Can't change data in GstBuffer - c

Question

I am trying to create a set of gstreamer plugins for image processing routines. I have successfully created a source element that reads the image and the metadata into a GstBuffer, and a sink element that writes the data in the buffer to disk (along with the accompanying metadata) as desired. I have successfully tested these, and achieve the desired output (identical to the input with no filters in place).
I have also created a stretching element, that utilizes an external library to fill the dynamic range available (ie a 16-bit image with only 12-bits used per pixel can be stretched to fill the whole 16-bits available).
If I simply push the unchanged buffer out on the srcpad for the Stretching element, I get what I would expect (an unchanged image). However, if I try to perform any sort of manipulation of the data in the buffer, the data in the buffer is set to 0's.
Here is the current implementation of the chain() function for my Stretching plugin:
static GstFlowReturn
gst_stretching_chain(GstPad *pad, GstObject *parent, GstBuffer *buf)
{
GstStretching *filter;
filter = GST_STRETCHING(parent);
g_print("Stretching...\n");
guint num_rows;
g_object_get(G_OBJECT(parent), "num_rows", &num_rows, NULL);
guint num_cols;
g_object_get(G_OBJECT(parent), "num_cols", &num_cols, NULL);
guint bit_depth;
g_object_get(G_OBJECT(parent), "bit_depth", &bit_depth, NULL);
guint sig_bits;
g_object_get(G_OBJECT(parent), "sig_bits", &sig_bits, NULL);
gchar *product;
g_object_get(G_OBJECT(parent), "product", &product, NULL);
GstMapInfo info_in;
gst_buffer_map(buf, &info_in, GST_MAP_WRITE);
guint8 *in = info_in.data;
GstMemory *mem;
mem = gst_allocator_alloc(NULL, num_rows*num_cols*bit_depth/8, NULL);
GstMapInfo info_out;
gst_memory_map(mem, &info_out, GST_MAP_WRITE);
guint8 *out = info_out.data;
float *rad_gain[4] = {NULL, NULL, NULL, NULL};
float *rad_offset[4] = {NULL, NULL, NULL, NULL};
StretchingImage((unsigned short int *)in, num_rows, num_cols, sig_bits,
bit_depth, rad_gain, rad_offset, 0, product, (unsigned short int *)out);
gst_buffer_unmap(buf, &info_in);
gst_buffer_replace_all_memory(buf, mem);
return gst_pad_push(filter->srcpad, buf);
}
When this did not work, I also tried a simple change of the data manually (to see if I would get the expected output):
static GstFlowReturn
gst_stretching_chain(GstPad *pad, GstObject *parent, GstBuffer *buf)
{
GstStretching *filter;
filter = GST_STRETCHING(parent);
g_print("Stretching...\n");
guint num_rows;
g_object_get(G_OBJECT(parent), "num_rows", &num_rows, NULL);
guint num_cols;
g_object_get(G_OBJECT(parent), "num_cols", &num_cols, NULL);
guint bit_depth;
g_object_get(G_OBJECT(parent), "bit_depth", &bit_depth, NULL);
guint sig_bits;
g_object_get(G_OBJECT(parent), "sig_bits", &sig_bits, NULL);
gchar *product;
g_object_get(G_OBJECT(parent), "product", &product, NULL);
GstMapInfo info_in;
gst_buffer_map(buf, &info_in, GST_MAP_WRITE);
guint8 *in = info_in.data;
GstMemory *mem;
mem = gst_allocator_alloc(NULL, num_rows*num_cols*bit_depth/8, NULL);
GstMapInfo info_out;
gst_memory_map(mem, &info_out, GST_MAP_WRITE);
guint8 *out = info_out.data;
int i;
for (i=0; i<num_rows*num_cols*bit_depth/8; i++) {
out[i] = 255;
}
float *rad_gain[4] = {NULL, NULL, NULL, NULL};
float *rad_offset[4] = {NULL, NULL, NULL, NULL};
StretchingImage((unsigned short int *)in, num_rows, num_cols, sig_bits,
bit_depth, rad_gain, rad_offset, 0, product, (unsigned short int *)out);
gst_buffer_unmap(buf, &info_in);
gst_buffer_replace_all_memory(buf, mem);
return gst_pad_push(filter->srcpad, buf);
}
Even with this, I still obtain all 0's when I examine the output. I am assuming I am doing something wrong when trying to access the data in the buffer, but haven't yet been able to figure out what it may be. Any ideas?

Answer 1

The
gst_buffer_map(buf, &info_in, GST_MAP_WRITE);
should be
gst_buffer_map(buf, &info_in, GST_MAP_READ);
Also fyi, you can simplfy the code
guint num_rows, num_cols, ...;
g_object_get(G_OBJECT(parent),
"num_rows", &num_rows,
"num_cols", &num_cols,
...
NULL);