分析ImageLoader图片压缩源码分析

ゞ浴缸里的玫瑰 2022-10-01 09:45 120阅读 0赞

/\*\*

* Computes sample size for downscaling image size (srcSize) to view size (targetSize). This sample
 * size is used during
 * {@linkplain BitmapFactory#decodeStream(java.io.InputStream, android.graphics.Rect, android.graphics.BitmapFactory.Options)
 * decoding image} to bitmap. 
 * 
 * Examples: 
 * 
 * <pre>
 * srcSize(100x100), targetSize(10x10), powerOf2Scale = true -> sampleSize = 8
 * srcSize(100x100), targetSize(10x10), powerOf2Scale = false -> sampleSize = 10
 *
 * srcSize(100x100), targetSize(20x40), viewScaleType = FIT_INSIDE -> sampleSize = 5
 * srcSize(100x100), targetSize(20x40), viewScaleType = CROP -> sampleSize = 2
 * </pre>
 * 
 * 
 * The sample size is the number of pixels in either dimension that correspond to a single pixel in the decoded
 * bitmap. For example, inSampleSize == 4 returns an image that is 1/4 the width/height of the original, and 1/16
 * the number of pixels. Any value <= 1 is treated the same as 1.
 *
 * @param srcSize Original (image) size
 * @param targetSize Target (view) size
 * @param viewScaleType {@linkplain ViewScaleType Scale type} for placing image in view
 * @param powerOf2Scale true - if sample size be a power of 2 (1, 2, 4, 8, ...)
 * @return Computed sample size
 */
 public static int computeImageSampleSize(ImageSize srcSize, ImageSize targetSize, ViewScaleType viewScaleType,
 		boolean powerOf2Scale) {
 	final int srcWidth = srcSize.getWidth();
 	final int srcHeight = srcSize.getHeight();
 	final int targetWidth = targetSize.getWidth();
 	final int targetHeight = targetSize.getHeight();
 
 	int scale = 1;
 
 	switch (viewScaleType) {
 		//标准是宽高压缩比例大的
 		case FIT_INSIDE:
 			if (powerOf2Scale) {
 				final int halfWidth = srcWidth / 2;
 				final int halfHeight = srcHeight / 2;
 				//scale两倍放大，直到压缩后的大小所以目标大小
 				while ((halfWidth / scale) > targetWidth || (halfHeight / scale) > targetHeight) { // ||
 					scale *= 2;
 				}
 			} else {
 				scale = Math.max(srcWidth / targetWidth, srcHeight / targetHeight); // max
 			}
 			break;
 		case CROP:
 		//标准是宽高压缩比例小的
 			if (powerOf2Scale) {
 				final int halfWidth = srcWidth / 2;
 				final int halfHeight = srcHeight / 2;
 				while ((halfWidth / scale) > targetWidth && (halfHeight / scale) > targetHeight) { // &&
 					scale *= 2;
 				}
 			} else {
 				scale = Math.min(srcWidth / targetWidth, srcHeight / targetHeight); // min
 			}
 			break;
 	}
 
 	if (scale < 1) {
 		scale = 1;
 	}
 	//压缩比例因在最大宽高比例内，压缩后的宽高不能超过最大宽高比。
 	scale = considerMaxTextureSize(srcWidth, srcHeight, scale, powerOf2Scale);
 
 	return scale;
 }
 
 private static int considerMaxTextureSize(int srcWidth, int srcHeight, int scale, boolean powerOf2) {
 	final int maxWidth = maxBitmapSize.getWidth();
 	final int maxHeight = maxBitmapSize.getHeight();
 	while ((srcWidth / scale) > maxWidth || (srcHeight / scale) > maxHeight) {
 		if (powerOf2) {
 			scale *= 2;
 		} else {
 			scale++;
 		}
 	}
 	return scale;
 }
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转载于:https://juejin.im/post/5ccff670f265da039444bee6