ncnn/docs/developer-guide/binaryop-broadcasting.md

### broadcasting rule

ncnn BinaryOp accepts blobs with different shape

C = BinaryOp(A, B)

shape notation convention is [w], [w,h], [w,h,c], [w,h,d,c]

* binaryop with scalar and scalar-like

|type|A|B|C|
|---|---|---|---|
|0|[2]|scalar / [1]|[2]|
|1|[2,3]|scalar / [1] / [1,1]|[2,3]|
|2|[2,3,4]|scalar / [1] / [1,1] / [1,1,1]|[2,3,4]|
|3|[2,3,4,5]|scalar / [1] / [1,1] / [1,1,1] / [1,1,1,1]|[2,3,4,5]|

* no broadcast

|type|A|B|C|
|---|---|---|---|
|4|[2]|[2]|[2]|
|5|[2,3]|[2,3]|[2,3]|
|6|[2,3,4]|[2,3,4]|[2,3,4]|
|7|[2,3,4,5]|[2,3,4,5]|[2,3,4,5]|

* broadcast B for inner axis

|type|A|B|C|
|---|---|---|---|
|8|[2,3]|[3] / [1,3]|[2,3]|
|9|[2,3,4]|[4] / [1,1,4]|[2,3,4]|
|10|[2,3,4]|[3,4] / [1,3,4]|[2,3,4]|
|11|[2,3,4,5]|[5] / [1,1,1,5]|[2,3,4,5]|
|12|[2,3,4,5]|[4,5] / [1,1,4,5]|[2,3,4,5]|
|13|[2,3,4,5]|[3,4,5] / [1,3,4,5]|[2,3,4,5]|

* broadcast B for outer axis

|type|A|B|C|
|---|---|---|---|
|14|[2,3]|[2,1]|[2,3]|
|15|[2,3,4]|[2,1,1]|[2,3,4]|
|16|[2,3,4]|[2,3,1]|[2,3,4]|
|17|[2,3,4,5]|[2,1,1,1]|[2,3,4,5]|
|18|[2,3,4,5]|[2,3,1,1]|[2,3,4,5]|
|19|[2,3,4,5]|[2,3,4,1]|[2,3,4,5]|

* some special broadcasting rule exists for model compatibility

|special type|A|B|C|
|---|---|---|---|
|20|[2,3,4]|[2,1,4]|[2,3,4]|