本文转自:http://blog.csdn.net/celerychen2009/article/details/9028145
在传统的图像,视频的后处理阶段,一般会涉及到图像大小的缩放问题。这样的操作是为了适配不同屏幕分辨率的大小。例如,对于高档相机拍摄的照片,一般都很大,而要在普通显示器上显示,则要在解码图像数据之后再做缩小操作才能显示到屏幕上。对于手机屏幕,更是如此。那么,能否在图像解码的过程中实现图片的缩小?答案是肯定的。网上开源的jpeg项目早就实现了频域下采样技术。美图秀秀,QQlive等多款桌面图片应用软件也采用了这项技术,并宣称对大图的加载速度快了好几倍。不过,QQlive居然直接使用了这个库!
通过在视频解码中使用频域下采样技术,我们在400M主频的手机上实现了720p的流畅解码,在400M主频的CPU上实现了1080p的视频解码,但不是很流畅。
频域下采样技术并不是多么高深的算法,下面的matlab代码十分简洁的描述了图像的频域下采样过程。
1. DCT域下采样算法
代码测试时,‘456.png’是一幅彩色图像
- % dct频率域下采样算法
- % 根据Dugad方法实现
- %程序作者: celery.chen@yahoo.com.cn
- %2010-10-12
-
I=imread('456.png');
- I=rgb2gray(I);
- figure(1);
- imshow(I);
- [width,height] = size(I);
- block_size_x = 8;
- block_size_y = 8;
- x_block_num = width/block_size_x;
- y_block_num = height/block_size_y;
- z_dst = zeros(width/2,height/2);
- pfun1 = @dct2;
- I_freq = blkproc(I,[8 8],pfun1);
- for i = 1: x_block_num
- for j = 1 : y_block_num
- P = I_freq((i-1)*8+1:(i-1)*8+4,(j-1)*8+1:(j-1)*8+4);
- P = P/2.0;
- z_dst( (i-1)*4 +1 : (i-1)*4 +4, (j-1)*4 +1 : (j-1)*4 +4) = P;
- end
- end
- pfun2 = @idct2;
- J = blkproc(z_dst,[4 4],pfun2);
- J = uint8(round(J));
- figure(2);
- imshow(J);
2. 另外一种DCT频域下采样算法
- % dct频率域下采样算法,根据下面论文提到的算法实现
- % 一种高效的DCT域图像下采样方法 中国图像图形学报 2005年4月
- %程序作者: celery.chen@yahoo.com.cn ,2010-10
- clc;
- clear;
- I=imread('456.png');
- I=rgb2gray(I);
- figure(1);
- imshow(I);
- [width,height] = size(I);
- block_size_x = 8*2;
- block_size_y = 8*2;
- x_block_num = width/block_size_x;
- y_block_num = height/block_size_y;
- C16 = dctmtx(16);
- C16LL = C16(1:8,1:8);
- C16LH = C16(1:8,9:16);
- C16HL = C16(9:16,1:8);
- C16HH = C16(9:16,9:16);
- C8 = dctmtx(8);
- P = C16LL*C8';
- P = P/sqrt(2);
- Q = C16LH*C8';
- Q = Q/sqrt(2);
- E = (P + Q)/2;
- F = (P - Q)/2;
- z_dst = zeros(width/2,height/2);
- pfun1 = @dct2;
- I_freq = blkproc(I,[8 8],pfun1);
- for i = 1: x_block_num
- for j = 1 : y_block_num
- X1 = I_freq((i-1)*16+1:(i-1)*16+8,(j-1)*16+1:(j-1)*16+8);
- X2 = I_freq((i-1)*16+1:(i-1)*16+8,(j-1)*16+9:(j-1)*16+16);
- X3 = I_freq((i-1)*16+9:(i-1)*16+16,(j-1)*16+1:(j-1)*16+8);
- X4 = I_freq((i-1)*16+9:(i-1)*16+16,(j-1)*16+9:(j-1)*16+16);
- A = E*(X1+X3)+F*(X1-X3);
- B = E*(X2+X4)+F*(X2-X4);
- XLL = (A + B) * E' + (A - B) * F';
- z_dst( (i-1)*8 +1 : (i-1)*8 +8, (j-1)*8 +1 : (j-1)*8 +8) = XLL;
- end
- end
- pfun2 = @idct2;
- J = blkproc(z_dst,[8 8],pfun2);
- J = uint8(round(J));
- figure(2);
- imshow(J);