SkImageInfo

• 整形的宽,高
这里的宽可以认为是图像的列数,即 column count
这里的高可以认为是图像的行数,

• 用于描述像素格式的SkColorType和SkAlphaType类型

• 用于描述颜色范围的SkColorSpace

• Make(int width, int height, SkColorType ct, SkAlphaType at, sk_sp<SkColorSpace> cs = nullptr)
传入的参数就是宽,高,颜色类型,透明度类型,颜色空间
这里的颜色空间cs是一个可选参数,如果颜色空间为null,默认的颜色空间则是sRGB.

• MakeN32(int width, int height, SkAlphaType at, sk_sp<SkColorSpace> cs = nullptr)
颜色类型指定为kN32_SkColorType,即kRGBA_8888_SkColorType.

• MakeN32Premul(int width, int height, sk_sp<SkColorSpace> cs = nullptr)
颜色类型为kN32_SkColorType,透明度类型为kPremul_SkAlphaType
Premul是单词premultiplied(预乘)的简称,即RGB分量的值其实已经和alpha相乘过了.

• MakeA8(int width, int height)

• MakeUnknown(int width, int height)
颜色类型和透明度类型都不设置的情况.

SkAlphaType

enum SkAlphaType {    
    kUnknown_SkAlphaType,                          //!< uninitialized    
    kOpaque_SkAlphaType,                           //!< pixel is opaque    
    kPremul_SkAlphaType,                           //!< pixel components are premultiplied by alpha   
    kUnpremul_SkAlphaType,                         //!< pixel components are independent of alpha    
    kLastEnum_SkAlphaType = kUnpremul_SkAlphaType, //!< last valid value
};

• kOpaque_SkAlphaType
透明度为1.0,即完全不透明

为SkBitmap分配内存

• allocPixels()

• allocPixels(const SkImageInfo& info, size_t rowBytes)
如果使用了这个方法,就不需要提前使用setInfo()方法设置SkImageInfo了,第二个参数即行字节数,也就是高度 * 每个像素包含的字节数,传入的值应该等于真实值或者比真实值稍大.

• tryAllocPixels(const SkImageInfo& info)

基本使用

#include "demo.h"
#include "include/core/SkBitmap.h"
#include "include/core/SkImageInfo.h"
#include "include/core/SkColor.h"
#include "utils/file_utils.h"

void drawDemoAlt() {
    SkBitmap bitmap;
    bitmap.setInfo(SkImageInfo::MakeN32(640, 640, kOpaque_SkAlphaType));
    bitmap.allocPixels();

    SkCanvas offscreen(bitmap);
    offscreen.clear(SK_ColorWHITE);

    SkPaint paint;
    paint.setAntiAlias(true);
    // 等分绘制圆形，颜色也按照规律改变
    for (int i = 0; i <= bitmap.width(); i = i + bitmap.width() / 10) {
        for (int j = 0; j <= bitmap.height(); j = j + bitmap.height() / 10) {
            float rRatio = (float) i / (float) bitmap.width();
            float gRatio = (float) j / (float) bitmap.height();
            float bRatio = (float) (i + j) / (float) (bitmap.width() + bitmap.height());
            paint.setColor(SkColorSetRGB(255 * rRatio, 255 * gRatio, 255 * bRatio));
            offscreen.drawCircle(i, j, 16, paint);
        }
    }
    saveBitmapAsPng("bitmap_allocPixels_alt.png", bitmap);
}

/*
 * 创建Bitmap的方法
 */
void drawDemo() {
    //创建Bitmap
    SkBitmap bitmap;
    SkImageInfo imageInfo = SkImageInfo::MakeN32(600, 320, kOpaque_SkAlphaType);
    bitmap.allocPixels(imageInfo, imageInfo.width() * imageInfo.bytesPerPixel());

    //Canva绘制
    SkCanvas offscreen(bitmap);
    offscreen.clear(SK_ColorWHITE);
    SkPaint paint;
    paint.setStrokeWidth(2);
    paint.setAntiAlias(true);
    paint.setColor(SK_ColorRED);
    for (int y : {100, 200, 300, 400, 500}) {
        offscreen.drawLine(0, y, 600, y, paint);
    }
    // 保存文件
    saveBitmapAsPng("bitmap_allocPixels.png", bitmap);
}

1. 设置SkImageInfo

2. 分配内存

源码地址