Avoiding processing on alpha channel

[yuguang-li]

I'm writing high-performance image processing filters in Halide. The input and output buffers are all defined in rgb-alpha four channels. When I'm processing the input image, the generated C code loops from channel 0 to channel 3. This happens in every single function in my code. I did tried to use out.bound(0,3) in the processing step and use final(x,y,c) = select(c<3,out(x,y,c),255) in the end. But the code cannot be compiled correctly. Does anyone have the same issue when doing optimization?

[abadams]

For simple pointwise filters, the number of channels that get evaluated is
determined by the number of channels in your output image. If you set
output.extent[2] == 3, it should only evaluate things over three channels.

In general Halide determines what to evaluate using the size of the output.

On Fri, Jul 15, 2016 at 8:18 PM, RileyLee [email protected] wrote:

I'm writing high-performance image processing filters in Halide. The input
and output buffers are all defined in rgb-alpha four channels. When I'm
processing the input image, the generated C code loops from channel 0 to
channel 3. This happens in every single function in my code. I did tried to
use out.bound(0,3) in the processing step and use final(x,y,c) =
select(c<3,out(x,y,c),255) in the end. But the code cannot be compiled
correctly. Does anyone have the same issue when doing optimization?

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[yuguang-li]

Thanks for your response Adam.
That's true. But what if my input has four channels, but I only want all my functions to work on RGB three channels? I'll add the alpha channel at the last step of my whole pipeline.

[abadams]

It doesn't matter how many channels the input has. The computation is
determined by how many channels the output has. Perhaps this is the pattern
you want:

output(x, y, c) = select(c < 3, input(x, y, clamp(c, 0, 2)), 255);

The clamp lets Halide know that you only need channels 0, 1, and 2 of all
previous pipeline stages.

On Fri, Jul 22, 2016 at 10:50 AM, RileyLee [email protected] wrote:

Thanks for your response Adam.
That's true. But what if my input has four channels, but I only want all
my functions to work on RGB three channels? I'll add the alpha channel at
the last step of my whole pipeline.

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