1. Interpretation of steps
1.1 SDL initialization, initialize the subsystems in SDL. The specific parameters are as follows:
1.2 Opening multimedia files:
Parameters: 1. ps: directly defines the pointer of the AVFormatContext class (null pointer)
2. filename: the path of the audio file
1.3 Get the message of the audio file:
1.4: Get stream encoding
By looping through the audio stream, obtain the encoding of the stream that needs to be decoded.
The types of streams are:
AVMEDIA_TYPE_UNKNOWN: Unknown type. Typically treated asAVMEDIA_TYPE_DATA(data type).AVMEDIA_TYPE_VIDEO: Video stream type. Used to represent video data in multimedia files.AVMEDIA_TYPE_AUDIO: Audio stream type. Used to represent audio data in multimedia files.AVMEDIA_TYPE_DATA: Data stream type. Used to represent opaque data information in multimedia files, usually continuous.AVMEDIA_TYPE_SUBTITLE: Subtitle stream type. Used to represent subtitle data in multimedia files.AVMEDIA_TYPE_ATTACHMENT: Attachment stream type. Used to represent opaque data information in multimedia files, usually sparse.
for(int i=0;i<(int)avformat_context->nb_streams; + + i)
{
if(avformat_context->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO)
{
videoStream = i;
}
}
1.5 Get the decoder
1.5.1 Create decoding context
pCodecCtx = avcodec_alloc_context3(NULL);
if (pCodecCtx == NULL)
{
printf("Could not allocate AVCodecContext\
");
return;
}
1.5.2 Copy codec parameters
if (avcodec_parameters_to_context(pCodecCtx, pFormatCtx->streams[audioindex]->codecpar) < 0)
{
printf("Could not init AVCodecContext\
");
return;
}
pFormatCtx->streams[audioindex]->codecpar represents the codec parameters of audio and video streams.
1.5.3 Get the decoder
pCodec = avcodec_find_decoder(pCodecCtx->codec_id);
if (pCodec == NULL) {
printf("Codec not found.\
");
return;
}
- Use the
avcodec_find_decoderfunction to find the corresponding decoder based on the encoder ID (pCodecCtx->codec_id) in the decoding context.
1.6 Open the decoder
1.7 SDL_CreateWindow` function is used to create a window
extern DECLSPEC SDL_Window * SDLCALL SDL_CreateWindow(const char *title,
int x, int y, int w,
int h, Uint32 flags);
parameter:
x, window x coordinate; y, window y coordinate; w, window width; h, window height;
flags, window attributes (select attributes on enumeration SDL_WindowFlags)
1.8 The `SDL_CreateRenderer` function is used to create a renderer.
extern DECLSPEC SDL_Renderer * SDLCALL SDL_CreateRenderer(SDL_Window * window,
int index, Uint32 flags);
parameter;
window: the rendered window
index: - `-1`: Index of the renderer, indicating to use the first supported renderer.
- `0`: The flag of the renderer, which means there is no special flag.
flags: Select attributes on the enumeration SDL_RendererFlags
1.9 The `SDL_CreateTexture` function is used to create a texture.
extern DECLSPEC SDL_Texture * SDLCALL SDL_CreateTexture(SDL_Renderer * renderer,
Uint32 format,
int access, int w,
int h);
parameter:
renderer: the renderer on which to create the texture
format: The pixel format of the texture, here IYUV format is used (enumeration SDL_PixelFormatEnum)
access: The access method of the texture, which means that the texture can be accessed through the stream (enumeration SDL_TextureAccess)
W: The width of the texture, using the width of the decoder context
h: The height of the texture, using the height of the decoder context
1.10 The `sws_getContext` function is used to create an image conversion context.
struct SwsContext *sws_getContext(int srcW, int srcH, enum AVPixelFormat srcFormat,
int dstW, int dstH, enum AVPixelFormat dstFormat,
int flags, SwsFilter *srcFilter,
SwsFilter *dstFilter, const double *param);
parameter:
srcW: width of input image
srcH: height of input image
enum AVPixelFormat srcFormat: pixel format of the input image
dstW: width of the output image
dstH: height of the output image
enum AVPixelFormat dstFormat: pixel format of the output image
flags: Specify the algorithm and options to use for rescaling
SwsFilter *srcFilter: Row size of source image (NULL is automatically calculated)
SwsFilter *dstFilter: row size of the target image (NULL is automatically calculated)
const double *param: additional options
1.11 The `av_image_alloc` function is used to allocate an image data buffer
int av_image_alloc(uint8_t *pointers[4], int linesizes[4],
int w, int h, enum AVPixelFormat pix_fmt, int align);
parameter:
pointers[4]: array of pointers pointing to image data
linesizes[4]: array pointing to image data line sizes
w: width of the image
h: height of the image
pix_fmt: pixel format of the image
align: Alignment of image data (align the value to be used for buffer size alignment)
1.12 Read data (12 to 18 are implemented in a loop)
int av_read_frame(AVFormatContext *s, AVPacket *pkt);
parameter:
s:AVFormatContext structure, representing the format context of the video file.
pkt: AVPacket structure, used to store read data packets. (Need to open up heap space)
1.13 Data decoding
int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt);
parameter:
avctx:AVCodecContext structure, representing the context of the decoder
avpkt:AVPacket structure, the data packet to be sent
Function: Submit the data packet to the decoder
int avcodec_receive_frame(AVCodecContext *avctx, AVFrame *frame);
parameter:
avctx:AVCodecContext structure, representing the context of the decoder
frame: AVFrame structure, used to store received frame data.
Function: Read from decoder, decode data
frame is created by av_frame_alloc()
1.14 Convert decoded frame data to target format
int sws_scale(struct SwsContext *c, const uint8_t *const srcSlice[],
const int srcStride[], int srcSliceY, int srcSliceH,
uint8_t *const dst[], const int dstStride[]);
parameter:
c: SwsContext structure, representing the image conversion context
srcSlice: decoded frame data
srcStride: The size of each line of decoded frame data
srcSliceY: the starting line of the source image
srcSliceH: Number of lines of source image
dst: array of pointers to target image data
dstStride: The size of each row of target image data
1.15. Update texture data
extern DECLSPEC int SDLCALL SDL_UpdateYUVTexture(SDL_Texture * texture,
const SDL_Rect * rect,
const Uint8 *Yplane, int Ypitch,
const Uint8 *Uplane, int Upitch,
const Uint8 *Vplane, int Vpitch);
parameter:
texture: the texture to be updated
rect: plane raw data
Yplane: Y component data
Ypitch: The size of each line of the Y component
Uplane: U component data
Upitch: The size of each row of the U component
Vplane:V component data
Vpitch: The size of each line of the V component
Function: YUV data texture update
1.16 Clear renderer
extern DECLSPEC int SDLCALL SDL_RenderClear(SDL_Renderer * renderer); Parameters: renderer: clear renderer name
1.17 Copy the texture to the renderer’s render target
extern DECLSPEC int SDLCALL SDL_RenderCopy(SDL_Renderer * renderer,
SDL_Texture * texture,
const SDL_Rect * srcrect,
const SDL_Rect * dstrect);
parameter:
renderer: renderer name
texture: the name of the texture to be copied
srcrect: source rectangle, indicating copying the entire texture
dstrect: target rectangle, indicating the position and size copied to the renderer
Note:
dstect needs to set coordinates (x, y) and length and width
1.18 The renderer’s render target is rendered to the window
SDL_RenderPresent(renderer); SDL_Delay(1000 / 30);
1.19 Destruction
SDL_DestroyTexture(texture); used to destroy SDL texture SDL_DestroyRenderer(renderer); used to destroy the SDL renderer SDL_DestroyWindow(window); used to destroy the SDL window SDL_Quit(); used to terminate the use of the SDL library and release the resources occupied by SDL