iPhone Objective C：UIImage - 感兴趣区域

Question

有没有办法获得包含 UIImage 的所有非透明部分的最小矩形区域？

我认为，逐像素读取以检查 alpha == 0 的位置......这不是一个好方法。

还有更好的办法吗？

非常感谢您的阅读

Answer 1

我认为没有办法在不逐像素检查图像的情况下做到这一点。图像来自哪里？如果你控制它们，你至少可以只逐个像素地执行一次，然后缓存信息或将其与图像一起分发（如果人们下载它们）。

Answer 2

好吧，这是我对这个问题的丑陋解决方案。我希望有更好的方法来做到这一点。

- (CGRect) getROIRect:(UIImage*)pImage {

CGRect roiRect = {{0,0}, {0,0}};
int vMaxX = -999;
int vMinX = 999;
int vMaxY = -999;
int vMinY = 999;
int x,y;

CGImageRef inImage = pImage.CGImage;
// Create off screen bitmap context to draw the image into. Format ARGB is 4 bytes for each pixel: Alpa, Red, Green, Blue
CGContextRef cgctx = [self createARGBBitmapContextFromImage:inImage];
if (cgctx == NULL) { return roiRect; /* error */ }

size_t w = CGImageGetWidth(inImage);
size_t h = CGImageGetHeight(inImage);
CGRect rect = {{0,0},{w,h}}; 

// Draw the image to the bitmap context. Once we draw, the memory
// allocated for the context for rendering will then contain the
// raw image data in the specified color space.
CGContextDrawImage(cgctx, rect, inImage); 

// Now we can get a pointer to the image data associated with the bitmap
// context.
unsigned char* data ;
BOOL tSet = NO;

    data= CGBitmapContextGetData (cgctx);
if (data != NULL) {
    for (x=0;x<w;x++) {
        for (y=0;y<h;y++) {

            //offset locates the pixel in the data from x,y.
            //4 for 4 bytes of data per pixel, w is width of one row of data.
            int offset = 4*((w*round(y))+round(x));
            int alpha =  data[offset];
            if (alpha > 0) {
                tSet = YES;
                if (x > vMaxX) {
                    vMaxX = x;
                }
                if (x < vMinX) {
                    vMinX = x;
                }
                if (y > vMaxY) {
                    vMaxY = y;
                }
                if (y < vMinY) {
                    vMinY = y;
                }
            }
        }
    }
}

if (!tSet) {
    vMaxX = w;
    vMinX = 0;
    vMaxY = h;
    vMinY = 0;
}
// When finished, release the context
CGContextRelease(cgctx);
// Free image data memory for the context
if (data) { free(data); }

CGRect roiRect2 = {{vMinX,vMinY},{vMaxX - vMinX,vMaxY - vMinY}}; 

return roiRect2;
}

- (CGContextRef) createARGBBitmapContextFromImage:(CGImageRef) inImage {

CGContextRef    context = NULL;
CGColorSpaceRef colorSpace;
void *          bitmapData;
int             bitmapByteCount;
int             bitmapBytesPerRow;

// Get image width, height. We'll use the entire image.
size_t pixelsWide = CGImageGetWidth(inImage);
size_t pixelsHigh = CGImageGetHeight(inImage);

// Declare the number of bytes per row. Each pixel in the bitmap in this
// example is represented by 4 bytes; 8 bits each of red, green, blue, and
// alpha.
bitmapBytesPerRow   = (pixelsWide * 4);
bitmapByteCount     = (bitmapBytesPerRow * pixelsHigh);

// Use the generic RGB color space.
//colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);

colorSpace = CGColorSpaceCreateDeviceRGB();

if (colorSpace == NULL)
{
    fprintf(stderr, "Error allocating color space\n");
    return NULL;
}

// Allocate memory for image data. This is the destination in memory
// where any drawing to the bitmap context will be rendered.
bitmapData = malloc( bitmapByteCount );
if (bitmapData == NULL)
{
    fprintf (stderr, "Memory not allocated!");
    CGColorSpaceRelease( colorSpace );
    return NULL;
}

// Create the bitmap context. We want pre-multiplied ARGB, 8-bits
// per component. Regardless of what the source image format is
// (CMYK, Grayscale, and so on) it will be converted over to the format
// specified here by CGBitmapContextCreate.
context = CGBitmapContextCreate (bitmapData, pixelsWide, pixelsHigh, 8,    bitmapBytesPerRow, colorSpace,    kCGImageAlphaPremultipliedFirst);
if (context == NULL)
{
    free (bitmapData);
    fprintf (stderr, "Context not created!");
}

// Make sure and release colorspace before returning
CGColorSpaceRelease( colorSpace );

return context;
}