hacker99 created SINGA-249:
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Summary: Convolution BP
Key: SINGA-249
URL: https://issues.apache.org/jira/browse/SINGA-249
Project: Singa
Issue Type: Wish
Environment: ubuntu 14.04，singa 1.0
Reporter: hacker99
I'm curious about how to calculate the gradient of the back propagation
algorithm eg. Convolution layer. Can anyone explain to me the details of the
implementation of the formula and the code? Very grateful, if there is some
documents or just tell why dw += Mult(grad_b, col_data.T())?
#code from src/model/layer/convolution.cc
const std::pair<Tensor, vector<Tensor>> Convolution::Backward(
int flag, const Tensor &grad) {
CHECK_EQ(grad.device()->lang(), kCpp);
CHECK_EQ(grad.nDim(), 4u);
CHECK(!buf_.empty());
Tensor src_data = buf_.top();
buf_.pop();
vector<Tensor> param_grad;
Tensor dx;
Tensor db, dw;
dx.ResetLike(src_data);
db.ResetLike(bias_);
dw.ResetLike(weight_);
dw.SetValue(0.0f);
size_t batchsize = grad.shape(0);
size_t imagesize = src_data.Size() / batchsize;
if (bias_term_) {
Tensor tmp1 =
Reshape(grad, Shape{batchsize * num_filters_,
grad.Size() / (batchsize * num_filters_)});
Tensor tmp2(Shape{batchsize * num_filters_});
SumColumns(tmp1, &tmp2);
Tensor tmp3 = Reshape(tmp2, Shape{batchsize, num_filters_});
SumRows(tmp3, &db);
}
auto in_data = src_data.data<float>();
Tensor col_data(Shape{col_height_, col_width_});
float *data_col = new float[col_height_ * col_width_];
float *dx_b = new float[imagesize];
for (size_t b = 0; b < batchsize; b++) {
Im2col(in_data + b * imagesize, channels_, height_, width_, kernel_h_,
kernel_w_, pad_h_, pad_w_, stride_h_, stride_w_, data_col);
col_data.CopyDataFromHostPtr(data_col, col_height_ * col_width_);
Tensor grad_b(Shape{num_filters_, conv_height_ * conv_width_});
CopyDataToFrom(&grad_b, grad, grad_b.Size(), 0, b * grad_b.Size());
dw += Mult(grad_b, col_data.T());
Tensor dcol_b = Mult(weight_.T(), grad_b);
auto dcol_data = dcol_b.data<float>();
Col2im(dcol_data, channels_, height_, width_, kernel_h_, kernel_w_, pad_h_,
pad_w_, stride_h_, stride_w_, dx_b);
dx.CopyDataFromHostPtr(dx_b, imagesize, b * imagesize);
}
param_grad.push_back(dw);
param_grad.push_back(db);
delete[] data_col;
delete[] dx_b;
return std::make_pair(dx, param_grad);
}
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