trainFastRCNNObjectDetector requires the input lgraph to be a valid Fast R-CNN network, that's why it errors when a GoogLeNet network is passed in, as it's not a Fast R-CNN network.
However trainFastRCNNObjectDetector can convert a pretrained network into a Fast R-CNN network if you pass in the name of the pretrained network as a string, in this case, "googlenet". This is why the second call you showed runs without errors.
If you are unable to install the GoogLeNet support package then you would need to manually convert it into a valid Fast R-CNN network before passing it in to trainFastRCNNObjectDetector.
Here's an example of creating a Fast R-CNN network using GoogLeNet as its backbone.
lgraph = googlenet("Weights", "none");
Replace classification layers
numClasses = 1; % Change this to your number of classes
newFCLayer = fullyConnectedLayer(numClasses + 1);
lgraph = replaceLayer(lgraph, 'loss3-classifier', newFCLayer);
newClsLayer = classificationLayer();
newSoftmax = softmaxLayer();
lgraph = replaceLayer(lgraph, 'prob', newSoftmax);
lgraph = replaceLayer(lgraph, 'output', newClsLayer);
Add regression layers
regLayers = [fullyConnectedLayer(4 * (numClasses), 'Name',"fcBoxDeltas",...
'WeightLearnRateFactor', 1, ...
'BiasLearnRateFactor', 2, ...
'WeightL2Factor', 1,...
'BiasL2Factor', 0),...
rcnnBoxRegressionLayer('Name', "boxDeltas")];
lgraph = addLayers(lgraph, regLayers);
lgraph = connectLayers(lgraph, "pool5-drop_7x7_s1", "fcBoxDeltas");
Add ROI Pooling
outLayers = ["inception_4e-1x1","inception_4e-3x3_reduce","inception_4e-5x5_reduce","inception_4e-pool"];
for i = 1:numel(outLayers)
lgraph = disconnectLayers(lgraph, 'inception_4d-output', outLayers{i});
end
roiPooling = roiMaxPooling2dLayer([14 14], "Name", "roiPooling");
lgraph = addLayers(lgraph, roiPooling);
% Connect feature layer to the ROI pooling layer
lgraph = connectLayers(lgraph, 'inception_4d-output', "roiPooling" + "/in");
for i = 1:numel(outLayers)
% Connect ROI Pooling layer to disconnected layers.
lgraph = lconnectLayers(lgraph, "roiPooling", outLayers{i});
end
% Add ROI input layer and connect to ROI pooling layer.
roiInput = roiInputLayer('Name',"roiInput");
lgraph = addLayers(lgraph, roiInput);
lgraph = connectLayers(lgraph, "roiInput", "roiPooling" + '/roi');
View the Fast R-CNN network architecture
analyzeNetwork(lgraph);
See this example on creating an R-CNN network using ResNet-50 and this documentation page for more information about designing Fast R-CNN networks. It's worth noting that converting a pretrained network into a Fast R-CNN can be done more easily using Deep Network Designer.
