ios - 如何在 iOS8 中以原始分辨率显示自制位图

Question

我正在为 iOS 开发一个通用应用程序，它将动态生成它自己的全屏位图(指向字节缓冲区中 32 位像素数据的指针)。它对触摸事件使用react，并且需要在用户触摸时以响应方式进行绘图(例如缩放/平移)。在我的应用程序开始时，我可以看到我的视网膜 iPad 和 iPod Touch 上的显示缩放了 2 倍。我的代码当前正确创建和显示位图，但分辨率为显示器原始分辨率的 1/2。我可以使用 View 的 nativeBounds 查看原始分辨率，但我想以原始分辨率创建和显示我的位图而不进行任何缩放。我尝试在 drawRect() 方法中更改变换比例，但它没有正常工作。下面是我的 drawRect 代码:

- (void)drawRect:(CGRect)rect
{
    UInt32 * pixels;
    pixels = (UInt32 *)[TheFile thePointer];
    NSUInteger width = [TheFile iScreenWidth];
    NSUInteger height = [TheFile iScreenHeight];
    NSUInteger borderX = [TheFile iBorderX];
    NSUInteger borderY = [TheFile iBorderY];
    NSUInteger bytesPerPixel = 4;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef gtx = CGBitmapContextCreate(pixels, width-borderX*2, height-borderY*2, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaNoneSkipLast | kCGBitmapByteOrder32Big);
    CGImageRef myimage = CGBitmapContextCreateImage(gtx);

    CGContextSetInterpolationQuality(gtx, kCGInterpolationNone); // does this speed it up?

    // Create a rect to display
    CGRect imageRect = CGRectMake(borderX, borderY, width - borderX*2, height - borderY * 2);

    // Need to repaint the background that would show through (black)
    if (borderX != 0 || borderY != 0)
    {
        [[UIColor blackColor] set];
        UIRectFill(rect);
    }

    // Transform image (flip right side up)
    CGContextTranslateCTM(context, 0, height);
    CGContextScaleCTM(context, 1.0, -1.0);

    // Draw the image
    CGContextDrawImage(context, imageRect, myimage); //image.CGImage);

    CGColorSpaceRelease(colorSpace);
    CGContextRelease(gtx);
    CGImageRelease(myimage);

} /* drawRect() */

编辑: 下面的答案通过使用 UIImageView 解决了性能问题，通过在 UIImage 的初始化中设置适当的显示比例解决了缩放问题。当 UIImage 比例与显示比例匹配时，它将以设备的原始分辨率以 1:1 的比例显示位图。

Answer 1

您的代码的问题在于，在创建结果图像后，代码将图像绘制到为 drawRect: 配置的当前图形上下文中。绘制图像的是 CPU。这就是为什么需要 70 毫秒。使用 UIImageView 渲染图像或将其设置为层的内容不由 CPU 处理，这样，它由 GPU 处理。 GPU 擅长这样的事情，所以在这种情况下它要快得多。由于 drawRect: 导致 Core Animation 创建一个在这种情况下无用的支持位图，因此您应该创建没有 drawRect: 的图像:

- (UIImage *)drawBackground {
    UIImage *output;
    UInt32 * pixels;
    pixels = (UInt32 *)[TheFile thePointer];
    NSUInteger borderX = [TheFile iBorderX];
    NSUInteger borderY = [TheFile iBorderY];
    NSUInteger width = [TheFile iScreenWidth] - 2 * borderX;
    NSUInteger height = [TheFile iScreenHeight] - 2 * borderY;
    NSUInteger bytesPerPixel = 4;
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    CGFloat scale = [UIScreen mainScreen].scale;

    // create bitmap graphic context
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef gtx = CGBitmapContextCreate(pixels, 
                                             width, 
                                             height,
                                             bitsPerComponent,
                                             bytesPerRow, 
                                             colorSpace, 
                                             kCGImageAlphaNoneSkipLast | 
                                             kCGBitmapByteOrder32Big);

    // create image
    CGImageRef myimage = CGBitmapContextCreateImage(gtx);
    output = [UIImage imageWithCGImage:myimage 
                                 scale:[UIScreen mainScreen].scale 
                           orientation:UIImageOrientationUp];

    // clean up
    CGColorSpaceRelease(colorSpace);
    CGContextRelease(gtx);
    CGImageRelease(myimage);

    return output;
}

当用户触发事件时，假设您使用手势识别器:

- (IBAction)handleTap:(UITapGestureRecognizer *)tap {
    UIImage *background = [self drawBackground];

    // when background view is an UIImageView
    self.backgroundView.image = background;
    // self.backgroundView should have already set up in viewDidLoad
    // based on your code snippet, you may need to configure background color
    // self.backgroundView.backgroundColor = [UIColor blackColor];
    // do other configuration if needed...

    // when background view is an UIView or subclass of UIView
    self.backgroundView.layer.contents = (id)background.CGImage;
    // not like UIImageView, the size of background view must exactly equals 
    // to the size of background image. Otherwise the image will be scaled.
}

---

我在一个测试项目中写了这个函数:

static UIImage *drawBackground() {
    // allocate bitmap buffer
    CGFloat scale = [UIScreen mainScreen].scale;
    CGRect screenBounds = [UIScreen mainScreen].bounds;
    NSUInteger borderWidth = 1 * 2; // width of border is 1 pixel
    NSUInteger width = scale * CGRectGetWidth(screenBounds) - borderWidth;
    NSUInteger height = scale * CGRectGetHeight(screenBounds) - borderWidth;
    NSUInteger bytesPerPixel = 4;

    // test resolution begin
    // tested on a iPhone 4 (320 x 480 points, 640 x 960 pixels), iOS 7.1
    // the image is rendered by an UIImageView which covers whole screen.
    // the content mode of UIImageView is center, which doesn't cause scaling.
    width = scale * 310; 
    height = scale * 240;
    // test resolution end

    UInt32 *pixels = malloc((size_t)(width * height * bytesPerPixel));

    // manipulate bitmap buffer
    NSUInteger count = width * height;
    unsigned char *byte = (unsigned char *)pixels;
    for (int i = 0; i < count; i = i + 1) {
        byte[0] = 100;
        byte = byte + 1;
        byte[0] = 100;
        byte = byte + 1;
        byte[0] = 0;
        byte = byte + 2;
    }

    // create bitmap grahpic context
    NSUInteger bytesPerRow = bytesPerPixel * width;
    NSUInteger bitsPerComponent = 8;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef gtx = CGBitmapContextCreate(pixels,
                                             width,
                                             height,
                                             bitsPerComponent,
                                             bytesPerRow,
                                             colorSpace,
                                             kCGImageAlphaNoneSkipLast |
                                             kCGBitmapByteOrder32Big);

    // create image
    CGImageRef myimage = CGBitmapContextCreateImage(gtx);
    UIImage *output = [UIImage imageWithCGImage:myimage 
                                          scale:scale 
                                    orientation:UIImageOrientationUp];

    // clean up
    CGColorSpaceRelease(colorSpace);
    CGContextRelease(gtx);
    CGImageRelease(myimage);
    free(pixels);

    return output;
}

我在 iPhone 4 设备上对其进行了测试。对我来说似乎没问题。这是屏幕截图: