Re: [FFmpeg-devel] [PATCH v9 3/6] libavcodec/webp: add support for animated WebP

[Thilo Borgmann via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

Am 31.12.23 um 13:56 schrieb Tomas Härdin:
>> +    for (int y = 0; y < height; y++) {
>> +        const uint8_t *src1 = src1_data[0] + y * src1_linesize[0];
>> +        const uint8_t *src2 = src2_data[0] + (y + pos_y) *
>> src2_linesize[0] + pos_x * src2_step[0];
>> +        uint8_t       *dest = dest_data[0] + (y + pos_y) *
>> dest_linesize[0] + pos_x * sizeof(uint32_t);
>> +        for (int x = 0; x < width; x++) {
>> +            int src1_alpha = src1[0];
>> +            int src2_alpha = src2[0];
>> +
>> +            if (src1_alpha == 255) {
>> +                memcpy(dest, src1, sizeof(uint32_t));
>> +            } else if (src1_alpha + src2_alpha == 0) {
>> +                memset(dest, 0, sizeof(uint32_t));
>> +            } else {
>> +                int tmp_alpha = src2_alpha - ROUNDED_DIV(src1_alpha
>> * src2_alpha, 255);
>> +                int blend_alpha = src1_alpha + tmp_alpha;
>> +
>> +                dest[0] = blend_alpha;
>> +                dest[1] = ROUNDED_DIV(src1[1] * src1_alpha + src2[1]
>> * tmp_alpha, blend_alpha);
>> +                dest[2] = ROUNDED_DIV(src1[2] * src1_alpha + src2[2]
>> * tmp_alpha, blend_alpha);
>> +                dest[3] = ROUNDED_DIV(src1[3] * src1_alpha + src2[3]
>> * tmp_alpha, blend_alpha);
>> +            }
>
> Is branching and a bunch of function calls (which I hope get optimized
> out) really faster than just always doing the blending?

If I trust my START_TIMER/STOP_TIMER interpretation, I'd say so:

With branches:
253315 UNITS in blend_alpha_yuva,     128 runs,      0 skips

Always blending:
351104 UNITS in blend_alpha_yuva,     128 runs,      0 skips



> It mgiht also be worthwhile to check 8 bytes at a time against
> UINT64_MAX and 0. That doesn't need to hold up this patch though. Same
> with the YUVA version.
>
>> +static int blend_frame_into_canvas(WebPContext *s)
>> +{
>> +    AVFrame *canvas = s->canvas_frame.f;
>> +    AVFrame *frame  = s->frame;
>> +    int width, height;
>> +    int pos_x, pos_y;
>> +
>> +    if ((s->anmf_flags & ANMF_BLENDING_METHOD) ==
>> ANMF_BLENDING_METHOD_OVERWRITE
>> +        || frame->format == AV_PIX_FMT_YUV420P) {
>> +        // do not blend, overwrite
>> +
>> +        if (canvas->format == AV_PIX_FMT_ARGB) {
>> +            width  = s->width;
>> +            height = s->height;
>> +            pos_x  = s->pos_x;
>> +            pos_y  = s->pos_y;
>> +
>> +            for (int y = 0; y < height; y++) {
>> +                const uint32_t *src = (uint32_t *) (frame->data[0] +
>> y * frame->linesize[0]);
>> +                uint32_t *dst = (uint32_t *) (canvas->data[0] + (y +
>> pos_y) * canvas->linesize[0]) + pos_x;
>> +                memcpy(dst, src, width * sizeof(uint32_t));
>> +            }
>
> This could be reduced to a single memcpy() when linesizes are equal.
> Same for the other memcpy()s

Its a subimage copied into a canvas (see pos_x and pos_y).
Has to be copied line-by-line.

Same for the other loops.

-Thilo

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