SkImageDecoder::kDecodePixels_Mode);
surface->lock(&outBuffer,NULL);
bpr = outBuffer.stride * bytesPerPixel(outBuffer.format);
bitmap.setConfig(convertPixelFormat(outBuffer.format), 320, 240, bpr);
bitmap.setPixels(outBuffer.bits);
canvas.drawBitmap(bmp, SkIntToScalar(200), SkIntToScalar(300));
surface->unlockAndPost();
sleep(3);
}
#endif
return 0;
}
设置字体
SkTypeface *font = SkTypeface::CreateFromFile("xxx.otf");
if(font){
paint.setARGB(255, 255, 0, 0);
paint.setTypeface(font);
paint.setTextSize(25);
canvas.drawText("abc", 3, 20, 20, paint);
}
还可以使用SkTypeface::CreateFromName创建face，skia会遍历SK_FONT_FILE_PREFIX宏所指示的目录下所有*.ttf字库文件，并将其加载内存。
以上我们简单的介绍了Flutter中的Skia引擎图像处理技术；在Android开发中啊Skia技术是很重要的存在，对于Skia的技术学习是需要更加深入学习，有关更多Skia技术和Flutter学习进阶。可以前往获取《Flutter3.0学习手册》；海量Android开发技术知识进阶也可以点击查看获取方式！
小结
目前，Skia 已然是 Android 官方的图像渲染引擎了，因此 Flutter Android SDK 无需内嵌 Skia 引擎就可以获得天然的 Skia 支持；而对于 iOS 平台来说，由于 Skia 是跨平台的，因此它作为 Flutter iOS 渲染引擎被嵌入到 Flutter 的 iOS SDK 中，替代了 iOS 闭源的 Core Graphics/Core Animation/Core Text，这也正是 Flutter iOS SDK 打包的 App 包体积比 Android 要大一些的原因。
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