先看視頻轉換320x240的效果比較圖：
採用的是:Two Pass Scaling using Filters圖片縮放算法
http://www.codeproject.com/KB/graphics/2_pass_scaling.aspx
把它改到mingw c語言下，用gcc編譯時遇到了許多問題，開始總出現異常，儘管在vc下測試是正常的。
目前還存在字幕顯示有點模糊，縮小情況下不能調整字幕顏色了，這個與視頻合併時有關,合併前字幕截圖是正常的。
由於字幕顏色格式採用的是rgba，而轉換出來的視頻顏色格式採用的可能是其它格式，如YUV420P, YV12等，
如何有效地轉換成一致，這個暫時不太清楚，儘管可用sws_scale轉換，但效果如何有待確認。
編譯異常主要遇到的情況：
1。聲明變量類型分配空間大小問題。由於原來用的是c++中的new,不用考慮int8, int32等大小，但用malloc則需要。
2。由於vc下有采用的變量類型在c語言下可能沒有，需要自己定義並替換原來的。
3。語法問題，如for (int i=0; i<10; i++){....}, i在後面就不能直接用了等。
有關Pass Scaling源代碼,裏面函數Filter,其實就是它原來的CBilinearFilter, 把原來的class全部改成了function
Pass Scaling的設計思路還是相當不錯的.
#ifndef _2_PASS_SCALE_H_
#define _2_PASS_SCALE_H_
#include <math.h>
typedef struct
{
double *Weights; // Normalized weights of neighboring pixels
int Left,Right; // Bounds of source pixels window
} ContributionType; // Contirbution information for a single pixel
typedef struct
{
ContributionType *ContribRow; // Row (or column) of contribution weights
UINT WindowSize, // Filter window size (of affecting source pixels)
LineLength; // Length of line (no. or rows / cols)
} LineContribType; // Contribution information for an entire line (row or column)
COLORREF * AllocAndScale (
COLORREF *pOrigImage,
UINT uOrigWidth,
UINT uOrigHeight,
UINT uNewWidth,
UINT uNewHeight);
COLORREF * Scale (
COLORREF *pOrigImage,
UINT uOrigWidth,
UINT uOrigHeight,
COLORREF *pDstImage,
UINT uNewWidth,
UINT uNewHeight);
LineContribType *AllocContributions (UINT uLineLength, UINT uWindowSize)
{
LineContribType *res = (LineContribType *) malloc(sizeof(LineContribType));
// Init structure header
res->WindowSize = uWindowSize;
res->LineLength = uLineLength;
// Allocate list of contributions
res->ContribRow = (ContributionType *) malloc(uLineLength * sizeof(ContributionType));
for (UINT u = 0 ; u < uLineLength ; u++)
{
// Allocate contributions for every pixel
res->ContribRow[u].Weights = (double *)malloc(uWindowSize * sizeof(double));
}
return res;
}
void FreeContributions (LineContribType * p)
{
for (UINT u = 0; u < p->LineLength; u++)
{
// Free contribs for every pixel
free(p->ContribRow[u].Weights);
}
free(p->ContribRow); // Free list of pixels contribs
free(p); // Free contribs header
}
double Filter (double dVal)
{
dVal = fabs(dVal);
return (dVal < 1.0 ? 1.0 - dVal : 0.0);
}
LineContribType *CalcContributions (UINT uLineSize, UINT uSrcSize, double dScale)
{
double dWidth;
double dFScale = 1.0;
double dFilterWidth = 1.0;
if (dScale < 1.0)
{ // Minification
dWidth = dFilterWidth / dScale;
dFScale = dScale;
}
else
{ // Magnification
dWidth= dFilterWidth;
}
// Window size is the number of sampled pixels
int iWindowSize = 2 * (int)ceil(dWidth) + 1;
// Allocate a new line contributions strucutre
LineContribType *res = AllocContributions (uLineSize, iWindowSize);
for (UINT u = 0; u < uLineSize; u++)
{ // Scan through line of contributions
double dCenter = (double)u / dScale; // Reverse mapping
// Find the significant edge points that affect the pixel
int iLeft = max (0, (int)floor (dCenter - dWidth));
res->ContribRow[u].Left = iLeft;
int iRight = min (ceil (dCenter + dWidth), uSrcSize - 1);
res->ContribRow[u].Right = iRight;
// Cut edge points to fit in filter window in case of spill-off
if (iRight - iLeft + 1 > iWindowSize)
{
if (iLeft < (uSrcSize - 1 / 2))
{
iLeft++;
}
else
{
iRight--;
}
}
int iSrc;
double dTotalWeight = 0.0; // Zero sum of weights
for (iSrc = iLeft; iSrc <= iRight; iSrc++)
{ // Calculate weights
dTotalWeight += (res->ContribRow[u].Weights[iSrc-iLeft] =
dFScale * Filter (dFScale * (dCenter - (double)iSrc)));
}
//assert (dTotalWeight >= 0.0); // An error in the filter function can cause this
if (dTotalWeight > 0.0)
{ // Normalize weight of neighbouring points
for (iSrc = iLeft; iSrc <= iRight; iSrc++)
{ // Normalize point
res->ContribRow[u].Weights[iSrc-iLeft] /= dTotalWeight;
}
}
}
return res;
}
void ScaleRow(COLORREF *pSrc,
UINT uSrcWidth,
COLORREF *pRes,
UINT uResWidth,
UINT uRow,
LineContribType *Contrib)
{
COLORREF *pSrcRow = &(pSrc[uRow * uSrcWidth]);
COLORREF *pDstRow = &(pRes[uRow * uResWidth]);
for (UINT x = 0; x < uResWidth; x++)
{ // Loop through row
BYTE r = 0;
BYTE g = 0;
BYTE b = 0;
int iLeft = Contrib->ContribRow[x].Left; // Retrieve left boundries
int iRight = Contrib->ContribRow[x].Right; // Retrieve right boundries
//printf("%d, left=%d, right=%d/n", x,iLeft, iRight);
if (iRight - iLeft >= 5) {
// printf("%d, left=%d, right=%d===============================/n", x,iLeft, iRight);
}
for (int i = iLeft; i <= iRight; i++)
{ // Scan between boundries
// Accumulate weighted effect of each neighboring pixel
r += (BYTE)(Contrib->ContribRow[x].Weights[i-iLeft] * (double)(GetRValue(pSrcRow[i])));
g += (BYTE)(Contrib->ContribRow[x].Weights[i-iLeft] * (double)(GetGValue(pSrcRow[i])));
b += (BYTE)(Contrib->ContribRow[x].Weights[i-iLeft] * (double)(GetBValue(pSrcRow[i])));
}
pDstRow[x] = RGB(r,g,b); // Place result in destination pixel
}
}
void HorizScale (COLORREF *pSrc,
UINT uSrcWidth,
UINT uSrcHeight,
COLORREF *pDst,
UINT uResWidth,
UINT uResHeight)
{
if (uResWidth == uSrcWidth)
{ // No scaling required, just copy
memcpy (pDst, pSrc, sizeof (COLORREF) * uSrcHeight * uSrcWidth);
}
// Allocate and calculate the contributions
LineContribType * Contrib = CalcContributions (uResWidth, uSrcWidth, (double)uResWidth/uSrcWidth);
for (UINT u = 0; u < uResHeight; u++)
{
printf("noRow=%d, w=%d/n", u, uResWidth);
ScaleRow ( pSrc,
uSrcWidth,
pDst,
uResWidth,
u,
Contrib); // Scale each row
}
FreeContributions (Contrib); // Free contributions structure
}
void ScaleCol (COLORREF *pSrc,
UINT uSrcWidth,
COLORREF *pRes,
UINT uResWidth,
UINT uResHeight,
UINT uCol,
LineContribType *Contrib)
{
for (UINT y = 0; y < uResHeight; y++)
{ // Loop through column
BYTE r = 0;
BYTE g = 0;
BYTE b = 0;
int iLeft = Contrib->ContribRow[y].Left; // Retrieve left boundries
int iRight = Contrib->ContribRow[y].Right; // Retrieve right boundries
//printf("%d, left=%d, right=%d/n", y,iLeft, iRight);
for (int i = iLeft; i <= iRight; i++)
{ // Scan between boundries
// Accumulate weighted effect of each neighboring pixel
COLORREF pCurSrc = pSrc[i * uSrcWidth + uCol];
r += BYTE(Contrib->ContribRow[y].Weights[i-iLeft] * (double)(GetRValue(pCurSrc)));
g += BYTE(Contrib->ContribRow[y].Weights[i-iLeft] * (double)(GetGValue(pCurSrc)));
b += BYTE(Contrib->ContribRow[y].Weights[i-iLeft] * (double)(GetBValue(pCurSrc)));
}
pRes[(uResHeight-1-y) * uResWidth + uCol] = RGB (r,g,b); // Place result in destination pixel
}
}
void VertScale (COLORREF *pSrc,
UINT uSrcWidth,
UINT uSrcHeight,
COLORREF *pDst,
UINT uResWidth,
UINT uResHeight)
{
if (uSrcHeight == uResHeight)
{ // No scaling required, just copy
memcpy (pDst, pSrc, sizeof (COLORREF) * uSrcHeight * uSrcWidth);
}
// Allocate and calculate the contributions
LineContribType * Contrib = CalcContributions (uResHeight, uSrcHeight, (double)uResHeight/uSrcHeight);
for (UINT u = 0; u < uResWidth; u++)
{ // Step through columns
printf("noCol=%d, w=%d/n", u, uResWidth);
ScaleCol ( pSrc,
uSrcWidth,
pDst,
uResWidth,
uResHeight,
u,
Contrib); // Scale each column
}
FreeContributions (Contrib); // Free contributions structure
}
COLORREF *AllocAndScale (
COLORREF *pOrigImage,
UINT uOrigWidth,
UINT uOrigHeight,
UINT uNewWidth,
UINT uNewHeight)
{
// Scale source image horizontally into temporary image
COLORREF *pTemp = (COLORREF*)malloc(uNewWidth * uOrigHeight * sizeof(COLORREF));
HorizScale (pOrigImage,
uOrigWidth,
uOrigHeight,
pTemp,
uNewWidth,
uOrigHeight);
// Scale temporary image vertically into result image
COLORREF *pRes = (COLORREF*)malloc(uNewWidth * uNewHeight * sizeof(COLORREF));
VertScale ( pTemp,
uNewWidth,
uOrigHeight,
pRes,
uNewWidth,
uNewHeight);
free(pTemp);
return pRes;
}
COLORREF *Scale (
COLORREF *pOrigImage,
UINT uOrigWidth,
UINT uOrigHeight,
COLORREF *pDstImage,
UINT uNewWidth,
UINT uNewHeight)
{
// Scale source image horizontally into temporary image
COLORREF *pTemp = (COLORREF*)malloc(uNewWidth * uOrigHeight * sizeof(COLORREF));
HorizScale (pOrigImage,
uOrigWidth,
uOrigHeight,
pTemp,
uNewWidth,
uOrigHeight);
// Scale temporary image vertically into result image
VertScale ( pTemp,
uNewWidth,
uOrigHeight,
pDstImage,
uNewWidth,
uNewHeight);
free(pTemp);
return pDstImage;
}
#endif // _2_PASS_SCALE_H_
參考文檔：
http://www.compuphase.com/graphic/scale2.htm