Expr(clip clips, string expr[, int format])¶
Expr evaluates an expression per pixel for up to 26 input clips. The expression, expr, is written using reverse polish notation and can be specified for each plane individually. The expression given for the previous plane is used if the expr array contains fewer expressions than the input clip has planes. In practice this means that a single expression will be applied to all planes by default.
Specifying an empty string as the expression enables a fast plane copy from the first specified clip, when possible. If it is not possible due to the output format being incompatible, the plane contents will be undefined.
Since the expression is evaluated at runtime, there are a few pitfalls. In order to keep speed up, the input ranges are not normalized to the usual floating point ranges. Instead they are left as is, meaning that an 8 bit clip will have values in the 0-255 range and a 10 bit clip will have values in the 0-1023 range. Note that floating point clips are even more difficult, as most channels are stored in the 0-1 range with the exception of U, V, Co and Cg planes, which are in the -0.5-0.5 range. If you mix clips with different input formats this must be taken into consideration.
When the output format uses 1 byte per sample, the result of the expression is clamped to [0, 255] before it is stored in the output frame. When the output format uses 2 bytes per sample, the result of the expression is clamped to [0, 65535], irrespective of the number of bits actually used. When the output format uses float samples, the result of the expression is stored without any clamping.
By default the output format is the same as the first input clip’s format. You can override it by setting format. The only restriction is that the output format must have the same subsampling as the input clips and be 8..16 bit integer or 32 bit float. 16 bit float is also supported on cpus with the f16c instructions.
Logical operators are also a bit special, since everything is done in floating point arithmetic. All values greater than 0 are considered true for the purpose of comparisons. Logical operators return 0.0 for false and 1.0 for true in their operations.
Since the expression is being evaluated at runtime, there are also the stack manipulation operators, swap and dup. The former swaps the topmost and second topmost values, and the latter duplicates the topmost stack value.
These operators also have swapN and dupN forms that allow a value N steps up in the stack to be swapped or duplicated. The top value of the stack has index zero meaning that dup is equivalent to dup0 and swap is equivalent to swap1. This is because swapN always swaps with the topmost value at index 0.
Clip load operators:
The operators taking one argument are:
exp log sqrt abs not dup dupN
The operators taking two arguments are:
+ - * / max min pow > < = >= <= and or xor swap swapN
The operators taking three arguments are:
For example these operations:
a b c ? d e < f abs
Are equivelent to these operations in C:
a ? b : c d < e abs(f)
How to average the Y planes of 3 YUV clips and pass through the UV planes unchanged (assuming same format):
std.Expr(clips=[clipa, clipb, clipc], expr=["x y + z + 3 /", "", ""])
How to average the Y planes of 3 YUV clips and pass through the UV planes unchanged (different formats):
std.Expr(clips=[clipa16bit, clipb10bit, clipa8bit], expr=["x y 64 * + z 256 * + 3 /", ""])
Setting the output format because the resulting values are illegal in a 10 bit clip (note that the U and V planes will contain junk since direct copy isn’t possible):
std.Expr(clips=[clipa10bit, clipb16bit, clipa8bit], expr=["x 64 * y + z 256 * + 3 /", ""], format=vs.YUV420P16)