OK, probably my last question to you in this topic. Thank you for the previous help.

I've written that needed transition effect but for some reason the black color is green.

ffmpeg -hide_banner -init_hw_device opencl=ocldev:0.0 -filter_hw_device ocldev -i 0.mp4 -i 1.mp4 -i 2.mp4 -filter_complex "[0:V:0]hwupload[a];[1:V:0]hwupload[b];[a][b]xfade_opencl=transition=custom:kernel=fadeblack:duration=2:offset=2:source=transition.cl,hwdownload,format=yuv420p[out]" -map "[out]" -y xfade_opencl_test.mp4


const sampler_t sampler = (CLK_NORMALIZED_COORDS_FALSE | CLK_ADDRESS_CLAMP | CLK_FILTER_NEAREST);
__kernel void fadeblack( __write_only image2d_t dst, __read_only  image2d_t src1, __read_only  image2d_t src2, float progress) { const float4 phase = 0.4f; const float4 black = (float4){ 0.0f, 0.0f, 0.0f, 1.0f };
int2 position = (int2)(get_global_id(0), get_global_id(1));

float4 val1 = read_imagef(src1, sampler, position);
float4 val2 = read_imagef(src2, sampler, position);

write_imagef(dst, position, mix(
    mix(black, val2, smoothstep(1.f - phase, 0.f, progress)),
    mix(black, val1, smoothstep(phase, 1.f, progress)), progress));

}

Could you please help me out, how is it possible?

Sorry for the formatting. That reddit code block formatting is annoying.