Calca app is a tool that will help you transfer to paper any image you have on your device or any image you capture with your camera making your device act as a camera lucida or as a projector.
    
    HOW DOES IT WORK?
     You need a support, such as a glass, a jar, a box or buy one that fits your needs, there are some that I recommend on my website erbolamm.com. 
     
We will choose an image: 
You can do it from your camera
Searching for one from the internet
Taking a screenshot
One that you already have on your device 
It can be a photo, a drawing, a text, a sketch, as long as it is an image file.  

Then you would go to the editing area:
 Using the buttons on the top bar you can rotate left or right in 90 degree sections.
By dragging over one of the corners you can select the area you want to draw.
Note: The crop size is set to vertical because in the next step the image is scaled to the height in order not to create empty areas and thus not to confuse the user.
    
Finally click on the top save button.

The result of the selected or final edited image will be 50% transparent. 

You can adjust the opacity by means of a slider bar at the bottom, until you get the most comfortable result to start tracing.

After giving permission to your device's camera, you will be able to see through it and the selected image the area where you are going to draw.

Note: Your device MUST FOCUS ON THE DRAWING AREA, be it paper, wall, floor, etc...FOR ITS CORRECT FUNCTIONING, AVOID PASSING YOUR HAND BETWEEN THE DRAWING AREA AND THE DEVICE.

You can draw on paper yes, but you can also make a small mural on the wall if you hold the phone upright and look through it at the image you have chosen on the wall.

You can also use a basic stand to support the device at a 45 degree tilt or whatever you choose, and you can make a 3D effect on the drawing. examples with videos and images in the help section (?)

You can also look for any drawing to colour and start practising.

I recommend relaxing music and a sufficiently lit environment and you will have a wonderful time.
In the help (?) I will be publishing videos of examples, direct that I do in my youtube channel Tutograti tricks, drawings that I have made, etc...

IMPORTANT: 
-This app is not magic (It needs practice, there are demonstrative videos to see results).
-You can search on the internet for any drawing to colour and start practising.
-You can remove advertising with a point system and at no cost. 
-It does not project the image (For this you need a projector). 
-To avoid autofocus you have to prevent your hand from passing under the camera (explained with images in the help blog). 
-You do not need to register with the app, you can download sample images from a secure folder in the cloud or search for any you want on the internet.
-This app is under continuous development, comments and suggestions are welcome at erbolamm.com. 

It is important and necessary to use a support, such as a glass, a jar, a box or the ones I recommend in the TRICKS & TIPS section, in the "about" section (!).

You can draw on a piece of paper yes, but you can also make a small mural on the wall if you hold the phone upright and look through it at the image you have chosen on the wall.

It would be helpful to get your feedback, to know if I'm doing it right or if I need to improve. 

Thank you for using my app.

Draw any photograph you have on your device As if it were an image projector