Reversing ProRes: Part1 (Bitrate)

One of the main problem I met with 3rd parties Prores encoders is bitrate. Approx bitrate could be found in Prores white papers.

But I talk logic of calculation max possible size. Why max? Because Apple doesnt really care about lower bound, so with black frame as source you will never be even close to bitrate mentioned in white papers.

Basically Apple has simple algorithm and they calc size of frame that encoder never exceed and second seems ideal size to fulfill declared bitrate.

so max/avg size depends on couple input conditions:

  1. resolution
  2. quality
  3. alpha

Alpha case is quite simple and weird same time, if you say to Prores encoder that you going to encode alpha it automatically increase max size by 3* width * height

Main logic is resolution based:

  • if resolution less or equal of SD_NTSC its 288 * 1024
  • if res less or equal of SD_PAL its 336 * 1024
  • so on

that actually was second weird thing I found, no I didnt mean link resolution and bitrate, but code how its implemented:

int size = width * height;
int rate = 0;

if (size <= 720 * 486) {
    rate = 288 * 1024;
else if (size <= 720 * 576) {
    rate = 336 * 1024;
else if (size <= 960 * 720) {
    rate = 432 * 1024;
// so on

my first question was “have they ever heard about binary search?” 🙂

Nevertheless when base value found they  tune it with respect to quality:

if (qual == proxy) {
    rate = 13 * rate / 63;
else if (qual == lt) {
    rate = 13 * rate / 28
//so on

thats basically it for max frame size. second value which I named avg value calculated even easier we just multiply previously found rate by 8 and divide by 9

P.S. there are also some color space tricks for example when we encode 422 frame to 4444 quality, but I wont cover it as its a bit artificial situation

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