Canon Camera Hackers Manual

Contents

4.7 Edge overlay 634.7 Edge overlayThe CHDK Edge Overlay function is able to extract the edges from an imageas soon as you half-press the shutter button and overlay the resulting dia-gram with the current content of the display. This allows viewing the previ-ous shot and the current shot in context, making it easy to register imagesand apply techniques such as onion skinning7.Edge overlay has applications in panorama photography, stereo pho-tography (with a single camera), time-lapse photography, bracketingphotography, and more.Of course, it is possible to choose the color of the edge overlay (menuentry Edge overlay color) or to show the edge overlay in Replay mode. Athreshold value (Edge overlay threshold) determines which brightness dif-ferences are regarded as edge. The lower this value, the more and wideredge lines you get; higher values create fewer and finer lines.With the menu entry Lock Edge Overlay, you can temporarily freeze anexisting edge diagram so that it is not replaced by a new one when youhalf-press the shutter button. Typically, you would use this option when youwant to register several images with one and the same reference image.Edge overlays can be saved (Save Edge Overlay) to the folder CHDK/EDGE/on the memory card. This allows you to use them at a later time. The func-tion Load Edge Overlay opens the file browser (section 4.10.1), allowing youto select an edge overlay file.For example, if you want to photograph a tree at different timesthroughout the year, you could save an edge diagram of the first pictureand reload it when you return to the scene. By registering the diagram withthe image in the display, you can be sure that the tree will always have thesame position in all images.The zoom level is saved with the edge overlay as well, so when you re-load an edge overlay (Load Edge Overlay), the camera is optionally set to thestored zoom level if option Load+Set Zoom is enabled.7 The term onion skinning refers to a technique of creating animated car-toons by overlaying individual frames drawn on onionskin paper.Figure 4-40Edge overlay during a panorama shot.When the first image is taken, theCHDK extracts the edges of thesubject. Then the camera is turned.Because the first image is still visible asa “ghost”, the photographer gets theopportunity to register the next shotwith the previous one.Figure 4-41The Edge overlay submenu