Working with the Histogram

This document introduces how to work with a histogram.


This document describes the various APIs for working with a Histogram.


A histogram provides details about the exposure and light on a scene. You can use the histogram to adjust the screen settings accordingly.

The code in this tutorial can be found in ecamextendedfunctest.cpp .

Working with a Histogram

The high level steps to work with histogram are shown here:

  • Make sure that you call CCameraV2Histogram::PrepareClientHistogramL before calling CCameraV2Histogram::StartHistogram API.

  • Use CCameraV2Histogram::StartHistogram call to start getting histogram notifications once a direct or a client based histogram is prepared.

  • Use CCamera::CCameraV2Histogram::GetHistogramStateL to get the list of histograms that are currently active. A histogram continues to be in an active state if StartHistogram() has been called on it and has not been yet stopped.

  • Use GetDirectHistogramPropertiesL API to get the current properties used by a direct histogram.

  • Call UpdateDirectHistogramPropertiesL API to update the properties of a direct histogram with the new values and call GetDirectHistogramPropertiesL to verify it.

Receiving Histograms

The histogram data is received as follows:

  • Use MDirectViewFinderObserver::ClientHistogramReady call back to notify the client about availability of the histogram data.

  • If a call to CCameraV2Histogram::StartHistogram is made without a previous successful call to CCameraV2Histogram:: PrepareClientHistogramL , the callback returns KErrBadHandle .

  • This provides a pointer to MHistogramV2Buffer that retrieves a single histogram.

  • If the histogram requested is of direct type, it will be directly embedded into the direct viewfinder. If a non direct histogram is requested, the histogram data will be passed to the camera client.


The following example illustrates the sequence for working with a histogram for client viewfinder.

  1. Start a histogram.

  2. Confirm whether a specific histogram is active.

             TBool histogramActive = EFalse;
  3. Get the current properties of a direct histogram.

             CCamera::CCameraV2Histogram::TDirectHistogramParameters histogramParameters;
  4. Update the properties of histogram with the new values.

             CCamera::CCameraV2Histogram::TDirectHistogramParameters histogramParameters;