Method and device for intra-coding an image frame

A method, performed by an image pipeline, for intra-coding an image frame, wherein the image pipeline comprises a processing part followed by an encoding part supporting a plurality of intra-prediction modes used for intra-coding image frames, and wherein the method comprises:

• by means of the processing part performing the steps of:
• receiving an image frame;
• for at least one block of pixel values in the image frame:
• predicting pixel values of the at least one block of pixel values using an intra-prediction mode supported by the encoding part;
• reducing a residual between at least one pixel value within the at least one block of pixel values and a corresponding predicted pixel value by adjusting the at least one pixel value within the at least one block of pixel values by a predefined amount up within a pixel value range when the corresponding predicted pixel value is larger than the at least one pixel value or by the predefined amount down within the pixel value range when the corresponding predicted pixel value is smaller than the at least one pixel value, and
• updating the image frame with the adjusted at least one pixel value within the at least one block of pixel values;
• and
• by means of the encoding part performing the step of:
• intra-coding the updated image frame using at least one out of the plurality of intra-prediction modes, wherein the intra-coding comprises:
• for each block of pixel values in the updated image frame, predicting pixel values and calculating residuals using one or more of the intra-prediction modes supported, and
• selecting, for intra-coding of each respective block of pixel values, the intra-prediction mode giving the lowest residual for the respective block of pixel values.