[FFmpeg-devel] [PATCH 2/7] lavc/pngdec: perform APNG blending in-place

[James Almer]

On 4/2/2021 11:40 AM, Anton Khirnov wrote:
> Saves an allocation+free and two frame copies per each frame.
> ---
>   libavcodec/pngdec.c | 51 +++++++++++++++++++++++++--------------------
>   1 file changed, 28 insertions(+), 23 deletions(-)
> 
> diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
> index 63c22063d9..095e4e86c2 100644
> --- a/libavcodec/pngdec.c
> +++ b/libavcodec/pngdec.c
> @@ -1068,8 +1068,12 @@ static void handle_p_frame_png(PNGDecContext *s, AVFrame *p)
>   static int handle_p_frame_apng(AVCodecContext *avctx, PNGDecContext *s,
>                                  AVFrame *p)
>   {
> +    uint8_t       *dst        = p->data[0];
> +    ptrdiff_t      dst_stride = p->linesize[0];
> +    const uint8_t *src        = s->last_picture.f->data[0];
> +    ptrdiff_t      src_stride = s->last_picture.f->linesize[0];
> +
>       size_t x, y;
> -    uint8_t *buffer;
>   
>       if (s->blend_op == APNG_BLEND_OP_OVER &&
>           avctx->pix_fmt != AV_PIX_FMT_RGBA &&
> @@ -1089,26 +1093,32 @@ static int handle_p_frame_apng(AVCodecContext *avctx, PNGDecContext *s,
>           if (ret < 0)
>               return ret;
>   
> +        src        = s->last_picture.f->data[0];
> +        src_stride = s->last_picture.f->linesize[0];

Is calling av_frame_make_writable(s->last_picture.f) valid at all? 
Shouldn't the frame's buffer be freed with ff_thread_release_buffer()? 
Especially if a non threading safe get_buffer2() callback was used.

Considering you can't call ff_thread_get_buffer() at this point to get a 
new writable buffer to av_frame_copy() to, since this is long after 
ff_thread_finish_setup() was called, not sure what else could be done.

> +
>           for (y = s->last_y_offset; y < s->last_y_offset + s->last_h; y++) {
> -            memset(s->last_picture.f->data[0] + s->image_linesize * y +
> +            memset(s->last_picture.f->data[0] + src_stride * y +
>                      s->bpp * s->last_x_offset, 0, s->bpp * s->last_w);
>           }
>       }
>   
> -    buffer = av_memdup(s->last_picture.f->data[0], s->image_linesize * s->height);
> -    if (!buffer)
> -        return AVERROR(ENOMEM);
> +    // copy unchanged rectangles from the last frame
> +    for (y = 0; y < s->y_offset; y++)
> +        memcpy(dst + y * dst_stride, src + y * src_stride, p->width * s->bpp);
> +    for (y = s->y_offset; y < s->y_offset + s->cur_h; y++) {
> +        memcpy(dst + y * dst_stride, src + y * src_stride, s->x_offset * s->bpp);
> +        memcpy(dst + y * dst_stride + (s->x_offset + s->cur_w) * s->bpp,
> +               src + y * src_stride + (s->x_offset + s->cur_w) * s->bpp,
> +               (p->width - s->cur_w - s->x_offset) * s->bpp);
> +    }
> +    for (y = s->y_offset + s->cur_h; y < p->height; y++)
> +        memcpy(dst + y * dst_stride, src + y * src_stride, p->width * s->bpp);
>   
> -    // Perform blending
> -    if (s->blend_op == APNG_BLEND_OP_SOURCE) {
> -        for (y = s->y_offset; y < s->y_offset + s->cur_h; ++y) {
> -            size_t row_start = s->image_linesize * y + s->bpp * s->x_offset;
> -            memcpy(buffer + row_start, p->data[0] + row_start, s->bpp * s->cur_w);
> -        }
> -    } else { // APNG_BLEND_OP_OVER
> +    if (s->blend_op == APNG_BLEND_OP_OVER) {
> +        // Perform blending
>           for (y = s->y_offset; y < s->y_offset + s->cur_h; ++y) {
> -            uint8_t *foreground = p->data[0] + s->image_linesize * y + s->bpp * s->x_offset;
> -            uint8_t *background = buffer + s->image_linesize * y + s->bpp * s->x_offset;
> +            uint8_t       *foreground = dst + dst_stride * y + s->bpp * s->x_offset;
> +            const uint8_t *background = src + src_stride * y + s->bpp * s->x_offset;
>               for (x = s->x_offset; x < s->x_offset + s->cur_w; ++x, foreground += s->bpp, background += s->bpp) {
>                   size_t b;
>                   uint8_t foreground_alpha, background_alpha, output_alpha;
> @@ -1135,18 +1145,17 @@ static int handle_p_frame_apng(AVCodecContext *avctx, PNGDecContext *s,
>                       break;
>                   }
>   
> -                if (foreground_alpha == 0)
> +                if (foreground_alpha == 255)
>                       continue;
>   
> -                if (foreground_alpha == 255) {
> -                    memcpy(background, foreground, s->bpp);
> +                if (foreground_alpha == 0) {
> +                    memcpy(foreground, background, s->bpp);
>                       continue;
>                   }
>   
>                   if (avctx->pix_fmt == AV_PIX_FMT_PAL8) {
>                       // TODO: Alpha blending with PAL8 will likely need the entire image converted over to RGBA first
>                       avpriv_request_sample(avctx, "Alpha blending palette samples");
> -                    background[0] = foreground[0];
>                       continue;
>                   }
>   
> @@ -1164,15 +1173,11 @@ static int handle_p_frame_apng(AVCodecContext *avctx, PNGDecContext *s,
>                       }
>                   }
>                   output[b] = output_alpha;
> -                memcpy(background, output, s->bpp);
> +                memcpy(foreground, output, s->bpp);
>               }
>           }
>       }
>   
> -    // Copy blended buffer into the frame and free
> -    memcpy(p->data[0], buffer, s->image_linesize * s->height);
> -    av_free(buffer);
> -
>       return 0;
>   }
>   
>