Photographic Substitution is a technique in which a computer generated 3D model is superimposed  (kept on the top of) a realistic photograph to conduct a convincing and realistic presentation .  When we try this we get a better understating about whether the conceptual design properly  merges with the actual background in terms of proportion , colour , texture , etc. To create such a perfect presentation you have to perform the following activities .
1. Capture a photograph from the actual background using a digital camera.
2. Photo editing of the background image to remove unwanted objects using Adobe Photoshop .
3. Create the 3D Model of the object (to be superimposed on the background image) using AutoCAD .
4. Apply finishes (materials) , lights and camera (matching camera angle of the actual photograph) , using AutoCAD or 3DS Max
5. Render this model , save the image and overlay it on  the  background image using Adobe Photoshop .
A college has constructed a pavilion in connection with a basket ball court and they need to protect this place by providing a slopped roof with shingles . The college has provided the following photograph captured from the site .
Pavilion shelter
As you can see , this background image needs some photo editing in the first place because there are some unwanted objects like pump set , support for painting etc. to be removed from the photograph .Then some additional details like flowers , pavement tiles etc are to be added on the image .This work is done in Photoshop . Click to enlarge this following image to see the image after photo editing .
After this photo editing work the 3D model of the protection for the pavilion is created using AutoCAD . A sloped roof is made and it is supported using frame work to protect the area below . Here is the image of the 3D Model .
sabeercad pavilion cad model1
This file is taken to 3DStudio max  and a camera is placed which matches with the camera view of the background image . Perspective matching is done through trail and error method . (Preferably both these cameras should have the same parameters such as focal length , distance from the target etc. ) The rendered model without the background appears as shown below.
sabeercad pavilion3
The following shingle tile was used to create the material
brown shingles
This image is taken to Adobe photoshop ,  pasted against the background image and the black transparent areas are deleted to get the realistic feel of the pavilion area protected by the framed roof .
sabeercad pavilion3a
Such a presentation would be always be more effective and successful because seeing is believing.
Click on the following video to see the three stages :


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