{"id":991,"date":"2011-05-06T12:59:06","date_gmt":"2011-05-06T03:29:06","guid":{"rendered":"http:\/\/www.clearchain.com\/blog\/?p=991"},"modified":"2011-05-06T12:59:06","modified_gmt":"2011-05-06T03:29:06","slug":"libdc1394_video_set_mode-doesnt-change-the-mode-a-solution","status":"publish","type":"post","link":"https:\/\/www.clearchain.com\/blog\/posts\/libdc1394_video_set_mode-doesnt-change-the-mode-a-solution","title":{"rendered":"libdc1394_video_set_mode doesn’t change the mode – A solution!"},"content":{"rendered":"

This article details a reason libdc1394_video_set_mode\/libdc1394_video_set_framerate\u00a0 may not work.<\/em><\/p>\n

Whilst working on an application that makes use of a 1394a\/firewire camera I found a strange bug\/feature with libdc1394<\/a>. The problem was dc1394 was being used to the mode of the camera to 800×600 resolution. However, when querying the frame size after dequeuing the frame, the size was only 640×480. Ie: See the below snippits:<\/p>\n

..\r\ndc1394error_t result = dc1394_video_set_mode( this->camera, DC1394_VIDEO_MODE_800x600_RGB8);\r\nif( result != DC1394_SUCCESS )\r\nexit(0);\r\n\r\n....\r\ndc1394video_frame_t *frame=NULL;\r\ndc1394error_t result;\r\n\r\n\/\/ capture one frame\r\nresult = dc1394_capture_dequeue( this->camera, DC1394_CAPTURE_POLICY_WAIT, &frame );\r\nif( result != DC1394_SUCCESS )\r\nexit(0);\r\n\r\n\/\/ Allocate image memory if required\r\nif( this->lastFrame.image == NULL ){\r\nmemcpy(this->lastFrame,frame, sizeof(dc1394video_frame_t));\r\nthis->lastFrame.image = (uint8_t *)malloc(frame->total_bytes * sizeof(uint8_t));\r\n\r\n..<\/pre>\n
At this point total_bytes was 921600 which works out to be 640x480x3 (RGB);<\/p>\n
I was stumped. I asked for one mode and got another back. All with no errors what so ever from libdc1394.<\/p>\n
After over an hour of debugging, google searching and more,\u00a0 I eventually found the reason.\u00a0 The comment that ended up in the code explains the cause:<\/p>\n
\/\/ Shutdown any isotransports that are setup for the camera.\r\n\/\/ It turns out that libDC1394 will quite happily let you set modes,\r\n\/\/ framerates, etc but if there is an iso stream running none of them\r\n\/\/ will actually take affect. Hence you can set a 800x600 res and get\r\n\/\/ back 640x480 video frame - really nice libdc1394. It could at least\r\n\/\/ indicate there was a active stream when setting the mode!\r\n\/\/ Anyway to deal with this we force the camera to stop capturing\r\n\/\/ before we change the settings, if the camera was capturing prior to\r\n\/\/ setConfiguration being called, we restart the capture<\/pre>\n
So the solution is to make sure the iso transmission stream has been shutdown. this can be achieved using:<\/p>\n
dc1394_video_set_transmission( this->camera, DC1394_OFF );\r\ndc1394_capture_stop( this->camera );<\/pre>\n
An easy fix but certainly an annoying setup. I’ve not tested it but I’m guessing many of the other functions:<\/p>\n